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Papers by Michael Durako

Research paper thumbnail of Assessment of the toxicity of Kuwait crude oil on the photosynthesis and respiration of seagrasses of the northern Gulf

Marine Pollution Bulletin, 1993

Photosynthetic and respiratory responses of leaf tissues of the seagrasses Halophila ovalis, H. s... more Photosynthetic and respiratory responses of leaf tissues of the seagrasses Halophila ovalis, H. stipulacea, and Halodule uninervis exposed for 12-18 h to unweathered Kuwait crude oil were measured using an oxygen electrode system to assess the possibility of acute toxicity. Leaf tissues were incubated in natural seawater (control treatment) or in the water-soluble fraction of a 1% (weight:volume) solution of Kuwait crude oil in seawater (oil treatment). Photosynthesis vs. irradiance (PI) responses exhibited typical lightsaturation kinetics. One-way analysis of variance detected no significant treatment effects on the PI characteristics: c~, Pm~, Ik, or I c. Respiration rates were also not significantly affected by short-term exposure to the oil treatment. In addition, no significant amongspecies differences in PI characteristics or respiration were detected, possibly reflecting the low metabolic state for these subtropical species during Leg II sampling when water temperatures were 16-19°C. These results support our observations that the Gulf War oil spill primarily impacted intertidal communities rather than the submergent plant communities of the northern Gulf region.

Research paper thumbnail of Megagametogenesis in Halophila johnsonii, a threatened seagrass with no known seeds, and the seed-producing Halophila decipiens (Hydrocharitaceae)

Aquatic Botany, May 1, 2008

This article was published in an Elsevier journal. The attached copy is furnished to the author f... more This article was published in an Elsevier journal. The attached copy is furnished to the author for non-commercial research and education use, including for instruction at the author's institution, sharing with colleagues and providing to institution administration. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier's archiving and manuscript policies are encouraged to visit: http://www.elsevier.com/copyright

Research paper thumbnail of Distribution, abundance, and species composition of seagrasses at several sites in Oman

Aquatic Botany, Apr 1, 1996

... Ecology and productivity of the sublittoral algae Ecklonia radiata and Sargassopsis zanardini... more ... Ecology and productivity of the sublittoral algae Ecklonia radiata and Sargassopsis zanardinii. Part I. Ecological studies of southern Oman kelp communi 212 BP Jupp et al. ... Phillips, RC, 1960. Observations on the ecology and distribution of the Florida seagrasses. ...

Research paper thumbnail of A comparative seasonal study of two populations of Hypnea musciformis from the East and West Coasts of Florida, USA II. Photosynthetic and respiratory rates

Marine Biology, 1980

Dry weight levels of the red alga Hypnea musciformis (Wulfen) Lamouroux from Atlantic and Gulf of... more Dry weight levels of the red alga Hypnea musciformis (Wulfen) Lamouroux from Atlantic and Gulf of Mexico coastal sites in Florida, USA were lowest in the late winter and early spring, increasing through the summer and highest in the fall. There was a two-month lag in the Gulf coast population's dry weight pattern, indicating differing growth patterns. Chlorophyll a, phycoerythrin and phycoerythrin/chlorophyll a ratios were highest in the winter and lowest in the summer for both populations. Total pigment levels for//, musciformis from the Atlantic coast site were significantly greater than those of the Gulf coast. Protein and carbohydrate percentages were inversely related in both populations, with carbohydrate levels highest in summer and protein levels highest in winter. The Gulf coast population contained significantly more protein than the Atlantic coast plants. Carrageenan levels were highest in spring and lowest in fall, the Atlantic coast population generally had higher levels than the Gulf coast population. The differences in seasonal patterns and levels of the chemical constituents were reflected by distinct morphological characteristics for each population. The Atlantic coast population was larger, darker, more coarse in texture and possessed more crozier branch tips than the Gulf coast plants. These distinctions represent acclimitization responses that relate to habitat differences.

Research paper thumbnail of Malate and lactate dehydrogenase activities in the pyloric caeca of Luidia clathrata (Echinodermata: Asteroidea)

Comparative biochemistry and physiology. B. Comparative biochemistry, 1979

... Biol. 36, 31-36. ELLINGTON WR & LAWRENCE JM (1973) Malic and lactatedehydrogenase... more ... Biol. 36, 31-36. ELLINGTON WR & LAWRENCE JM (1973) Malic and lactatedehydrogenase activities in regular and irregular echinoids. Comp. Biochem. ... Mar. Biol. 37, 279-289. SIMPSON JW 8l. AWAPARA J. (1966) The pathway of glu-cose degradation in some invertebrates. ...

Research paper thumbnail of Particulate matter resuspension via metabolically produced gas bubbles from benthic estuarine microalgae communities

Limnology and Oceanography, Jul 1, 1982

Gas bubbles originating from interstitial and epibenthic organisms in Thalassia testudinum cultur... more Gas bubbles originating from interstitial and epibenthic organisms in Thalassia testudinum culture and field systems were observed to rise and resuspend adsorbed particulate matter. In culture, 307.9 ml of gas were evolved and 0.97 g dry wt of particulate matter was resuspended·m−2·24 h−1. In situ studies in Tampa Bay, Florida, yielded similar values for gas production, but higher particulate loads. Chromatographic analyses of the bubbles indicated that the major component was photosynthetically produced oxygen. The resuspended particulate matter was 70–96% inorganic; the organic fraction consisted of living organisms and detrital material. This phenomenon may be important in the sediment kinetics and nutrient cycles of estuarine systems.

Research paper thumbnail of Flavones and flavone glycosides from Halophila johnsonii

Phytochemistry, Oct 1, 2008

Halophila johnsonii Eiseman is a shallow-water marine angiosperm which contains UV-absorbing meta... more Halophila johnsonii Eiseman is a shallow-water marine angiosperm which contains UV-absorbing metabolites. Studies on methanol extracts of H. johnsonii by means of HPLC-UV, NMR, HPLC-MS resulted in isolation and identification of seven previously unknown flavone glycosides: 5,6,7,3 0 ,4 0 ,5 0-hexahydroxyflavone-7-O-b-glucopyranoside (1), 5,6,7,3 0 ,4 0 ,5 0-hexahydroxyflavone-7-O-(6 00-O-acetyl)-b-glucopyranoside (2), 6-hydroxyluteolin-7-O-(6 00-O-acetyl)-b-glucopyranoside (3), 6-hydroxyapigenin-7-O-(6 00-Oacetyl)-b-glucopyranoside (4), 6-hydroxyapigenin-7-O-(6 00-O-[E]-coumaroyl)-b-glucopiranoside (5), 6-hydroxyapigenin-7-O-(6 00-O-[E]-caffeoyl)-b-glucopyranoside (6) and 6-hydroxyluteolin-7-O-(6 00-O-[E]coumaroyl)-b-glucopyranoside (7). Also isolated were three known flavone glycosides, 6-hydroxyluteolin 7-O-b-glucopyranoside (8), scutellarein-7-O-b-glucopyranoside (9), and spicoside (10), and five known flavones, pedalitin (11), ladanetin (12), luteolin (13), apegenin (14) and myricetin (15). Qualitative comparison of the flavonoid distribution in the leaf and rhizome-root portions of the plant was also investigated, with the aim of establishing the UV-protecting roles that flavonoids played in the sea grass.

Research paper thumbnail of Photosynthetic characteristics of Thalassia testudinum measured in situ by pulse-amplitude modulated (PAM) fluorometry: methodological and scale-based considerations

Aquatic Botany, Jun 1, 2002

Shoot-to-landscape scale sources of variation in photosynthetic characteristics of Thalassia test... more Shoot-to-landscape scale sources of variation in photosynthetic characteristics of Thalassia testudinum, as measured in situ using a submersible pulse-amplitude modulated fluorometer (diving-PAM), were investigated. Shoot-scale variation was assessed to develop a standard-methods protocol for this species. Significant within-shoot and among-shoot scale variation was observed for several PAM-fluorescence parameters. The fraction of incident photosynthetically active radiation (PAR) absorbed by rank 1 (youngest) leaves, 0.67 ± 0.03, was significantly lower than the PAR absorbed by rank 2 and 3 leaves (0.78 ± 0.04 and 0.77 ± 0.04, respectively). Quantum yields (Y) and photosynthetic efficiency (F v /F m) exhibited greater variability and generally decreased with increasing leaf age (Y = 0.78 ± 0.02, 0.78 ± 0.03, and 0.76 ± 0.06 and F v /F m = 0.79 ± 0.02, 0.77 ± 0.5, and 0.75 ± 0.06 for rank 1, 2, and 3 leaves, respectively). Maximum fluorescence of light-acclimated leaves (F m) significantly decreased from the base to the tip of leaves (1032 ± 203, 1059 ± 139, and 793 ± 107 for the base, middle and tip of the leaf) and Y was significantly reduced from 0.68-0.76 to 0.27-0.62 when measured where leaf lesions were present. However, paired comparisons (with versus without lesions at the tip of the leaf) for the base, middle, and tip areas of rank 2 leaves indicated that lesions effects on photosynthesis were localized only to the area of the lesion (tip). The Y exhibited relatively high variability for short-shoots within seagrass die-off patches and were significantly reduced for short-shoots exhibiting die-off symptoms (rotten leaf base grayish-green leaf color, but no lesions). Negative response slopes for Y and F v /F m , measured as part of a landscape-scale sampling program, revealed the presence of significant diurnal (time-of-day) and photosynthetic photon flux density (PPFD) related variation, with a more negative slope for Y. These results indicate that the selection of leaf tissue, short-shoot location, and time of

Research paper thumbnail of Spatial Influences on Temporal Variations in Leaf Growth and Chemical Composition of Thalassia testudinum Banks Ex König in Tampa Bay, Florida

Gulf Research Reports, 1985

The importance of spatial influences on seasonal fluctuations in Thalassia testudinum leaf blade ... more The importance of spatial influences on seasonal fluctuations in Thalassia testudinum leaf blade lengths and chemical constituents was demonstrated. Differences between samples from fringe and mid-bed for several constituents were significant and, if not accounted for, could affect the measurement of apparent seasonal cycles. Fringe-shoots, reflecting the influence of more intense grazing activity, had shorter leaf blade lengths, lower dry weights and carbohydrate levels, and higher protein levels than mid-bed shoots. Mid-bed rhizomes and roots had highest protein and ash levels reflecting possible sediment influence. Percent ash and protein in the rhizomes, and percent carbohydrate in the roots exhibited seasonal fluctuations, but the levels were different between fringe and mid-bed samples. Protein levels were greatest in shoots and roots, while carbohydrate levels were highest in rhizomes, illustrating the respective partitioning of biosynthetic and storage functions. The spatial differences seem to reflect gradients in biological and chemical interactions, and they may play an important role in trophic interactions in seagrass systems.

Research paper thumbnail of Chemical fluctuations due to seasonal and cropping effects on an algal-seagrass community

Research paper thumbnail of Recurrence of Thalassia testudinum seagrass die-off in Florida Bay, USA: initial observations

Marine Ecology Progress Series, 2016

Widespread mortality of Thalassia testudinum was first documented in Florida Bay, USA, during the... more Widespread mortality of Thalassia testudinum was first documented in Florida Bay, USA, during the summer of 1987. This unprecedented event spanned 3 yr, affected 40 km 2 of seagrass and resulted in more than a decade of ecological disturbances. Initial putative causes for seagrass die-off ranged from climatic anomalies and watershed changes to wasting disease and eutrophication. Subsequent experimental research suggested that hypoxic plant tissue, caused by low water column oxygen content or reduced photosynthesis, allowed intrusion of sulfide leading to plant death. Contributing factors included high temperatures, salinities and T. testudinum biomass, together causing lower oxygen water solubility, higher community respiration rates and elevated nighttime oxygen demand. The Fisheries Habitat Assessment Program (FHAP) has tracked the system's slow recovery since 1995. Recent FHAP data (2012) indicated that T. testudinum had returned to pre-die-off densities in even the most severely affected locations. During the summer of 2015, following several months of drought, National Park Service researchers reported hypersaline conditions and a recurrence of seagrass die-off in north-central Florida Bay. An interagency effort is presently underway to document the duration, extent, impacts and possible factors responsible for the current mortality. Initial field surveys indicate that there is high spatial coincidence between the current and the 1987−1990 events and that hypersalinity, water column stratification and bottom-water anoxia might have once again resulted in mass mortality of T. testudinum in Florida Bay. The goal of this report is to alert the scientific community to the recurrence of this important ecological event.

Research paper thumbnail of Landscape-scale variation in a sulfur-based sediment stress indicator for the seagrass Thalassia testudinum in Florida Bay, USA

Marine Ecology Progress Series

Intrusion of sediment-derived hydrogen sulfide into above-ground tissues of seagrasses is correla... more Intrusion of sediment-derived hydrogen sulfide into above-ground tissues of seagrasses is correlated with reduced growth and has been linked to large-scale die-offs of Thalassia testudinum in Florida Bay, USA. In May of 2019, leaves from T. testudinum short shoots at 350 sites within 13 basins across Florida Bay were collected to investigate bay-wide variation in a novel, stable sulfur isotope-based indicator of hydrogen sulfide intrusion [(δ34Sleaf + 30)/total sulfur content]. The components of this sediment stress indicator (SSI), total sulfur content (% dry weight) and δ34S, were negatively correlated (R2 = -0.24), indicating greater sediment sulfide exposure in plants with higher sulfur content. Generalized additive model selection revealed that SSI was best predicted by a model which included T. testudinum cover, sediment depth, and Halodule wrightii cover (R2 = 0.24, weight = 0.48). Macrophyte communities dominated by dense T. testudinum climax communities and with deep sedime...

Research paper thumbnail of Interspecific variation in diurnal photo-protective strategies of Thalassia testudinum and Halophila johnsonii

PAM fluorometer data of diurnal changes in 2013 + 2014HPLC diurnal photopigment data in 2014Trolo... more PAM fluorometer data of diurnal changes in 2013 + 2014HPLC diurnal photopigment data in 2014Trolox equivalent antioxidant capacity (TEAC) diurnal data for 2013Physical water quality data 2013 + 2014

Research paper thumbnail of Leaf Spectral Reflectance Shows Thalassia testudinum Seedlings More Sensitive to Hypersalinity than Hyposalinity

Frontiers in Plant Science, 2017

Thalassia testudinum (turtle grass) is the dominant and climax-successional seagrass species in t... more Thalassia testudinum (turtle grass) is the dominant and climax-successional seagrass species in the subtropical/tropical Atlantic and Caribbean region. Two die-offs of T. testudinum in Florida Bay, United States have raised concerns regarding the resilience of this species to environmental disturbances. Seedlings are important in recovery of T. testudinum, following disturbance events. Leaf spectral reflectance [R(λ)] was measured in T. testudinum seedlings exposed for 2 weeks to three salinities (20, 35, and 50) and two light levels (full sun and 50-70% light reduction) in experimental mesocosms. Multivariate analyses indicated that hypersalinity had a greater effect on spectral reflectance than hyposalinity or light reduction. There was an increase in variability and flattening of reflectance spectra at the highest salinity. All three salinity treatments had distinct reflectance spectra across green wavelengths (530-580 nm), with additional discrimination between 20 versus 50 and 35 versus 50 treatments across red wavelengths (630-690 nm). Red:Green reflectance ratios were highest and photochemical reflective index values were lowest for the salinity 50 treatment, but were not significantly different between the salinity 20 and 35 treatments. The changes in the R(λ) spectra for the salinity 50 seedlings were consistent with previously observed reductions in leaf pigments and maximum photochemical efficiency of photosystem II. These observations indicate that leaf spectral reflectance is a sensitive indicator of plant stress in T. testudinum seedlings and that seedlings are more sensitive to short-term exposures to hypersalinity than hyposalinity.

Research paper thumbnail of Assessment of the toxicity of Kuwait crude oil on the photosynthesis and respiration of seagrasses of the northern Gulf

Marine Pollution Bulletin, 1993

Photosynthetic and respiratory responses of leaf tissues of the seagrasses Halophila ovalis, H. s... more Photosynthetic and respiratory responses of leaf tissues of the seagrasses Halophila ovalis, H. stipulacea, and Halodule uninervis exposed for 12-18 h to unweathered Kuwait crude oil were measured using an oxygen electrode system to assess the possibility of acute toxicity. Leaf tissues were incubated in natural seawater (control treatment) or in the water-soluble fraction of a 1% (weight:volume) solution of Kuwait crude oil in seawater (oil treatment). Photosynthesis vs. irradiance (PI) responses exhibited typical lightsaturation kinetics. One-way analysis of variance detected no significant treatment effects on the PI characteristics: c~, Pm~, Ik, or I c. Respiration rates were also not significantly affected by short-term exposure to the oil treatment. In addition, no significant amongspecies differences in PI characteristics or respiration were detected, possibly reflecting the low metabolic state for these subtropical species during Leg II sampling when water temperatures were 16-19°C. These results support our observations that the Gulf War oil spill primarily impacted intertidal communities rather than the submergent plant communities of the northern Gulf region.

Research paper thumbnail of Ecology of seagrasses in northeastern Saudi Arabia one year after the Gulf War oil spill

Marine Pollution Bulletin, 1993

Approximately 1 year after the Gulf War oil spill we examined the distribution, species compositi... more Approximately 1 year after the Gulf War oil spill we examined the distribution, species composition, abundance and productivity of seagrasses in several oil contaminated bays along the northeastern coastline of Saudi Arabia. The most abundant seagrass, Halodule uninervis, grew from the intertidal down to depths of 10 m with a standing crop ranging from 2 to 32 gdw m-2. Specific leaf productivity in a heavily oiled shallow site (1.5 m depth) was 2.2% d-1 and areal production was 0.172 gdw m-2 d-1. Specific productivity was similar to other reported rates for healthy populations of Halodule species but the low areal productivity reflected late winter water temperatures which were between 16 and 19"C. Halophila twalis was intermediate in abundance followed by Halophila stipulacea. Leaf morphology and

Research paper thumbnail of Recruitment of Thalassia testudinum seedlings into physically disturbed seagrass beds

Marine Ecology Progress Series, 2004

Thalassia testudinum seedling recruitment, survival, and growth were examined within physically d... more Thalassia testudinum seedling recruitment, survival, and growth were examined within physically disturbed seagrass beds in the Florida Keys. Two separate studies were conducted. In the first, a large-scale study, T. testudinum seedlings were surveyed and collected from a large seagrass disturbance (1560 m 2), 4.8 yr after the initial impact from a motor-vessel grounding. In the second, a smaller-scale study, T. testudinum seedling recruitment was examined over a 2 yr period within 9 smaller motor-vessel disturbances located within intact T. testudinum beds. In the large-scale study, we reconstructed the age of the seedlings based on shoot production rates from a previous study and from the small-scale study. A total of 79 seedlings were collected that varied in age from young of the year to 4.8 yr old; 6 different seedling cohorts were identified. The average density and rhizomeelongation rate for 1 yr old seedlings were 1 m-2 and 6.6 cm yr-1 , respectively. In the small-scale study, we surveyed and permanently marked all newly recruited seedlings; monitoring was conducted 5 more times over a 2 yr period. The average seedling survival after Year 1 was 42%; after Year 2, average survival dropped to 20%. The average seedling density after Year 1 was 0.071 m-2 ; after Year 2, average density dropped to 0.026 m-2. The average rhizome-elongation rate and shootproduction rate of 1 yr old seedlings were 6 cm yr-1 and 1.8 shoots yr-1 , respectively. The results of this study suggest that T. testudinum seedlings are a regular and reliable source of new recruits on seagrass banks in the Florida Keys.

Research paper thumbnail of Impact of light limitation on seagrasses

Journal of Experimental Marine Biology and Ecology, 2007

Seagrass distribution is controlled by light availability, especially at the deepest edge of the ... more Seagrass distribution is controlled by light availability, especially at the deepest edge of the meadow. Light attenuation due to both natural and anthropogenically-driven processes leads to reduced photosynthesis. Adaptation allows seagrasses to exist under these sub-optimal conditions. Understanding the minimum quantum requirements for growth (MQR) is revealed when light conditions are insufficient to maintain a positive carbon balance, leading to a decline in seagrass growth and distribution. Respiratory demands of photosynthetic and non-photosynthetic tissues strongly influence the carbon balance, as do resource allocations between above-and below-ground biomass. Seagrass light acclimation occurs on varying temporal scales, as well as across spatial scales, from the position along a single leaf blade to within the canopy and finally across the meadow. Leaf absorptance is regulated by factors such as pigment content, morphology and physical properties. Chlorophyll content and morphological characteristics of leaves such as leaf thickness change at the deepest edge. We present a series of conceptual models describing the factors driving the light climate and seagrass responses under current and future conditions, with special attention on the deepest edge of the meadow.

Research paper thumbnail of Dissolved oxygen stratification in two micro-tidal partially-mixed estuaries

Estuarine, Coastal and Shelf Science, 2006

The controlling physical factors for vertical oxygen stratification in micro-tidal, partially-mix... more The controlling physical factors for vertical oxygen stratification in micro-tidal, partially-mixed estuaries are discussed in this paper. A theoretical deduction shows that vertical stratification of dissolved oxygen (DO) concentration can be explained by the extended Hansen and Rattray's Central Region theory, which suggests that in addition to biological factors such as photosynthesis, biochemical oxygen demand (BOD), sediment oxygen demand (SOD), vertical DO profiles are mainly controlled by physical factors such as surface re-aeration, river flow, and estuarine gravitational circulation. Vertical mixing of DO from surface re-aeration and photosynthesis sets a DO profile of higher concentration near the surface and lower near the bottom. With a positive seaward longitudinal DO gradient, strong river flow and estuarine gravitational circulation can cause lower DO concentrations near the surface and higher near the bottom. The actual vertical oxygen profile is then determined by the relative magnitude of the above-mentioned mechanisms. It is sensitive to two parameters: (1) the strength of the gravitational circulation (u E); and (2) the relative importance between biochemical oxygen demand and vertical diffusivity (a). Vertical DO stratification usually becomes weaker as u E increases. The impact of gravitational circulation on vertical oxygen distribution becomes more important for a larger a. The impact of a on oxygen stratification is profound. As u E (and river flow) increases, DO stratification appears to be less sensitive to the value of a. Surface-to-bottom differences in DO concentrations (DDO) is negligible when a is small (a < 0.5). As a increases, DDO increases under a weak to moderate gravitational circulation mode (u E 5 cm s À1). Under a strong gravitational circulation mode, DDO becomes negative with a small a (a < 2), and as a continues to increase, DDO becomes positive. The newly-deduced governing equation for vertical oxygen stratification is applied to two micro-tidal, partially-mixed estuarine systems: the Cape Fear River Estuary (CFRE) and the Pamlico River Estuary (PRE) of North Carolina. In the CFRE, although strong vertical salinity stratification exists, DO concentrations are usually well mixed. De-coupling between salinity stratification and oxygen stratification is mainly due to a relatively stronger estuarine gravitational circulation and higher freshwater inflow in the system. It appears that river flow and gravitational circulation are the dominant factors in controlling oxygen stratification in the CFRE. In contrast, vertical stratification of DO concentrations is closely correlated with that of salinity in the PRE. In the PRE, the estuarine gravitational mode and river flow are often both very weak, and DO stratification is very sensitive to the value of a. With negligible influence from tidal mixing, the system is more sensitive to vertical mixing regulated by salinity stratification and wind. As a result, vertical DO stratification is closely correlated with salinity stratification in the PRE.

Research paper thumbnail of Population-based variation in resilience to hyposalinity stress in <i>Halophila johnsonii</i>

Bulletin of Marine Science, 2014

Plants of the threatened seagrass, Halophila johnsonii Eiseman, from a riverine and marine popula... more Plants of the threatened seagrass, Halophila johnsonii Eiseman, from a riverine and marine population were exposed to a series of salinity treatments within mesocosms. Survival and maximum photochemical efficiencies of PSII (F v /F m) were measured in response to varied frequency, duration, and amplitude of hyposalinity exposures. Both populations exhibited high survival after two cycles of short, pulsed hyposalinity treatments (100% and 89%, respectively). However, two cycles of long pulses of low salinity resulted in 100% mortality for marine H. johnsonii and >50% mortality for riverine plants. After two cycles of gradual salinity reduction to a salinity of 5, survival for marine and riverine plants was also low (22% and 33%, respectively). F v /F m values of riverine H. johnsonii were relatively high (0.65-0.70) after a single short pulse or gradual reduction to salinity of 10. F v /F m values of marine plants were lower in these two treatments and exhibited greater declines following a single, long pulse to a salinity of 10 or after a single gradual reduction to a salinity of 5. F v / F m values showed riverine plants were also more resistant to repeated pulses of moderate low salinity than marine plants. Our results indicate wide tolerances of H. johnsonii to short pulses of hyposalinity, but suggest that repeated or prolonged hyposalinity stress at near-tolerance salinity levels can have additive effects on photosynthetic health and survival. Differences in resilience of marine and riverine H. johnsonii populations are consistent with previous suggestions that estuarine ecophenes are more tolerant to hyposalinity than marine populations. Halophila johnsonii Eiseman is a small-bodied seagrass with an extremely limited geographic range. Its distribution is restricted to about 200 km of coastal lagoons and estuaries of southeastern Florida from Sebastian Inlet (27°51´N, 80°27´W) in northern Indian River Lagoon to northern Biscayne Bay (25°45´N, 80°07´W), which includes man-made navigation channels (Intracoastal Waterway) that connect these larger water bodies and stormwater drainage canals managed by state water management districts (Kenworthy 1997). Halophila johnsonii is the least abundant seagrass within its range and is considered an opportunistic species, often occurring intertidally or at the upper and lower margins of seagrass beds where other seagrass research paper

Research paper thumbnail of Assessment of the toxicity of Kuwait crude oil on the photosynthesis and respiration of seagrasses of the northern Gulf

Marine Pollution Bulletin, 1993

Photosynthetic and respiratory responses of leaf tissues of the seagrasses Halophila ovalis, H. s... more Photosynthetic and respiratory responses of leaf tissues of the seagrasses Halophila ovalis, H. stipulacea, and Halodule uninervis exposed for 12-18 h to unweathered Kuwait crude oil were measured using an oxygen electrode system to assess the possibility of acute toxicity. Leaf tissues were incubated in natural seawater (control treatment) or in the water-soluble fraction of a 1% (weight:volume) solution of Kuwait crude oil in seawater (oil treatment). Photosynthesis vs. irradiance (PI) responses exhibited typical lightsaturation kinetics. One-way analysis of variance detected no significant treatment effects on the PI characteristics: c~, Pm~, Ik, or I c. Respiration rates were also not significantly affected by short-term exposure to the oil treatment. In addition, no significant amongspecies differences in PI characteristics or respiration were detected, possibly reflecting the low metabolic state for these subtropical species during Leg II sampling when water temperatures were 16-19°C. These results support our observations that the Gulf War oil spill primarily impacted intertidal communities rather than the submergent plant communities of the northern Gulf region.

Research paper thumbnail of Megagametogenesis in Halophila johnsonii, a threatened seagrass with no known seeds, and the seed-producing Halophila decipiens (Hydrocharitaceae)

Aquatic Botany, May 1, 2008

This article was published in an Elsevier journal. The attached copy is furnished to the author f... more This article was published in an Elsevier journal. The attached copy is furnished to the author for non-commercial research and education use, including for instruction at the author's institution, sharing with colleagues and providing to institution administration. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier's archiving and manuscript policies are encouraged to visit: http://www.elsevier.com/copyright

Research paper thumbnail of Distribution, abundance, and species composition of seagrasses at several sites in Oman

Aquatic Botany, Apr 1, 1996

... Ecology and productivity of the sublittoral algae Ecklonia radiata and Sargassopsis zanardini... more ... Ecology and productivity of the sublittoral algae Ecklonia radiata and Sargassopsis zanardinii. Part I. Ecological studies of southern Oman kelp communi 212 BP Jupp et al. ... Phillips, RC, 1960. Observations on the ecology and distribution of the Florida seagrasses. ...

Research paper thumbnail of A comparative seasonal study of two populations of Hypnea musciformis from the East and West Coasts of Florida, USA II. Photosynthetic and respiratory rates

Marine Biology, 1980

Dry weight levels of the red alga Hypnea musciformis (Wulfen) Lamouroux from Atlantic and Gulf of... more Dry weight levels of the red alga Hypnea musciformis (Wulfen) Lamouroux from Atlantic and Gulf of Mexico coastal sites in Florida, USA were lowest in the late winter and early spring, increasing through the summer and highest in the fall. There was a two-month lag in the Gulf coast population's dry weight pattern, indicating differing growth patterns. Chlorophyll a, phycoerythrin and phycoerythrin/chlorophyll a ratios were highest in the winter and lowest in the summer for both populations. Total pigment levels for//, musciformis from the Atlantic coast site were significantly greater than those of the Gulf coast. Protein and carbohydrate percentages were inversely related in both populations, with carbohydrate levels highest in summer and protein levels highest in winter. The Gulf coast population contained significantly more protein than the Atlantic coast plants. Carrageenan levels were highest in spring and lowest in fall, the Atlantic coast population generally had higher levels than the Gulf coast population. The differences in seasonal patterns and levels of the chemical constituents were reflected by distinct morphological characteristics for each population. The Atlantic coast population was larger, darker, more coarse in texture and possessed more crozier branch tips than the Gulf coast plants. These distinctions represent acclimitization responses that relate to habitat differences.

Research paper thumbnail of Malate and lactate dehydrogenase activities in the pyloric caeca of Luidia clathrata (Echinodermata: Asteroidea)

Comparative biochemistry and physiology. B. Comparative biochemistry, 1979

... Biol. 36, 31-36. ELLINGTON WR &amp;amp; LAWRENCE JM (1973) Malic and lactatedehydrogenase... more ... Biol. 36, 31-36. ELLINGTON WR &amp;amp; LAWRENCE JM (1973) Malic and lactatedehydrogenase activities in regular and irregular echinoids. Comp. Biochem. ... Mar. Biol. 37, 279-289. SIMPSON JW 8l. AWAPARA J. (1966) The pathway of glu-cose degradation in some invertebrates. ...

Research paper thumbnail of Particulate matter resuspension via metabolically produced gas bubbles from benthic estuarine microalgae communities

Limnology and Oceanography, Jul 1, 1982

Gas bubbles originating from interstitial and epibenthic organisms in Thalassia testudinum cultur... more Gas bubbles originating from interstitial and epibenthic organisms in Thalassia testudinum culture and field systems were observed to rise and resuspend adsorbed particulate matter. In culture, 307.9 ml of gas were evolved and 0.97 g dry wt of particulate matter was resuspended·m−2·24 h−1. In situ studies in Tampa Bay, Florida, yielded similar values for gas production, but higher particulate loads. Chromatographic analyses of the bubbles indicated that the major component was photosynthetically produced oxygen. The resuspended particulate matter was 70–96% inorganic; the organic fraction consisted of living organisms and detrital material. This phenomenon may be important in the sediment kinetics and nutrient cycles of estuarine systems.

Research paper thumbnail of Flavones and flavone glycosides from Halophila johnsonii

Phytochemistry, Oct 1, 2008

Halophila johnsonii Eiseman is a shallow-water marine angiosperm which contains UV-absorbing meta... more Halophila johnsonii Eiseman is a shallow-water marine angiosperm which contains UV-absorbing metabolites. Studies on methanol extracts of H. johnsonii by means of HPLC-UV, NMR, HPLC-MS resulted in isolation and identification of seven previously unknown flavone glycosides: 5,6,7,3 0 ,4 0 ,5 0-hexahydroxyflavone-7-O-b-glucopyranoside (1), 5,6,7,3 0 ,4 0 ,5 0-hexahydroxyflavone-7-O-(6 00-O-acetyl)-b-glucopyranoside (2), 6-hydroxyluteolin-7-O-(6 00-O-acetyl)-b-glucopyranoside (3), 6-hydroxyapigenin-7-O-(6 00-Oacetyl)-b-glucopyranoside (4), 6-hydroxyapigenin-7-O-(6 00-O-[E]-coumaroyl)-b-glucopiranoside (5), 6-hydroxyapigenin-7-O-(6 00-O-[E]-caffeoyl)-b-glucopyranoside (6) and 6-hydroxyluteolin-7-O-(6 00-O-[E]coumaroyl)-b-glucopyranoside (7). Also isolated were three known flavone glycosides, 6-hydroxyluteolin 7-O-b-glucopyranoside (8), scutellarein-7-O-b-glucopyranoside (9), and spicoside (10), and five known flavones, pedalitin (11), ladanetin (12), luteolin (13), apegenin (14) and myricetin (15). Qualitative comparison of the flavonoid distribution in the leaf and rhizome-root portions of the plant was also investigated, with the aim of establishing the UV-protecting roles that flavonoids played in the sea grass.

Research paper thumbnail of Photosynthetic characteristics of Thalassia testudinum measured in situ by pulse-amplitude modulated (PAM) fluorometry: methodological and scale-based considerations

Aquatic Botany, Jun 1, 2002

Shoot-to-landscape scale sources of variation in photosynthetic characteristics of Thalassia test... more Shoot-to-landscape scale sources of variation in photosynthetic characteristics of Thalassia testudinum, as measured in situ using a submersible pulse-amplitude modulated fluorometer (diving-PAM), were investigated. Shoot-scale variation was assessed to develop a standard-methods protocol for this species. Significant within-shoot and among-shoot scale variation was observed for several PAM-fluorescence parameters. The fraction of incident photosynthetically active radiation (PAR) absorbed by rank 1 (youngest) leaves, 0.67 ± 0.03, was significantly lower than the PAR absorbed by rank 2 and 3 leaves (0.78 ± 0.04 and 0.77 ± 0.04, respectively). Quantum yields (Y) and photosynthetic efficiency (F v /F m) exhibited greater variability and generally decreased with increasing leaf age (Y = 0.78 ± 0.02, 0.78 ± 0.03, and 0.76 ± 0.06 and F v /F m = 0.79 ± 0.02, 0.77 ± 0.5, and 0.75 ± 0.06 for rank 1, 2, and 3 leaves, respectively). Maximum fluorescence of light-acclimated leaves (F m) significantly decreased from the base to the tip of leaves (1032 ± 203, 1059 ± 139, and 793 ± 107 for the base, middle and tip of the leaf) and Y was significantly reduced from 0.68-0.76 to 0.27-0.62 when measured where leaf lesions were present. However, paired comparisons (with versus without lesions at the tip of the leaf) for the base, middle, and tip areas of rank 2 leaves indicated that lesions effects on photosynthesis were localized only to the area of the lesion (tip). The Y exhibited relatively high variability for short-shoots within seagrass die-off patches and were significantly reduced for short-shoots exhibiting die-off symptoms (rotten leaf base grayish-green leaf color, but no lesions). Negative response slopes for Y and F v /F m , measured as part of a landscape-scale sampling program, revealed the presence of significant diurnal (time-of-day) and photosynthetic photon flux density (PPFD) related variation, with a more negative slope for Y. These results indicate that the selection of leaf tissue, short-shoot location, and time of

Research paper thumbnail of Spatial Influences on Temporal Variations in Leaf Growth and Chemical Composition of Thalassia testudinum Banks Ex König in Tampa Bay, Florida

Gulf Research Reports, 1985

The importance of spatial influences on seasonal fluctuations in Thalassia testudinum leaf blade ... more The importance of spatial influences on seasonal fluctuations in Thalassia testudinum leaf blade lengths and chemical constituents was demonstrated. Differences between samples from fringe and mid-bed for several constituents were significant and, if not accounted for, could affect the measurement of apparent seasonal cycles. Fringe-shoots, reflecting the influence of more intense grazing activity, had shorter leaf blade lengths, lower dry weights and carbohydrate levels, and higher protein levels than mid-bed shoots. Mid-bed rhizomes and roots had highest protein and ash levels reflecting possible sediment influence. Percent ash and protein in the rhizomes, and percent carbohydrate in the roots exhibited seasonal fluctuations, but the levels were different between fringe and mid-bed samples. Protein levels were greatest in shoots and roots, while carbohydrate levels were highest in rhizomes, illustrating the respective partitioning of biosynthetic and storage functions. The spatial differences seem to reflect gradients in biological and chemical interactions, and they may play an important role in trophic interactions in seagrass systems.

Research paper thumbnail of Chemical fluctuations due to seasonal and cropping effects on an algal-seagrass community

Research paper thumbnail of Recurrence of Thalassia testudinum seagrass die-off in Florida Bay, USA: initial observations

Marine Ecology Progress Series, 2016

Widespread mortality of Thalassia testudinum was first documented in Florida Bay, USA, during the... more Widespread mortality of Thalassia testudinum was first documented in Florida Bay, USA, during the summer of 1987. This unprecedented event spanned 3 yr, affected 40 km 2 of seagrass and resulted in more than a decade of ecological disturbances. Initial putative causes for seagrass die-off ranged from climatic anomalies and watershed changes to wasting disease and eutrophication. Subsequent experimental research suggested that hypoxic plant tissue, caused by low water column oxygen content or reduced photosynthesis, allowed intrusion of sulfide leading to plant death. Contributing factors included high temperatures, salinities and T. testudinum biomass, together causing lower oxygen water solubility, higher community respiration rates and elevated nighttime oxygen demand. The Fisheries Habitat Assessment Program (FHAP) has tracked the system's slow recovery since 1995. Recent FHAP data (2012) indicated that T. testudinum had returned to pre-die-off densities in even the most severely affected locations. During the summer of 2015, following several months of drought, National Park Service researchers reported hypersaline conditions and a recurrence of seagrass die-off in north-central Florida Bay. An interagency effort is presently underway to document the duration, extent, impacts and possible factors responsible for the current mortality. Initial field surveys indicate that there is high spatial coincidence between the current and the 1987−1990 events and that hypersalinity, water column stratification and bottom-water anoxia might have once again resulted in mass mortality of T. testudinum in Florida Bay. The goal of this report is to alert the scientific community to the recurrence of this important ecological event.

Research paper thumbnail of Landscape-scale variation in a sulfur-based sediment stress indicator for the seagrass Thalassia testudinum in Florida Bay, USA

Marine Ecology Progress Series

Intrusion of sediment-derived hydrogen sulfide into above-ground tissues of seagrasses is correla... more Intrusion of sediment-derived hydrogen sulfide into above-ground tissues of seagrasses is correlated with reduced growth and has been linked to large-scale die-offs of Thalassia testudinum in Florida Bay, USA. In May of 2019, leaves from T. testudinum short shoots at 350 sites within 13 basins across Florida Bay were collected to investigate bay-wide variation in a novel, stable sulfur isotope-based indicator of hydrogen sulfide intrusion [(δ34Sleaf + 30)/total sulfur content]. The components of this sediment stress indicator (SSI), total sulfur content (% dry weight) and δ34S, were negatively correlated (R2 = -0.24), indicating greater sediment sulfide exposure in plants with higher sulfur content. Generalized additive model selection revealed that SSI was best predicted by a model which included T. testudinum cover, sediment depth, and Halodule wrightii cover (R2 = 0.24, weight = 0.48). Macrophyte communities dominated by dense T. testudinum climax communities and with deep sedime...

Research paper thumbnail of Interspecific variation in diurnal photo-protective strategies of Thalassia testudinum and Halophila johnsonii

PAM fluorometer data of diurnal changes in 2013 + 2014HPLC diurnal photopigment data in 2014Trolo... more PAM fluorometer data of diurnal changes in 2013 + 2014HPLC diurnal photopigment data in 2014Trolox equivalent antioxidant capacity (TEAC) diurnal data for 2013Physical water quality data 2013 + 2014

Research paper thumbnail of Leaf Spectral Reflectance Shows Thalassia testudinum Seedlings More Sensitive to Hypersalinity than Hyposalinity

Frontiers in Plant Science, 2017

Thalassia testudinum (turtle grass) is the dominant and climax-successional seagrass species in t... more Thalassia testudinum (turtle grass) is the dominant and climax-successional seagrass species in the subtropical/tropical Atlantic and Caribbean region. Two die-offs of T. testudinum in Florida Bay, United States have raised concerns regarding the resilience of this species to environmental disturbances. Seedlings are important in recovery of T. testudinum, following disturbance events. Leaf spectral reflectance [R(λ)] was measured in T. testudinum seedlings exposed for 2 weeks to three salinities (20, 35, and 50) and two light levels (full sun and 50-70% light reduction) in experimental mesocosms. Multivariate analyses indicated that hypersalinity had a greater effect on spectral reflectance than hyposalinity or light reduction. There was an increase in variability and flattening of reflectance spectra at the highest salinity. All three salinity treatments had distinct reflectance spectra across green wavelengths (530-580 nm), with additional discrimination between 20 versus 50 and 35 versus 50 treatments across red wavelengths (630-690 nm). Red:Green reflectance ratios were highest and photochemical reflective index values were lowest for the salinity 50 treatment, but were not significantly different between the salinity 20 and 35 treatments. The changes in the R(λ) spectra for the salinity 50 seedlings were consistent with previously observed reductions in leaf pigments and maximum photochemical efficiency of photosystem II. These observations indicate that leaf spectral reflectance is a sensitive indicator of plant stress in T. testudinum seedlings and that seedlings are more sensitive to short-term exposures to hypersalinity than hyposalinity.

Research paper thumbnail of Assessment of the toxicity of Kuwait crude oil on the photosynthesis and respiration of seagrasses of the northern Gulf

Marine Pollution Bulletin, 1993

Photosynthetic and respiratory responses of leaf tissues of the seagrasses Halophila ovalis, H. s... more Photosynthetic and respiratory responses of leaf tissues of the seagrasses Halophila ovalis, H. stipulacea, and Halodule uninervis exposed for 12-18 h to unweathered Kuwait crude oil were measured using an oxygen electrode system to assess the possibility of acute toxicity. Leaf tissues were incubated in natural seawater (control treatment) or in the water-soluble fraction of a 1% (weight:volume) solution of Kuwait crude oil in seawater (oil treatment). Photosynthesis vs. irradiance (PI) responses exhibited typical lightsaturation kinetics. One-way analysis of variance detected no significant treatment effects on the PI characteristics: c~, Pm~, Ik, or I c. Respiration rates were also not significantly affected by short-term exposure to the oil treatment. In addition, no significant amongspecies differences in PI characteristics or respiration were detected, possibly reflecting the low metabolic state for these subtropical species during Leg II sampling when water temperatures were 16-19°C. These results support our observations that the Gulf War oil spill primarily impacted intertidal communities rather than the submergent plant communities of the northern Gulf region.

Research paper thumbnail of Ecology of seagrasses in northeastern Saudi Arabia one year after the Gulf War oil spill

Marine Pollution Bulletin, 1993

Approximately 1 year after the Gulf War oil spill we examined the distribution, species compositi... more Approximately 1 year after the Gulf War oil spill we examined the distribution, species composition, abundance and productivity of seagrasses in several oil contaminated bays along the northeastern coastline of Saudi Arabia. The most abundant seagrass, Halodule uninervis, grew from the intertidal down to depths of 10 m with a standing crop ranging from 2 to 32 gdw m-2. Specific leaf productivity in a heavily oiled shallow site (1.5 m depth) was 2.2% d-1 and areal production was 0.172 gdw m-2 d-1. Specific productivity was similar to other reported rates for healthy populations of Halodule species but the low areal productivity reflected late winter water temperatures which were between 16 and 19"C. Halophila twalis was intermediate in abundance followed by Halophila stipulacea. Leaf morphology and

Research paper thumbnail of Recruitment of Thalassia testudinum seedlings into physically disturbed seagrass beds

Marine Ecology Progress Series, 2004

Thalassia testudinum seedling recruitment, survival, and growth were examined within physically d... more Thalassia testudinum seedling recruitment, survival, and growth were examined within physically disturbed seagrass beds in the Florida Keys. Two separate studies were conducted. In the first, a large-scale study, T. testudinum seedlings were surveyed and collected from a large seagrass disturbance (1560 m 2), 4.8 yr after the initial impact from a motor-vessel grounding. In the second, a smaller-scale study, T. testudinum seedling recruitment was examined over a 2 yr period within 9 smaller motor-vessel disturbances located within intact T. testudinum beds. In the large-scale study, we reconstructed the age of the seedlings based on shoot production rates from a previous study and from the small-scale study. A total of 79 seedlings were collected that varied in age from young of the year to 4.8 yr old; 6 different seedling cohorts were identified. The average density and rhizomeelongation rate for 1 yr old seedlings were 1 m-2 and 6.6 cm yr-1 , respectively. In the small-scale study, we surveyed and permanently marked all newly recruited seedlings; monitoring was conducted 5 more times over a 2 yr period. The average seedling survival after Year 1 was 42%; after Year 2, average survival dropped to 20%. The average seedling density after Year 1 was 0.071 m-2 ; after Year 2, average density dropped to 0.026 m-2. The average rhizome-elongation rate and shootproduction rate of 1 yr old seedlings were 6 cm yr-1 and 1.8 shoots yr-1 , respectively. The results of this study suggest that T. testudinum seedlings are a regular and reliable source of new recruits on seagrass banks in the Florida Keys.

Research paper thumbnail of Impact of light limitation on seagrasses

Journal of Experimental Marine Biology and Ecology, 2007

Seagrass distribution is controlled by light availability, especially at the deepest edge of the ... more Seagrass distribution is controlled by light availability, especially at the deepest edge of the meadow. Light attenuation due to both natural and anthropogenically-driven processes leads to reduced photosynthesis. Adaptation allows seagrasses to exist under these sub-optimal conditions. Understanding the minimum quantum requirements for growth (MQR) is revealed when light conditions are insufficient to maintain a positive carbon balance, leading to a decline in seagrass growth and distribution. Respiratory demands of photosynthetic and non-photosynthetic tissues strongly influence the carbon balance, as do resource allocations between above-and below-ground biomass. Seagrass light acclimation occurs on varying temporal scales, as well as across spatial scales, from the position along a single leaf blade to within the canopy and finally across the meadow. Leaf absorptance is regulated by factors such as pigment content, morphology and physical properties. Chlorophyll content and morphological characteristics of leaves such as leaf thickness change at the deepest edge. We present a series of conceptual models describing the factors driving the light climate and seagrass responses under current and future conditions, with special attention on the deepest edge of the meadow.

Research paper thumbnail of Dissolved oxygen stratification in two micro-tidal partially-mixed estuaries

Estuarine, Coastal and Shelf Science, 2006

The controlling physical factors for vertical oxygen stratification in micro-tidal, partially-mix... more The controlling physical factors for vertical oxygen stratification in micro-tidal, partially-mixed estuaries are discussed in this paper. A theoretical deduction shows that vertical stratification of dissolved oxygen (DO) concentration can be explained by the extended Hansen and Rattray's Central Region theory, which suggests that in addition to biological factors such as photosynthesis, biochemical oxygen demand (BOD), sediment oxygen demand (SOD), vertical DO profiles are mainly controlled by physical factors such as surface re-aeration, river flow, and estuarine gravitational circulation. Vertical mixing of DO from surface re-aeration and photosynthesis sets a DO profile of higher concentration near the surface and lower near the bottom. With a positive seaward longitudinal DO gradient, strong river flow and estuarine gravitational circulation can cause lower DO concentrations near the surface and higher near the bottom. The actual vertical oxygen profile is then determined by the relative magnitude of the above-mentioned mechanisms. It is sensitive to two parameters: (1) the strength of the gravitational circulation (u E); and (2) the relative importance between biochemical oxygen demand and vertical diffusivity (a). Vertical DO stratification usually becomes weaker as u E increases. The impact of gravitational circulation on vertical oxygen distribution becomes more important for a larger a. The impact of a on oxygen stratification is profound. As u E (and river flow) increases, DO stratification appears to be less sensitive to the value of a. Surface-to-bottom differences in DO concentrations (DDO) is negligible when a is small (a < 0.5). As a increases, DDO increases under a weak to moderate gravitational circulation mode (u E 5 cm s À1). Under a strong gravitational circulation mode, DDO becomes negative with a small a (a < 2), and as a continues to increase, DDO becomes positive. The newly-deduced governing equation for vertical oxygen stratification is applied to two micro-tidal, partially-mixed estuarine systems: the Cape Fear River Estuary (CFRE) and the Pamlico River Estuary (PRE) of North Carolina. In the CFRE, although strong vertical salinity stratification exists, DO concentrations are usually well mixed. De-coupling between salinity stratification and oxygen stratification is mainly due to a relatively stronger estuarine gravitational circulation and higher freshwater inflow in the system. It appears that river flow and gravitational circulation are the dominant factors in controlling oxygen stratification in the CFRE. In contrast, vertical stratification of DO concentrations is closely correlated with that of salinity in the PRE. In the PRE, the estuarine gravitational mode and river flow are often both very weak, and DO stratification is very sensitive to the value of a. With negligible influence from tidal mixing, the system is more sensitive to vertical mixing regulated by salinity stratification and wind. As a result, vertical DO stratification is closely correlated with salinity stratification in the PRE.

Research paper thumbnail of Population-based variation in resilience to hyposalinity stress in <i>Halophila johnsonii</i>

Bulletin of Marine Science, 2014

Plants of the threatened seagrass, Halophila johnsonii Eiseman, from a riverine and marine popula... more Plants of the threatened seagrass, Halophila johnsonii Eiseman, from a riverine and marine population were exposed to a series of salinity treatments within mesocosms. Survival and maximum photochemical efficiencies of PSII (F v /F m) were measured in response to varied frequency, duration, and amplitude of hyposalinity exposures. Both populations exhibited high survival after two cycles of short, pulsed hyposalinity treatments (100% and 89%, respectively). However, two cycles of long pulses of low salinity resulted in 100% mortality for marine H. johnsonii and >50% mortality for riverine plants. After two cycles of gradual salinity reduction to a salinity of 5, survival for marine and riverine plants was also low (22% and 33%, respectively). F v /F m values of riverine H. johnsonii were relatively high (0.65-0.70) after a single short pulse or gradual reduction to salinity of 10. F v /F m values of marine plants were lower in these two treatments and exhibited greater declines following a single, long pulse to a salinity of 10 or after a single gradual reduction to a salinity of 5. F v / F m values showed riverine plants were also more resistant to repeated pulses of moderate low salinity than marine plants. Our results indicate wide tolerances of H. johnsonii to short pulses of hyposalinity, but suggest that repeated or prolonged hyposalinity stress at near-tolerance salinity levels can have additive effects on photosynthetic health and survival. Differences in resilience of marine and riverine H. johnsonii populations are consistent with previous suggestions that estuarine ecophenes are more tolerant to hyposalinity than marine populations. Halophila johnsonii Eiseman is a small-bodied seagrass with an extremely limited geographic range. Its distribution is restricted to about 200 km of coastal lagoons and estuaries of southeastern Florida from Sebastian Inlet (27°51´N, 80°27´W) in northern Indian River Lagoon to northern Biscayne Bay (25°45´N, 80°07´W), which includes man-made navigation channels (Intracoastal Waterway) that connect these larger water bodies and stormwater drainage canals managed by state water management districts (Kenworthy 1997). Halophila johnsonii is the least abundant seagrass within its range and is considered an opportunistic species, often occurring intertidally or at the upper and lower margins of seagrass beds where other seagrass research paper