Impact of light limitation on seagrasses (original) (raw)
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Scientific Reports
Analyses of the integrated seagrass response to depth support the previously documented low plasticity and consistent shade-adapted leaf physiology of a habitat-builder that dominates wellilluminated reef environments. two structural responses, "canopy-opening" and "below-groundmass-depletion", govern the photoacclimatory response and facilitate, respectively, light penetration within the canopy and functional adjustments in whole-plant carbon balances. conversely, "canopyclosing" may also explain dense canopies formed close to the waterline, as they provide shade and photoprotection to a susceptible leaf physiology under highlight. canopy light attenuation is primarily regulated by the leaf area index (LAi), which is governed by changes in shoot size and density. Shoot density diminishes non-linearly with depth, while shoot size increases to a maximum followed by a decline. the initial increase in shoot size, which resembles a self-thinning response, increases LAi and meadow production in shallow depths. these seagrass structural adjustments have relevant ecological implications. canopy-thinning allows macrophyte diversity to increase with depth, while seagrass production and carbon storage diminish exponentially, and are maximal only in a shallow coastal fringe. the results support the universality of plant self-thinning, from phytoplankton to complex canopies, likely the consequence of simple physical laws related to light limitation and pigment self-shading within photosynthetic structures and communities. Seagrasses build highly productive coastal habitats across all continents except Antarctica 1,2. Despite only occupying 0.1% of the global ocean surface, seagrass meadows are considered important oceanic sinks for atmospheric CO 2 3 , being responsible for the sequestration of 20% of the global organic carbon present in marine sediments 4. High burial rates and slow organic carbon decomposition in the sediment 5 are consistent with this interpretation. However, a large variability in carbon stock and accumulation rates among species and habitats has been documented 6 , as well as a wide spatial and temporal variation in seagrass standing biomass and daily leaf production 2,7. For example, in oligotrophic reef environments seagrasses often colonize coarse carbonate sediments where their capacity to retain the organic carbon produced is small 8,9. For the climax species in the Caribbean and Gulf of Mexico, Thalassia testudinum, a variation of up to 100-fold in standing biomass has been documented 10. Nutrient limitations such as nitrogen 11 , phosphorus 12 , or iron 13 , have been proposed as the main limiting factors for seagrass productivity in reef environments; while highlight stress 14,15 , and freshwater inputs 16 , have been also suggested to affect seagrass growth and production.
Photoacclimation and Light Thresholds for Cold Temperate Seagrasses
2022
Water quality deterioration is expected to worsen the light conditions in shallow coastal waters with increasing human activities. Temperate seagrasses are known to tolerate a highly fluctuating light environment. However, depending on their ability to adjust to some decline in light conditions, decreases in daily light quantity and quality could affect seagrass physiology, productivity, and, eventually, survival if the Minimum Quantum Requirements (MQR) are not reached. To better understand if, how, and to what extent photosynthetic adjustments contribute to light acclimation, eelgrass (Zostera marina L.) shoots from the cold temperate St. Lawrence marine estuary (Rimouski, QC, Canada) were exposed to seven light intensity treatments (6, 36, 74, 133, 355, 503, and 860 µmol photons m −2 s −1 , 14:10 light:dark photoperiod). Photosynthetic capacity and efficiency were quantified after five and 25 days of light exposure by Pulse Amplitude Modulated (PAM) fluorometry to assess the rapid response of the photosynthetic apparatus and its acclimation potential. Photoacclimation was also studied through physiological responses of leaves and shoots (gross and net primary production, pigment content, and light absorption). Shoots showed proof of photosynthetic adjustments at irradiances below 200 µmol photons m −2 s −1 , which was identified as the threshold between limiting and saturating irradiances. Rapid Light Curves (RLC) and net primary production (NPP) rates revealed sustained maximal photosynthetic rates from the highest light treatments down to 74 µmol photons m −2 s −1 , while a compensation point (NPP = 0) of 13.7 µmol photons m −2 s −1 was identified. In addition, an important package effect was observed, since an almost threefold increase in chlorophyll content in the lowest compared to the highest light treatment did not change the leaves' light absorption. These results shed new light on photosynthetic and physiological processes, triggering light acclimation in cold temperate eelgrass. Our study documents an MQR value for eelgrass in the St. Lawrence estuary, which is highly pertinent in the context of conservation and restoration of eelgrass meadows.
PLoS ONE, 2014
A three-dimensional computer model of canopies of the seagrass Amphibolis griffithii was used to investigate the consequences of variations in canopy structure and benthic light environment on leaf-level photosynthetic saturation state. The model was constructed using empirical data of plant morphometrics from a previously conducted shading experiment and validated well to in-situ data on light attenuation in canopies of different densities. Using published values of the leaflevel saturating irradiance for photosynthesis, results show that the interaction of canopy density and canopy-scale photosynthetic response is complex and non-linear, due to the combination of self-shading and the non-linearity of photosynthesis versus irradiance (P-I) curves near saturating irradiance. Therefore studies of light limitation in seagrasses should consider variation in canopy structure and density. Based on empirical work, we propose a number of possible measures for canopy scale photosynthetic response that can be plotted to yield isoclines in the space of canopy density and light environment. These plots can be used to interpret the significance of canopy changes induced as a response to decreases in the benthic light environment: in some cases canopy thinning can lead to an equivalent leaf level light environment, in others physiological changes may also be required but these alone may be inadequate for canopy survival. By providing insight to these processes the methods developed here could be a valuable management tool for seagrass conservation during dredging or other coastal developments.
Estuarine, Coastal and Shelf Science, 2021
We show that the effect of temperature on photosynthesis of a widely distributed and ecological important seagrass species was not consistent among locations, with some evidence of acclimation to the local temperature range. This has important implications for modelling seagrass productivity and the impacts of light reduction on seagrass ecosystems. Reduced light availability is one of the main pressures negatively impacting on seagrass meadows worldwide. Our knowledge of seagrass photosynthetic characteristics is critical to predicting and managing impacts of light reducing activities but suffers from two critical information gaps: first, data is overwhelmingly derived from studies of leaf tissue and not for whole plants, and is unlikely to reflect whole plant performance under light reduction stress; and second, few studies have looked at spatial and temporal variability in photosynthetic performance of whole seagrasses. We investigated temporal and spatial variation at a range of temperatures for whole plants of Halophila ovalis, a widely distributed species, by measuring oxygen exchange of intact plants collected from four locations across a latitudinal gradient (10 •) at three temperatures (17 • C, 23 • C, 28 • C). For all locations, temperature affected all photosynthetic parameters, with some parameters (NP max , R, I k) showing a distinct difference between tropical and temperate locations. For example, NP max ranged from 1.35 ± 0.12 to 5 ± 0.16 mg O₂. g DW − 1. hr − 1 in temperate locations and from 4 ± 0.3 to 12 ± 0.68 mg O₂. g DW − 1. hr − 1 in the tropical location. However, the effect of temperature on photosynthesis was not consistent among locations, and often the rate of photosynthesis was greatest at temperatures approaching the mean month maximum temperature for the location, suggestive of acclimation. Time of year also affected photosynthetic rates and how temperature influenced those rates. We conclude that the application of P-I parameters to model, predict or manage the effect of light reduction of H. ovalis, and likely other seagrass species, may require site-and timespecific knowledge of P-I relationships.
Thresholds of irradiance for seagrass Posidonia oceanica meadow metabolism
Meadows of the endemic seagrass Posidonia oceanica are threatened in the Mediterranean due to a general deterioration of the light environment that becomes critical when light irradiance is insufficient to meet the carbon requirements of the system. Here, we conduct a 3 wk, in situ shading experiment (8 levels plus controls) to determine the threshold of irradiance for balanced metabolism in a shallow P. oceanica meadow and further assess the recovery of the system 1 wk later. Reduced light irradiance decreased the net community production of the meadow, which may turn negative (i.e. respiration exceeded gross community primary production) under 338 μE m−2 s−1. Shading throughout the experiment did not appear to cause sustained physiological damage to the system since values of net community production after the cessation of shading were similar to pre-experimental, ambient levels. Sediment acid volatile sulfide pools ranged between 0.002 and 0.058 mol m−2 across shading treatments, and the highest pools were observed in the most shaded sediments. At high light impairment, meristematic cell divisions were low, and carbohydrate content in young rhizomes decreased throughout the experiment. Eight days after the cessation of shading, reduced rhizome carbohydrate stores and elevated sediment sulfide levels still persisted in the previously intensively shaded areas. The present study provides evidence of resistance and resilience of the seagrass Posidonia oceanica to light impairment for short (3 wk) periods of time. Although the compensation irradiance of the system varied by ~2-fold, it provides a quantitative estimate of the irradiance threshold at which seagrass meadows may shift from being coastal carbon sinks to CO2 sources.
Effects of irradiance, temperature, and nutrients on growth dynamics of seagrasses: A review
Productivity of seagrasses can be controlled by physiological processes, as well as various biotic and abiotic factors that influence plant metabolism. Light, temperature, and inorganic nutrients affect biochemical processes of organisms, and are considered as major factors controlling seagrass growth. Minimum light requirements for seagrass growth vary among species due to unique physiological and morphological adaptations of each species, and within species due to photo-acclimation to local light regimes. Seagrasses can enhance light harvesting efficiencies through photo-acclimation during low light conditions, and thus plants growing near their depth limit may have higher photosynthetic efficiencies. Annual temperatures, which are highly predictable in aquatic systems, play an important role in controlling site specific seasonal seagrass growth. Furthermore, both thermal adaptation and thermal tolerance contribute greatly to seagrass global distributions. The optimal growth temperature for temperate species range between 11.5 °C and 26 °C, whereas the optimal growth temperature for tropical/subtropical species is between 23 °C and 32 °C. However, productivity in persistent seagrasses is likely controlled by nutrient availability, including both water column and sediment nutrients. It has been demonstrated that seagrasses can assimilate nutrients through both leaf and root tissues, often with equal uptake contributions from water column and sediment nutrients. Seagrasses use HCO 3 − inefficiently as a carbon source, thus photosynthesis is not always saturated with respect to DIC at natural seawater concentrations leading to carbon limitation for seagrass growth. Our understanding of growth dynamics in seagrasses, as it relates to main environmental factors such as light, temperature, and nutrient availability, is critical for effective conservation and management of seagrass habitats.
Marine Environmental Research, 2018
Seagrasses inhabit environments where light varies at different timescales, nonetheless are acutely sensitive to reductions in light beyond some conditional bounds. Two tropical deep-water seagrasses, Halophila decipiens and Halophila spinulosa, from the Great Barrier Reef were tested for their response to defined light and temperature regimes to identify their growth requirements and potential thresholds of mortality. Species were exposed to two light intensities, saturating (75 µmol photons m-2 s-1) and limiting (25 µmol photons m-2 s-1) light and two temperature treatments (26°C and 30°C) over a four-week period. Wavelengthspecific parameters of PSII photochemistry were evaluated for seagrass leaves, as well as shoot density, gas exchange, and pigment content. Both species were sustained under saturating light levels (3.2 mol photons m-2 d-1) while limiting light led to decreased shoot density for H. decipiens and H. spinulosa after two and four weeks, respectively. Wavelength-specific photochemistry was also affected under light-limiting treatments for both species while the functional absorption cross section was highly conserved. Photoacclimation and physiological adjustments by either species was not adequate to compensate for reduced irradiance suggesting these plants reside at the margins of their functional limits. As such, relatively short periods of light attenuating events, like dredging or flood plumes, may be detrimental to deep-water seagrass populations.
Marine Ecology Progress Series, 2007
Morphological and growth characteristics of the meadow-forming seagrass Posidonia sinuosa (Cambridge et Kuo), were measured along a depth-related gradient of light to infer its response to long-term differences in light availability. Morphometric measurements were carried out at 6 depths between 1.6 and 9.0 m in summer and winter at Cockburn Sound and summer only at Warnbro Sound in south-western Australia. The minimum light requirement for P. sinuosa of 8.5% sub-surface light was among the lower range reported for seagrasses. Its slow growth rate (0.5-1.5 mg dry shoot -1 d -1 ), relative to similarly sized species, may contribute to the low light requirements of this species. Shoot density, leaf area index and biomass showed pronounced and consistent differences among depths (up to 88-fold reduction of above-ground biomass from shallow to deep sites). At the deeper sites, the reduced shoot density probably reduces respiratory demand and alleviates self-shading. Morphological differences (leaf length, width and thickness and number of leaves per shoot) did not follow a clear and consistent trend with depth. Despite a 70% reduction in light availability at the canopy level between the shallowest and deepest sites, leaf growth rate was unaffected by depth during summer, and in winter differed between only a few depths. We propose that the reduction in shoot density partially alleviates the effects of self-shading and permits comparable leaf growth rates across the depth range. These results suggest that for interpreting long-term responses to light availability, shoot density is the most sensitive of the morphological characteristics measured here.
Daily variation patterns in seagrass photosynthesis along a vertical gradient
Marine Ecology Progress Series, 2003
The relationship between the available photosynthetic active radiation (PAR) and the photosynthetic yield of Photosystem II (PSII), measured by chlorophyll a fluorescence, was assessed in the intertidal seagrasses of Ria Formosa, a coastal lagoon in southern Portugal. Most of the lagoon's intertidal is occupied by a monospecific population of Zostera noltii (Hornemann), which occupies a vertical gradient of about 2 m. The upper distribution limit of this species comprises the edge of a Spartina maritima (Curtis) Fernald saltmarsh and the lower limit the transition to the subtidal, dominated by Cymodocea nodosa (Ucria) Ascherson. Diurnal changes in the pattern of rapid-light curves (RLCs) was investigated with pulse amplitude-modulated (PAM) fluorometry in Z. noltii and in C. nodosa in the upper and lower intertidal. The light reactions of photosynthesis were assessed by fitting photosynthesis-irradiance (P-I) models to the RLCs. The photosynthetic parameters of Z. noltii revealed that this plant species exhibits sun-and shade-type responses in its upper and lower vertical distribution limits, respectively. At the lower distribution limit in low light, the initial slopes of all RLCs were significantly higher than at the upper site, but decreased with increasing irradiances, while at the upper distribution limit, although lower, the slopes were unaffected by increasing irradiance. C. nodosa presented a typical shade-type response, as evidenced by the daily variation and light dependence of both photosynthetic efficiency and optimal quantum yield. The relationship between the maximum electron-transport rate and irradiance suggests that this species is strongly light-limited. We suggest that attempts to characterize the photosynthetic behaviour of an intertidal meadow should consider both diurnal fluctuations in the plants' photosynthetic activity as well as its vertical distribution frequency.