Differential response of equatorial Pacific phytoplankton to iron fertilization (original) (raw)
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Marine Ecology Progress Series, 2000
During the IronEx II experiment in the eastern equatorial Pacific (May to June 1995), the response of the microplankton community to mesoscale iron fertilization was followed using a combination of marker-pigment, microscopical and flow cytometric techniques. Phytoplankton standing stock bloomed dramatically over a period of 6 d following 3 iron additions of 2 and 1 nM, respectively. Carbon biomass in the patch increased by a factor of 4, chlorophyll a by about a factor of 16 and diatoms by > 70-fold relative to contemporaneous levels in the ambient community. The bloom then plateaued sharply and remained at a more or less constant level for 4 d, despite the addition of more iron (1 nM) and physiological indices (low C:chl a ratio and elevated photochemical quantum efficiency) suggesting that the cells were healthy and growing rapidly. Relatively large pennate diatoms (Nitzschia spp., median length 20 to 24 µm) dominated the patch bloom, with smaller pennate species and centric diatoms declining in relative importance. Heterotrophic bacteria increased at a slow rate (0.08 d -1 ) for >10 d during the experiment, as did heterotrophic nanoflagellates. There were also indications of enhanced cell size, cellular pigment content and possibly growth rates of small phytoplankton. Nonetheless, little difference was observed between the ambient community and the peak patch bloom with respect to the size composition of auto-and heterotrophic populations <10 µm in cell size. The relative constancy of the smaller size fractions, the sharp curtailment of net growth of the bloom after 6 d, and > 3-fold increase in large heterotrophic dinoflagellates and ciliates suggest that protistan grazers may have played an active role in controlling the phytoplankton response to increased iron availability.
Different reactions of Southern Ocean phytoplankton size classes to iron fertilization
Limnology and Oceanography, 2006
During the European Iron Fertilisation Experiment (EIFEX), performed in the Southern Ocean, we investigated the reactions of different phytoplankton size classes to iron fertilization, applying measurements of size fractionated pigments, particulate organic matter, microscopy, and flow cytometry. Chlorophyll a (Chl a) concentrations at 20-m depth increased more than fivefold following fertilization through day 26, while concentrations of particulate organic carbon (POC), nitrogen (PON), and phosphorus (POP) roughly doubled through day 29. Concentrations of Chl a and particulate organic matter decreased toward the end of the experiment, indicating the demise of the iron-induced phytoplankton bloom. Despite a decrease in total diatom biomass at the end of the experiment, biogenic particulate silicate (bPSi) concentrations increased steadily due to a relative increase of heavily silicified diatoms. Although diatoms .20 mm were the main beneficiaries of iron fertilization, the growth of small diatoms (2-8 mm) was also enhanced, leading to a shift from a haptophyte-to a diatom-dominated community in this size fraction. The total biomass had lower than Redfield C : N, N : P, and C : P ratios but did not show significant trends after iron fertilization. This concealed various alterations in the elemental composition of the different size fractions. The microplankton (.20 mm) showed decreasing C : N and increasing N : P and C : P ratios, possibly caused by increased N uptake and the consumption of cellular P pools. The nanoplankton (2-20 mm) showed almost constant C : N and decreasing N : P and C : P ratios. Our results suggest that the latter is caused by a shift in composition of taxonomic groups.
Growth rates, grazing, sinking, and iron limitation of equatorial Pacific phytoplankton
Limnology and Oceanography, 1991
Concentrations of phytoplankton and NO, are consistently low (0.2-0.4 pg Chl a liter-') and high (4-12 PM) in surface waters of the oceanic eastern and central equatorial Pacific, and phytoplankton populations are dominated by small solitary phytoplankton (< 10 pm). Growth rates of natural phytoplankton populations, needed to assess the relative importance of many of the processes considered important in maintaining the high-nutrient, low-phytoplankton biomass condition observed in the equatorial Pacific, were estimated by several methods. The growth rates of natural phytoplankton populations were found to be -0.7 d-l or 1 biomass doubling d-' and were similar for all methods. To keep this system in its observed balance requires that loss rates approximate observed growth rates. Grazing rates, measured with a dilution grazing experiment, were high (-0.5 d-l), accounting for a large fraction of the daily production. Additions of various forms of Fe to 5-7-d incubations utilizing ultraclean techniques resulted in significant shifts in autotrophic (from picoplankton to diatom dominated) and heterotrophic assemblages between initial samples, controls, and Fe enrichments, which were presumably due to Fe, grazing by both protistan and metazoan components, and incubation artifacts. Estimated growth rates of small pennate diatoms (2 x 10 pm) showed increases in Fe enrichments (mean f SD = 1.6OkO.04 d-l) with respect to controls (mean -C SD = 1.2 1 kO.33 d-l). The growth rates of the pennate diatoms were similar to those estimated for the larger (~5 pm) size fraction of the natural populations (mean = 1.42 d-l). We hypothesize that biomass regulation of the larger size fraction of phytoplankton is by a combination of grazing, Fe limitation, and sinking and that understanding the controls over the larger phytoplankton is fundamental to unraveling the high-nutrient, low-biomass paradigm.
Deep Sea Research Part II: Topical Studies in Oceanography, 2001
The "rst in situ iron-fertilisation in polar Southern Ocean waters, SOIREE (Southern Ocean Iron RElease Experiment) was a mesoscale, 13-day experiment that resulted in a phytoplankton bloom (chlorophyll-a increased six-fold; algal carbon, three-fold). Just prior to the iron enrichment, community chlorophyll-a at the study site was dominated by picoplankton, with an &innoculum' of large diatoms. Iron enrichment resulted in a #oristic shift, with increased diatom abundances, and to a lesser extent elevated nanoplankton stocks. Picoplankton stocks initially increased in response to iron supply, but were then grazed down to initial levels by microzooplankton. The chain-forming pennate diatom Fragilariopsis kerguelensis was the dominant bloom species, although (20 m haptophytes also increased in abundance. Other algal responses to iron enrichment included elevated nanoplankton cell size, increased chain length of Fragilariopsis kerguelensis, and higher cellular chlorophyll-a levels within the pico-, nano-and microplankton. During the SOIREE bloom, chlorophyll-a levels of up to 3.0 mg m\ were recorded at depth and averaged 1.8 mg m\ in the 65-m mixed layer. The #oristic shift from a picoeukaryote-dominated community to one dominated by large diatoms was similar to that observed prior to and during algal blooms in the vicinity of open-ocean fronts in the Southern Ocean.
2011
Phytoplankton dynamics were investigated in the eastern equatorial Pacific at 32 stations sampled during two cruises (December 2004 and September 2005). Based on standing stock analyses from HPLC pigments, flow cytometry and microscopy, we used a modified 2-treatment approach of the seawater dilution method to estimate taxon-specific phytoplankton growth and mortality rates in 8-depth per station profiles. These data were complemented by contemporaneous measurements of dissolved iron (Fe). The stations encompassed an equatorial zonal gradient (110 to 1401W) of diminishing eastward Fe availability in the euphotic zone from upwelling of the Equatorial Undercurrent (EUC). Latitudinal variation was assessed by meridional transects at 110 and 1401W. Overall, euphotic zone averaged growth rates were 0.53 7 0.17 d À 1 (total chlorophyll a), 0.34 7 0.15 d À 1 (divinyl chlorophyll a) and 0.86 70.32 d À 1 (fucoxanthin). Microzooplankton grazing accounted for 50-60% of daily production of eukaryotic algae, whereas essentially all growth of phototrophic prokaryotes was consumed daily. Fucoxanthin, representing diatoms, was a minor component of the accessory pigments, but diatom growth rates were both significantly higher than other taxonomically defined groups and dropped off more sharply with depth (low light level). Strikingly, no spatial or temporal trends were seen in the 256 growth rate measurements for each measured pigment. However, the diminishing eastward equatorial Fe gradient was associated with deepening subsurface pigment maxima and decreasing surface-layer pigment stocks (down to the 8% light level). In addition, integrated standing stocks of total chlorophyll a and Prochlorococcus (divinyl chlorophyll a) were strongly correlated with integrated iron at equatorial upwelling stations, yet no correlation with Fe was seen for any of the eukaryotic groups, including diatoms. This latter result is contrary to expectations from previous Fe addition experiments (in situ or in bottles), where diatom biomass increased relative to other phytoplankton. We hypothesize that the natural supply of Fe to the base of the euphotic zone from the EUC is less favorable for diatoms because of light limitation. Rather, new Fe is rapidly incorporated into a small phytoplanktondominated community in the deep euphotic zone, and tightly coupled grazing control results in a system regulated by return of recycled Fe.
Deep Sea Research Part I: Oceanographic Research Papers, 2007
The dynamics of phytoplankton species populations recorded during the 3-week, iron-fertilization experiment EisenEx carried out in spring in the Antarctic Polar Frontal Zone are presented and discussed as the difference between growth and mortality rates. Only two cosmopolitan diatom species, the centric Chaetoceros debilis and the pennate Pseudo-nitzschia lineola, increased population density exponentially throughout the experiment to 150-and 90-fold of initial values, respectively. Because C. debilis initial abundance was tenfold lower than that of P. lineola, the two contributed 1% and 21% to bloom biomass, respectively at the end of the experiment, highlighting the role of seeding in bloom formation. The other significant species increased population size at a linear rate throughout the experiment or for a short spurt phase to 3-to 18-fold of initial values. Conservative estimates of mortality rates within diatom species populations were obtained by comparing net accumulation rates of full cells with those of empty and broken frustules. The ratios were consistent over time for the various species but varied widely between them. The species-specific variation can be explained by differences in both growth and mortality rates, the latter partly due to either selective grazing or avoidance by the large protozoo-and metazooplankton populations present. Selective predation by the abundant copepod populations on protistan grazers of diatoms (ciliates and heterotrophic dinoflagellates) apparently aided diatom biomass build-up. The response patterns of populations of the phytoplankton species present fall into six categories comprising disparate species, indicating that phylogeny is a poor predictor of ecology. The group that did not respond to fertilization was the most diverse and included both endemic and cosmopolitan as well as background and bloom-forming species. This lack of response to the advent of favorable growth conditions indicates that proximate factors during EisenEx triggered growth only in some species but had little effect on others. We attribute the differences in behavior to ultimate factors such as seasonal effects on life cycles and other internal constraints on growth rates. The implications for our understanding of the evolutionary ecology of phytoplankton and its impact on global biogeochemical cycles are pointed out.
Limnology and Oceanography, 1993
A 6-d nutrient enrichment experiment was performed in the tropical North Pacific Ocean at 9"N, 147"W with ultraclean techniques. Changes in phytoplankton biomass, C and N assimilation rates, growth rates, and species composition were monitored with HPLC pigment analyses and flow cytometry techniques, as well as 14C fixation into particulate C, pigments, and protein. Prochlorophyte specific growth rates (from divinyl Chl a labeling) increased from an initial value of 0.15 d -I to 0.96 d-l following macronutrient addition (N, P, and Si). Diatoms, however, were unable to grow without added Fe. Diatom populations were severely colimited by Fe and macronutrients but achieved a specific growth rate of 2.5 d -' following Fe and macronutrient additions. Results implied that grazing rates (g) on prochlorophytes were stimulated in approximate balance with prochlorophyte growth (p) after 6 d (g : p = O.SS), but that grazing processes were not as efficient (g: p = 0.40) at controlling the diatom standing stock. Our results suggest that grazing processes may be the most important factor regulating procaryotic biomass, but Fc limitation is the proximate control of diatom biomass and hence may limit the utilization of macronutrients in the equatorial Pacific Ocean. R. J. Olson), We thank N. Welschmeyer for the use of some sampling equipment. Support for K. W. Bruland and D. A. Hutchins was provided by NSF (OCE 90-000 15 1) and ONR NOOO14-92-J-1304 to K. W. Bruland. Support for G. R. DiTullio was provided by NSF (OCE 90-17543) to G. R. DiTullio and N. A. Welschmeyer. Finally, we acknowledge J. T. Hollibaugh and two anonymous reviewers whose comments improved the quality of the text.
2014
Dilution experiments were performed to quantify growth and mortality rates of phytoplankton groups (as defined by pigment markers) for 5 wk in an iron-induced phytoplankton bloom during the European Iron Fertilization Experiment (EIFEX) conducted in the Southern Ocean. Rates could be reliably measured for the 2 main groups, diatoms and prymnesiophytes. Mean phytoplankton intrinsic growth rates were around 0.23 d −1 , without a significant temporal trend. Mortality rates, however, decreased with time (from ~0.3 to ~0.06 d −1 ), leading to an increase in decoupling between phytoplankton growth and microzooplankton grazing. The decrease in grazing was correlated with the decrease in concentrations of small microprotozooplankton (< 60 µm). As a consequence, net growth in the dilution experiments increased from around 0 d −1 up to 0.13 d −1 in the last days of the experiment, 35 d after the initial iron fertilization. This pattern did not reflect the dynamics of net phytoplankton accumulation in the fertilized patch, which increased until Days 24 to 27 and decreased thereafter. The difference between experimental and natural phytoplankton net growth is the biomass that escapes microzooplankton grazing and does not accumulate in the surface mixed layer, i.e. the biomass that went to higher trophic levels plus that exported out of the mixed layer. It increased throughout EIFEX and suggests a shift from a predominantly recycling system towards a more exporting one.
Journal of Experimental Marine Biology and Ecology, 2007
Effects of nutrients on the size and composition of phytoplankton and the resulting changes in ecosystem structure were investigated using incubations of unaltered deep-sea waters from 400 m and 700 m depths of Suruga Bay Japan. This new approach allowed us to observe the succession of phytoplankton communities from conditions of high to low nitrogen availability and explore the effects of N source and availability on the composition and size of diatoms under low grazing pressure. Diatoms dominated the phytoplankton biomass throughout the duration of experiments in both incubations. However, among diatoms, the genus Thalassiosira dominated the abundance and biomass when nutrients were replete, and the genus Chaetoceros dominated the abundance and biomass when nitrate became depleted. Our results show that diatoms can dominate other phytoplankton groups not only under nutrient replete conditions, but also (particularly for Chaetoceros) under low concentrations of recycled nitrogen if silica and phosphate are not deficient. Diatoms b 15 μm dominated the biomass in the 700 m incubation whereas those N 15 μm dominated the biomass in the 400 m incubations during the formation of blooms under nitrate-replete conditions. After nitrate depletion, diatoms N 15 μm dominated the biomass in both incubations. Our results showed that availability of N determined the composition of diatoms if Si and P are not deficient. However, for size the results showed that N source and availability did not determine the size of phytoplankton. Our results also showed that the molar ratio of Si:N uptake was b 1 for diatoms under nutrient replete conditions. Further the Si:N drawdown ratio decreased with the increasing abundance of smaller diatoms (b 15 μm).