Availability of iron for phytoplankton growth in the north-east Atlantic (original) (raw)
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Availability of iron and major nutrients for phytoplankton in the north-east Atlantic Ocean
Limnology and Oceanography, 2004
Because of recent findings that Fe is a limiting factor for phytoplankton activity even at relatively high dissolved iron (DFe) concentrations, the potential importance of Fe limitation was revisited in the northeast Atlantic Ocean (39-45ЊN, 17-21ЊW). We report data gathered during deck incubation experiments performed at three stations in February-March 2001 with surface seawater containing DFe concentrations of ϳ0.40 nmol L Ϫ1. At all stations, Fe addition enhanced phytoplankton growth. Fe limitation was moderate and occurred simultaneously with limitation by major nutrients. This was clearly demonstrated for diatoms that were colimited by orthosilicic acid. Micro-, nano-, and picoplankton benefited from Fe enrichment. Experiments performed with the trihydroxamate siderophore desferrioxamine mesylate B (DFOB) indicated that Fe reserves exist within the cells, especially within the larger cells. This reserve could result from luxurious storage of Fe by colimited cells during episodic atmospheric deposition of Saharan dust. Simulating concentrations of dust resulting from aerosol deposition in well-stratified surface waters, we determined that the solubility of Saharan dust was very low (Ͻ0.1% w/w) but the amount of DFe released in seawater was sufficient to relieve the Fe limitation of the ambient phytoplankton community.
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.
Controlling iron availability to phytoplankton in iron-replete coastal waters
Marine Chemistry, 2004
Recent work demonstrates that the micronutrient iron may strongly influence the magnitude and character of algal production in nearshore waters due in part to the higher but variable iron requirements of neritic phytoplankton. However, ascertaining the direct effects of iron nutrition in coastal waters has been forestalled by our inability to experimentally regulate ambient iron availability independent of other factors. We present here results from size-fractionated iron uptake experiments showing that increasing concentrations of the siderophore Desferriferrioxime B (DFB) progressively decreases the biological availability of iron tracer added to natural seawater. These findings extend those of previous studies showing that high concentrations of DFB induce iron limitation of phytoplankton in coastal waters. Similar tests with two siderophores (P1P and PCC7002 No. 1) isolated from marine prokaryotes showed little or no impact on short-term iron uptake in these natural population cultures. DFB additions did not influence the short-term uptake of carbon indicating that its inhibitory effect was not due to general toxicity to the cells. Uptake rates of iron tracer in the large (>5.0 Am) phytoplankton fraction decreased linearly with increasing DFB concentrations, becoming undetectable at z 3 nM DFB, or f 5 Â over ambient dissolved iron concentrations. The decrease in iron availability with DFB addition was equally dramatic for the ultraplankton (0.2-5.0 Am), but in this case low-level tracer uptake (f 10%) persisted even at high DFB concentrations (3-500 nM). Our experimental findings are combined with a preliminary kinetic model to suggest that iron equilibration among the natural ligand classes (L 1 , L 2) and DFB may require an adjunctive (or associative) ligand substitution mechanism to explain the very rapid effect that DFB exerts on iron uptake when added to seawater. Even so, several hours to days likely are needed for the equilibration of added iron among DFB and natural ligands when low-level (e.g., 0.5 nM) DFB concentrations are employed. Model results provide indirect support to earlier suggestions that large eukaryotic phytoplankton extract iron from the weaker class of natural ligands (Fe(III)L 2). The combination of iron enrichment and DFB amendments provides a practical means for studying how iron influences algal production, carbon cycling and phytoplankton species composition in nearshore waters.
A chemical method for estimating the availability of iron to phytoplankton in seawater
Marine Chemistry, 1991
A technique that employs the complexing agent 8-hydroxyquinoline (oxine) to measure a labile portion of total Fe in seawater was used to test the hypothesis that the biological availability of colloidal Fe is a direct function of its chemical lability. Three colloidal ferrihydrites (amorphous FeOOH) were examined for their lability in seawater and for their ability to support growth of three species of neritic phytoplankton (Thalassiosira pseudonana, Isochrysis galbana and Dunaliella tertiolecta). Earlyphase (freshly precipitated) ferrihydrite was ~ 65% oxine-labile and proved to be an excellent source of available Fe, as determined by organism growth. Limited heating of this ferrihydrite, which accelerates low-temperature aging processes similar to those operating in nature, caused sharp decreases in both the algal growth responses and in the oxine-lability of the ferrihydrite. The correspondence between colloid lability and organism growth in culture suggests that Fe availability is related strongly to its chemical lability. It thus appears that the oxine technique provides an operational method for estimating the biological availability of Fe in seawater.
Global Biogeochemical Cycles, 2013
1] A closed eddy core in the Subantarctic Atlantic Ocean was fertilized twice with two tons of iron (as FeSO 4 ), and the 300 km 2 fertilized patch was studied for 39 days to test whether fertilization enhances downward particle flux into the deep ocean. Chlorophyll a and primary productivity doubled after fertilization, and photosynthetic quantum yield (F V /F M ) increased from 0.33 to ≥0.40. Silicic acid (<2 μmol L À1 ) limited diatoms, which contributed <10% of phytoplankton biomass. Copepods exerted high grazing pressure. This is the first study of particle flux out of an artificially fertilized bloom with very low diatom biomass. Net community production (NCP) inside the patch, estimated from O 2 :Ar ratios, averaged 21 mmol POC m À2 d À1 , probably ±20%. 234 Th profiles implied constant export of~6.3 mmol POC m À2 d À1 in the patch, similar to unfertilized waters. The difference between NCP and 234 Th-derived export partly accumulated in the mixed layer and was partly remineralized between the mixed layer and 100 m. Neutrally buoyant sediment traps at 200 and 450 m inside and outside the patch caught mostly <1.1 mmol POC m À2 d À1 , predominantly of fecal origin; flux did not increase upon fertilization. Our data thus indicate intense flux attenuation between 100 and 200 m, and probably between the mixed layer and 100 m. We attribute the lack of fertilization-induced export to silicon limitation of diatoms and reprocessing of sinking particles by detritus feeders. Our data are consistent with the view that nitrate-rich but silicate-deficient waters are not poised for enhanced particle export upon iron addition. Citation: Martin, P., et al. (2013), Iron fertilization enhanced net community production but not downward particle flux during the Southern Ocean iron fertilization experiment LOHAFEX, Global Biogeochem. Cycles, 27,