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  1. Free, publicly-accessible full text available November 1, 2023
  2. In addition to ocean acidification, a significant recent warming trend in Chinese coastal waters has received much attention. However, studies of the combined effects of warming and acidification on natural coastal phytoplankton assemblages here are scarce. We conducted a continuous incubation experiment with a natural spring phytoplankton assemblage collected from the Bohai Sea near Tianjin. Experimental treatments used a full factorial combination of temperature (7 and 11°C) and pCO 2 (400 and 800 ppm) treatments. Results suggest that changes in pCO 2 and temperature had both individual and interactive effects on phytoplankton species composition and elemental stoichiometry. Warming mainly favored the accumulation of picoplankton and dinoflagellate biomass. Increased pCO 2 significantly increased particulate organic carbon to particulate organic phosphorus (C:P) and particulate organic carbon to biogenic silica (C:BSi) ratios, and decreased total diatom abundance; in the meanwhile, higher pCO 2 significantly increased the ratio of centric to pennate diatom abundance. Warming and increased pCO 2 both greatly decreased the proportion of diatoms to dinoflagellates. The highest chlorophyll a biomass was observed in the high pCO 2 , high temperature phytoplankton assemblage, which also had the slowest sinking rate of all treatments. Overall, there were significant interactive effects of increased pCOmore »2 and warming on dinoflagellate abundance, pennate diatom abundance, diatom vs. dinoflagellates ratio and the centric vs. pennate ratio. These findings suggest that future ocean acidification and warming trends may individually and cumulatively affect coastal biogeochemistry and carbon fluxes through shifts in phytoplankton species composition and sinking rates.« less
  3. Abstract

    Throughout the open ocean, a minimum in dissolved iron concentration (dFe) overlaps with the deep chlorophyll maximum (DCM), which marks the lower limit of the euphotic zone. Maximizing light capture in these dim waters is expected to require upregulation of Fe-bearing photosystems, further depleting dFe and possibly leading to co-limitation by both iron and light. However, this effect has not been quantified for important phytoplankton groups likeProchlorococcus, which contributes most of the productivity in the oligotrophic DCM. Here, we present culture experiments withProchlorococcusstrain MIT1214, a member of the Low Light 1 ecotype isolated from the DCM in the North Pacific subtropical gyre. Under a matrix of iron and irradiance matching those found at the DCM, the ratio of Fe to carbon inProchlorococcusMIT1214 cells ranged from 10–40 × 10−6 mol Fe:mol C and increased with light intensity and growth rate. These results challenge theoretical models predicting highest Fe:C at lowest light intensity, and are best explained by a large photosynthetic Fe demand that is not downregulated at higher light. To sustain primary production in the DCM with the rigid Fe requirements of low-light-adaptedProchlorococcus, dFe must be recycled rapidly and at high efficiency.