Abstract
Growth of large phytoplankton is considered to be diffusion limited at low nutrient concentrations, yet their constraints and contributions to carbon (C) and nitrogen fluxes in field plankton communities are poorly quantified under this condition. Using secondary ion mass spectrometry (SIMS), we quantified cell-specific assimilation rates of C, nitrate, and ammonium in summer communities of large phytoplankton when dissolved inorganic nitrogen concentrations are low in temperate coastal regions. Chain-forming diatoms composed 6% of total particulate organic carbon, but contributed 20% of C assimilation, 54% of nitrate assimilation and 32% of ammonium assimilation within the plankton community. In contrast, large dinoflagellates composed 11% of total POC, and contributed 14% of the C assimilation, 4% of ammonium and 9% of nitrate assimilation within the plankton community. Measured cell-specific C and nitrate assimilation rate match the Redfield ratio and the maximal nitrate assimilation in Chaetoceros spp. predicted by mass transfer theory. However, average ammonium assimilation rates were 30 and 340% higher than predicted by mass transfer theory in Tripos/Ceratium and Chaetoceros, respectively, suggesting that microbial interactions in the phycosphere may facilitate substantial luxury ammonium uptake by Chaetoceros in environments with fluctuating nitrate concentrations.
Highlights
Aquatic primary production constitutes approximately 50% of global primary production[1]
We performed our study in a plankton community in a temperate coastal region with persistently low nutrient concentrations during late summer when turbulence was moderate
Species belonging to Tripos/ Ceratium spp., Asterionellopsis glacialis, Chaetoceros spp. (>8 μm) and other chain-forming diatoms ca. 5–20 μm (e.g., Skeletonema marinoi, Cerataulina pelagica, Dactyliosolen spp., Leptocylindrus danicus, Guinardia delicatula) were analyzed by Secondary ion mass spectrometry (SIMS) to assess their assimilation of C, ammonium, and nitrate
Summary
Aquatic primary production constitutes approximately 50% of global primary production[1]. The oligotrophic (sub)tropical ocean and summer blooms in temperate coastal regions are characterized by low DIN concentrations and low abundances of large, chain-forming diatoms, while large, supposedly mixotrophic dinoflagellates, and picoplankton (
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