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.

在低养分浓度下，大型浮游植物的生长被认为是扩散受限的，但是在这种条件下，它们对田间浮游生物群落的限制以及对碳（C）和氮通量的贡献很难被量化。使用二次离子质谱仪（SIMS），我们定量了在温带沿海地区溶解的无机氮浓度较低时，大型浮游植物夏季群落中C，硝酸盐和铵的细胞特异性同化率。在浮游生物群落中，成链硅藻占颗粒有机碳总量的6％，但贡献了20％的C同化，54％的硝酸盐同化和32％的铵同化。相比之下，大型鞭毛虫占总POC的11％，并贡献了14％的C同化，浮游生物群落中有4％的铵和9％的硝酸盐被吸收。测得的细胞特异性C和硝酸盐同化率与Redfield比率和最大硝酸盐同化率相匹配Chaetoceros spp。由传质理论预测。然而，平均质量的氨同化率分别比Tripos / Ceratium和Chaetoceros中的传质理论所预测的分别高30％和340％，这表明在硝酸盐浓度波动的环境中，藻圈中的微生物相互作用可能促进Chaetoceros大量吸收豪华的铵。