Many coastal oceans experience not only increased loads of nutrients but also changes in the stoichiometry of nutrient supply. Excess supply of nitrogen and stable or decreased supply of silicon lower silicon to nitrogen (Si:N) ratios, which may decrease diatom proportion in phytoplankton. To examine how Si:N ratios affect plankton community composition and food web structure, we performed a mesocosm experiment where we manipulated Si:N ratios and copepod abundance in a Baltic Sea plankton community. In high Si:N treatments, diatoms dominated. Some of them were likely spared from grazing unexpectedly resulting in higher diatom biomass under high copepod grazing. With declining Si:N ratios, dinoflagellates became more abundant under low and picoplankton under high copepod grazing. This altered plankton food web structure: under high Si:N ratios, edible diatoms were directly accessible food for copepods, while under low Si:N ratios, microzooplankton and phago-mixotrophs (mixoplankton) were a more important food source for mesograzers. The response of copepods to changes in the phytoplankton community was complex and copepod density-dependent. We suggest that declining Si:N ratios favor microzoo- and mixoplankton leading to increased complexity of planktonic food webs. Consequences on higher trophic levels will, however, likely be moderated by edibility, nutritional value or toxicity of dominant phytoplankton species.

中文翻译：

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在较低的Si：N比下，硅藻的优势降低，改变了浮游生物食物网

许多沿海海洋不仅营养增加，而且营养供应的化学计量也发生变化。氮的过量供应和硅的稳定或减少的供应降低了硅氮比（Si：N），这可能会降低浮游植物中硅藻的比例。为了研究Si：N比率如何影响浮游生物群落组成和食物网结构，我们进行了介观试验，在波罗的海浮游生物群落中我们操纵了Si：N比率和co足类动物的丰度。在高Si：N处理中，硅藻占主导地位。其中一些可能免于意外放牧，从而在高high足类放牧下导致更高的硅藻生物量。随着Si：N比值的降低，在低co足类放牧的情况下，低鞭毛藻和浮游动物的鞭毛藻变得更加丰富。这改变了浮游生物的食物网结构：在高Si：N比例下，食用硅藻是co足类动物的直接食物，而在低的Si：N比率下，微浮游动物和吞噬混合营养动物（浮游植物）是中食动物更重要的食物来源。co足类对浮游植物群落变化的响应是复杂的，且co足类密度依赖性。我们建议降低Si：N比例有利于微生动物和浮游生物，从而导致浮游食物网的复杂性增加。但是，营养水平较高的后果很可能会被主要浮游植物的可食性，营养价值或毒性所缓解。co足类对浮游植物群落变化的响应是复杂的，且co足类密度依赖性。我们建议降低Si：N比例有利于微生动物和浮游生物，从而导致浮游食物网的复杂性增加。但是，营养水平较高的后果很可能会被主要浮游植物的可食性，营养价值或毒性所缓解。co足类对浮游植物群落变化的响应是复杂的，且co足类密度依赖性。我们建议降低Si：N比例有利于微生动物和浮游生物，从而导致浮游食物网的复杂性增加。但是，营养水平较高的后果很可能会被主要浮游植物的可食性，营养价值或毒性所缓解。