Nutrient breakpoints are the concentration at which there is a fundamental change in phytoplankton responses to further increases in nutrient loading. The ecological implication is that nutrient breakpoints signal a community transition from a relatively stable state to an alternate or transition state with marked changes in structure and, by extension, function. Nutrient breakpoints for the total phytoplankton community and individual phytoplankton groups in response to increases in dissolved inorganic nitrogen (DIN) concentrations were determined for two contrasting estuarine systems. Phytoplankton were exposed to increasing N addition scenarios (0–100 μmol DIN L⁻¹) under high and low irradiance conditions. Responses of total algal biomass, group diversity, and individual algal groups were determined after 72‐h incubations in experimental bioassays. Phytoplankton from the low‐salinity estuary exhibited higher DIN breakpoints (>double) than the high‐salinity estuary while irradiance (20% vs. 40% of ambient) did not affect breakpoints. DIN additions changed algal group diversity, but community similarities were > 80%. Our results suggest that ambient DIN concentrations should not exceed breakpoint concentrations to prevent possible shifts from a stable to a transitional or alternate state. DIN thresholds should be 25 and 50 μmol L⁻¹ for the high‐ and low‐salinity estuaries, respectively. These levels should mitigate the risk of major alterations in phytoplankton community structure and function in these two estuarine systems.

中文翻译：

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河口浮游植物群落的营养断点

营养突破点是浮游植物对营养物负荷进一步增加的反应发生根本变化的浓度。生态学意义是营养素断裂点表明群落从相对稳定的状态过渡到交替或过渡状态，并在结构和功能上发生了明显变化。对于两个形成对比的河口系统，确定了总浮游植物群落和各个浮游植物群体对溶解性无机氮（DIN）浓度增加的营养折断点。浮游植物暴露于提高氮肥除了场景（0-100  μ摩尔DIN大号^-1）在高和低辐照度条件下。在实验生物测定中孵育72小时后，确定总藻类生物量，组多样性和单个藻类组的响应。低盐度河口的浮游植物比高盐度河口显示更高的DIN断点（>两倍），而辐照度（20％对环境的40％）不影响断点。DIN的添加改变了藻类的多样性，但社区相似度> 80％。我们的结果表明，环境DIN浓度不应超过断点浓度，以防止从稳定状态过渡到过渡状态或交替状态。DIN阈值应为25和 50μmol L ^-1分别用于高盐度和低盐度河口。这些水平应减轻这两个河口系统中浮游植物群落结构和功能发生重大变化的风险。