Seasonal nitrate drawdown transitions lake ecosystems from phosphorus to nitrogen deficiency in annual cycles

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Abstract Phytoplankton biomass is often limited by nitrogen (N) and/or phosphorus (P). Quantifying the relative availability of N and P in an ecosystem is useful in indicating nutrient limitation status. In subtropical lakes, nitrate availability generally follows a seasonal pattern with maxima in the late winter/early spring and minima in late summer/early autumn, also known as seasonal nitrate drawdown. Phytoplankton nutrient limitation in response to seasonal nitrate drawdown is largely understudied in lakes. We used a long‐term dataset of seasonal nitrate drawdown/accumulation alongside short‐term fertilization bioassay experiments and phytoplankton metatranscriptomes to quantify phytoplankton nutrient limitation and physiological response to seasonally changing nutrient availability in Beaver Lake, Arkansas. We hypothesized a seasonal shift in the phytoplankton community from P to N limitation. We found that the seasonal nitrate drawdown pattern in Beaver Lake has consistently occurred for at least 25 yr and is related to water temperature and season. We also found that Beaver Lake shifts from P limitation in spring to N and P co‐limitation in summer. A transition in phytoplankton physiological response from N assimilation and transport in spring to synthesis and fixation in summer/early autumn suggests that microbial activity and phytoplankton gene expression are responsive to seasonality and nutrient availability. We concluded that while N 2 fixation expression increases in response to N deficiency, it is not sufficient to offset N limitation. This study is the first to formally test the nitrate drawdown hypothesis , highlighting nitrate drawdown effects on phytoplankton and its implications for dual nutrient control in lakes.

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