<p>Mixotrophy is the ability of phytoplankton to acquire nutrients (specifically nitrogen) from both inorganic and organic sources, and it represents a potentially important process affecting phytoplankton production and corresponding nutrient cycling in estuarine environments. Understanding the balance between growth supported by inorganic versus organic nutrients is therefore crucial to improve simulation of phytoplankton dynamics and to assess the impacts of nutrient reduction management strategies at the estuary scale. Here we examine key processes affecting phytoplankton production (e.g., mixotrophic growth, nutrient limitation) and their spatiotemporal variation along a large-scale environmental gradient. We combine numerical modeling with <i>in situ</i> observations and results from laboratory experiments by calibrating various phytoplankton production models to multi-decadal light-saturated phytoplankton production and other ecological observations from the Chesapeake Bay (U.S.A.). When mixotrophy is excluded, the optimal model fit to the data suggests an absence of inorganic nitrogen limitation in the Bay and predicts higher carbon-to-chlorophyll-a ratios in the mesohaline than in the polyhaline region in summer. These results contradict observations and lead to an overestimation of autotrophic phytoplankton production by <InlineEquation ID="IEq1"> <EquationSource Format="TEX">\(\sim\)</EquationSource> </InlineEquation> 40 % in fall and <InlineEquation ID="IEq2"> <EquationSource Format="TEX">\(\sim\)</EquationSource> </InlineEquation> 75 % in spring in the polyhaline portion of the Bay compared to simulations that include mixotrophy. Mixotrophy is thus required to realistically simulate inorganic nitrogen limitation and spatial variability in carbon-to-chlorophyll-a ratios. The average contribution of mixotrophic growth to total phytoplankton production strongly varies depending on season and region in the Bay (<InlineEquation ID="IEq3"> <EquationSource Format="TEX">\(\sim\)</EquationSource> </InlineEquation> 10–75 %). Our study provides one of the first estimates of the relative contribution of mixotrophic growth to total phytoplankton production, with implications for both experimental scientists and modelers, both of whom often focus on autotrophic growth. Excluding mixotrophy may lead to underestimating phytoplankton sensitivity to nutrient reductions, potentially leading to inaccurate assessments of nutrient reduction management strategies.</p>

The Importance of Mixotrophy for Estuarine Phytoplankton Production and Nutrient Cycling

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2026年4月8日

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Dante M. L. Horemans,
Marjorie A. M. Friedrichs,
Pierre St-Laurent,
Raleigh R. Hood,
Christopher W. Brown

摘要

Mixotrophy is the ability of phytoplankton to acquire nutrients (specifically nitrogen) from both inorganic and organic sources, and it represents a potentially important process affecting phytoplankton production and corresponding nutrient cycling in estuarine environments. Understanding the balance between growth supported by inorganic versus organic nutrients is therefore crucial to improve simulation of phytoplankton dynamics and to assess the impacts of nutrient reduction management strategies at the estuary scale. Here we examine key processes affecting phytoplankton production (e.g., mixotrophic growth, nutrient limitation) and their spatiotemporal variation along a large-scale environmental gradient. We combine numerical modeling with in situ observations and results from laboratory experiments by calibrating various phytoplankton production models to multi-decadal light-saturated phytoplankton production and other ecological observations from the Chesapeake Bay (U.S.A.). When mixotrophy is excluded, the optimal model fit to the data suggests an absence of inorganic nitrogen limitation in the Bay and predicts higher carbon-to-chlorophyll-a ratios in the mesohaline than in the polyhaline region in summer. These results contradict observations and lead to an overestimation of autotrophic phytoplankton production by \(\sim\) 40 % in fall and \(\sim\) 75 % in spring in the polyhaline portion of the Bay compared to simulations that include mixotrophy. Mixotrophy is thus required to realistically simulate inorganic nitrogen limitation and spatial variability in carbon-to-chlorophyll-a ratios. The average contribution of mixotrophic growth to total phytoplankton production strongly varies depending on season and region in the Bay ( \(\sim\) 10–75 %). Our study provides one of the first estimates of the relative contribution of mixotrophic growth to total phytoplankton production, with implications for both experimental scientists and modelers, both of whom often focus on autotrophic growth. Excluding mixotrophy may lead to underestimating phytoplankton sensitivity to nutrient reductions, potentially leading to inaccurate assessments of nutrient reduction management strategies.