Abstract
Northern lakes are experiencing widespread increases in dissolved organic carbon (DOC) that are likely to lead to changes in pelagic phytoplankton biomass. Pelagic phytoplankton biomass responds to trade-offs between light and nutrient availability. However, the influence of DOC light absorbing properties and carbon–nutrient stoichiometry on phytoplankton biomass across seasonal or spatial gradients has not been assessed. Here, we analyzed data from almost 5000 lakes to examine how the carbon–phytoplankton biomass relationship is influenced by seasonal changes in light availability, DOC light absorbing properties (carbon-specific visual absorbance, SVA420), and DOC–nutrient [total nitrogen (TN) and total phosphorus (TP)] stoichiometry, using TOC as a proxy for DOC. We found evidence for trade-offs between light and nutrient availability in the relationship between DOC and phytoplankton biomass [chlorophyll (chl)-a], with the shape of the relationship varying with season. A clear unimodal relationship was found only in the fall, particularly in the subsets of lakes with the highest TOC:TP. Observed trends of increasing TOC:TP and decreasing TOC:TN suggest that the effects of future browning will be contingent on future changes in carbon–nutrient stoichiometry. If browning continues, phytoplankton biomass will likely increase in most northern lakes, with increases of up to 76% for a 1.7 mg L−1 increase in DOC expected in subarctic regions, where DOC, SVA420, DOC:TN, and DOC:TP are all low. In boreal regions with higher DOC and higher SVA420, and thus lower light availability, lakes may experience only moderate increases or even decreases in phytoplankton biomass with future browning.
Highlights
The highest concentration of lakes in terms of both total number and area is located in northern latitudes (Verpoorter and others 2014)
Because quadratic curves were identified as the best fits for all seasons, quadratic curves were fitted seasonal total organic carbon (TOC)–chl-a curves subsetted to quartiles of the TOC:total phosphorus (TP) ratio, TOC:total nitrogen (TN) ratio, and SVA420
The unimodal response was less consistent than we expected based on previous studies (Karlsson and others 2015; Kelly and others 2018; Bergstrom and Karlsson 2019), but our finding is supported by a recent analysis that found increases in primary production (PP) with increasing dissolved organic carbon (DOC) across large gradients in boreal Canada (Bogard and others 2020), by a recent study which identified no unimodal relationship between DOC and chl-a in a survey of lakes in central Ontario (Senar and others 2021), and by targeted experimental studies which commonly observed increases in GPP with increasing DOC (Klug 2002; Zwart and others 2016)
Summary
The highest concentration of lakes in terms of both total number and area is located in northern latitudes (Verpoorter and others 2014). The trade-off between light and nutrient availability may, as shown in empirical (Hanson and others 2003; Ask and others 2009) and theoretical (Kelly and others 2018; Vasconcelos and others 2019) studies, result in a unimodal response in pelagic primary production (PP) to increases in colored DOC, with peaks in PP observed at intermediate DOC concentrations of 10 to 12 mg L-1 It is not yet clear under what conditions a unimodal response is likely to occur, and what implications this has for the future trajectory of PP with future browning. Because the hypothesized mechanism for the unimodal response involves a trade-off between light and nutrient availability, the degree to which DOC absorbs light in the photosynthetically active spectrum should influence the position of the unimodal curve, affecting the DOC concentration at which peak phytoplankton biomass occurs (Kelly and others 2018) These theoretical models have been tested in several experimental studies, but have not often been examined using field datasets
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