Lake browning generates a spatiotemporal mismatch between dissolved organic carbon and limiting nutrients

The development of cyanobacteria blooms is of increasing concern in many lakes worldwide, and as a result, modeling their predictors is vital for understanding where and why they occur. In this study, we developed and analyzed a 640‐lake data set that spans Canada and 12 ecozones to identify the drivers of cyanobacteria biomass and of several key toxin‐ and bloom‐forming genera (Microcystis, Aphanizomenon, and Dolichospermum). The database consisted of an exhaustive list of potential predictors (n = 55), including water chemistry, land‐use, and zooplankton variables. We applied a series of empirical modeling approaches to identify significant predictors and thresholds (generalized linear and additive models, mixed effect regression trees), all while accounting for ecozone variability. Across all modeling approaches, and ecozones total phosphorus was identified as the most important predictor of total cyanobacterial and focal genera biomass. In addition, cyanobacteria across Canada showed significant associations with increasing dissolved organic and inorganic carbon, and several ions. Despite the widely held notion that cyanobacteria are often toxic and/or a poor food source for zooplankton, we found a positive relationship between cyanobacteria and zooplankton, particularly with daphnid and copepod biomass. Localized top‐down forces and evolutionary adaptations resulting from long‐term exposure in eutrophic lakes are among the possible explanations for this observed positive association. By considering a suite of complementary modeling approaches, we found that nonlinear models provided greater predictive power and the random ecozone effect was minor due to the overarching importance of local abiotic and biotic factors.

Section: Discussionsupporting

confidence: 88%

Both biotic and abiotic predictors explain significant variation in cyanobacteria biomass across lakes from temperate to subarctic zones

MacKeigan

Taranu

Pick

et al. 2023

“…Phytoplankton communities responded to different N forms, especially in the presence of P additions, suggesting phytoplankton communities have flexibility in their nutrient demands and can capitalize on the DON pool across a range of nutrient conditions. In this region and others around the world, lakes are experiencing an increase in dissolved organic matter (“browning”; Williamson et al 2015; Stetler et al 2021) and increasing DON concentrations (Stetler et al 2021). Expanding on these results to understand how various N forms, both labile and refractory, might affect phytoplankton might prove crucial for predicting algal blooms and nutrient limitation and setting load limits in freshwater ecosystems.…”

Section: Discussionmentioning

confidence: 99%

Nutrient function over form: Organic and inorganic nitrogen additions have similar effects on lake phytoplankton nutrient limitation

Volponi

Wander

Richardson

et al. 2022

The concentration of dissolved organic nitrogen (DON) is increasing in many northern hemisphere lakes, yet its use by phytoplankton and fate in the environment seldom have been quantified. We conducted 1 week, in situ, microcosm incubations across 25 lakes in northeastern North America to understand how DON, dissolved inorganic nitrogen (DIN), and dissolved inorganic phosphorus (P) affected phytoplankton biomass. In addition, we tested whether lakes were limited by single macronutrients (N or P) or colimited by both. Phytoplankton biomass in 80% of lakes responded similarly to DON and DIN additions. Of the lakes where N form produced differential responses, the majority of phytoplankton communities exhibited greater biomass accumulation with DON than DIN. Colimitation was the most common type of nutrient limitation among the study lakes, followed by P limitation. Limitation type shifted with N form in 40% of the study lakes, but without consistent patterns explaining how shifts occurred. Regardless of N form, lakes with watersheds more dominated by agriculture and higher total dissolved nitrogen (TDN) tended to show P‐limited phytoplankton responses, while lakes with less agricultural watersheds and lower TDN tended to show colimited phytoplankton responses. Finally, ambient TDN and total phosphorus (TP) nutrient concentrations were stronger predictors of limitation type than ambient TDN : TP ratios. The different contributions of DON and DIN to phytoplankton biomass in some of our study lakes suggest that DON loading from surrounding watersheds may be an overlooked component in predicting phytoplankton productivity and nutrient limitation dynamics in freshwater ecosystems.

“…In this context, differences in the direction and magnitude of trends across resources may signify shifts in aquatic nutrient limitation via changing N : P (Isles et al 2018) as well as a growing imbalance in the supply of organic energy vs. nutrients (e.g., DOC : inorganic N). These shifts and imbalances could constrain the growth of aquatic heterotrophs (Taylor and Townsend 2010) and regulate aquatic productivity (Stetler et al 2021).…”

mentioning

confidence: 99%

Co‐occurrence of browning and oligotrophication in a boreal stream network

Mosquera

Hasselquist

Sponseller

et al. 2022

The relative supply of carbon (C), nitrogen (N), and phosphorus (P) to freshwater ecosystems is of fundamental importance to aquatic productivity, nutrient cycling, and food web dynamics. In northern landscapes, ongoing climate change, as well as legacies from atmospheric deposition, have the potential to drive changes in how these elements are recycled on land and exported to streams. While it is well established that dissolved organic carbon (DOC) concentrations have increased in many high latitude streams, the simultaneous trends for N and P and the ratios among these resources, are not well documented. We used data from 13 sites in a boreal stream network to analyze decadal‐scale changes in dissolved inorganic N (DIN), dissolved organic N (DON), and dissolved inorganic P (DIP) concentrations and partition these trends seasonally. We observed widespread declines for DIP and DIN in streams, regardless of catchment characteristics. DIN decline was strongest during the growing season, and together with increases in DOC/DON at several sites, suggests increasing N retention by plants and soil microbes across this landscape. By contrast, declines for DIP occurred primarily during late autumn and winter, indicating that key biogeochemical changes are also occurring during non‐growing season. Linking these trends to increases in DOC concentration in streams revealed changes in the ratio of energy to nutrient supply for the majority of sites, becoming richer in carbon and poorer in limiting nutrients over time. Overall, our observations from this stream network point to ongoing oligotrophication, with possible consequences for aquatic ecosystems in boreal landscapes.