Recent Synchronous Declines in DIN:TP in Swedish Lakes

Abstract

Declining atmospheric nitrogen (N) deposition in northern Europe and parts of North America, coupled with ongoing changes in climate, has the potential to alter the nutrient limitation status of freshwater ecosystems. In this study we compared time series data of atmospheric N deposition, air temperature, and precipitation with corresponding estimates of dissolved inorganic nitrogen (DIN), total phosphorus (TP), DIN:TP, and total organic carbon from 78 headwater streams and 95 nutrient‐poor lakes in Sweden from 1998 to 2013 to assess trends in, and potential drivers of, lake N:P ratios. We found that trends in nutrients were variable at the scale of individual lakes but were highly synchronous at the regional scale, suggesting underlying control by broad‐scale environmental drivers mediated by site‐specific characteristics. Widespread declines in lake DIN throughout Sweden were correlated with declines in atmospheric N deposition, particularly in northern areas. TP did not have strong directional trends, but interannual variability was synchronous at regional scales, implying that broad‐scale climate drivers were affecting these trends. Overall, we observed a significant decline in DIN:TP throughout Sweden over the monitoring period. At the beginning of the study period, 32% of lakes were N limited and 45% colimited by N and P. Proportions increased to 63% of lakes N limited and 20% colimited by N and P at the end of the study period. These results suggest that N limitation is likely to become more widespread in subarctic and boreal areas of Europe in the future if recent trends continue.

Anthropogenic global shifts in biospheric N and P concentrations and ratios and their impacts on biodiversity, ecosystem productivity, food security, and human health

“…The cascades of effects due to anthropogenic shifts in N:P ratios are similar in aquatic systems (lakes, estuaries, streams) and terrestrial ecosystems, where water and planktonic N:P ratios tend to F I G U R E 5 Current N and P imbalances linked to human activity in river basins increase in response to atmospheric deposition, leading to lower "growth rates," complexity of community structure, and trophic diversity ( Figure 6; Table S1). Exceptions to these trends, however, have been recorded for aquatic systems, such as a decrease in N:P ratios in Japan due to the increasing deposition of P from dust dispersed from countries in southeastern Asia (Miyazako et al, 2015), and for European and North American lakes in areas with recent reductions in N deposition (Gerson et al, 2016;Isles et al, 2018). Although most studies of urban and crop wastes and leachate loads to rivers and estuaries (83.3%) have found increasing N:P ratios associated with increasing N:P ratios from human inputs, other studies (13.7%) tended to find decreasing ratios in areas with high livestock densities (Arbuckle & Downing, 2001;Johnson, Heck, & Fourqurean, 2006; Figure 6; Table S1).…”

“…The shifts in organisms' N:P ratio resulting from different environmental conditions are strongly related with shifts in ecosystems structure and function (Loladze & Elser, 2011;Penuelas et al, 2013;Sterner & Elser, 2002). Imbalances between these two nutrients, N and P in natural, seminatural, and managed ecosystems (Carnicer et al, 2015;Delgado-Baquerizo et al, 2017;Hu et al, 2018;Liu, Fu, Zheng, & Liu, 2010;Penuelas et al, 2013;Ulm, Hellmann, Cruz, & Máguas, 2016), reduce C capture and global Isles, Creed, & Bergstrom, 2018). Atmospheric P deposition is also increasing due to the rising levels of anthropogenic emissions of P to the atmosphere (3.5 Tg P/year), which have led to current net continental and oceanic rates of P deposition of 2.7 and 0.8 Tg P/year, respectively (Wang, Balkanski, et al, 2015).…”

Changes in nutritional quality and nutrient limitation regimes of phytoplankton in response to declining N deposition in mountain lakes

“…Being nutrient poor, these lake ecosystems may respond strongly to even small environmental changes that affect the nutrient limitation regimes of phytoplankton. Recent studies from eastern North America and Scandanavia indicate that nutrient concentrations [dissolved inorganic nitrogen (DIN) and total phosphorus (TP)] in northern nutrient poor lakes are declining (Eimers et al 2009;Canham et al 2012;Huser et al 2018;Isles et al 2018), while concentrations of dissolved organic carbon (DOC) are increasing (Monteith et al 2007;de Wit et al 2016;Kritzberg 2017). The effect of declining trends in nutrient concentrations is a general decline in DIN:TP ratios, implying that phytoplankton N limitation is becoming more widespread in nutrient poor lakes (Isles et al 2018).…”

“…Recent studies from eastern North America and Scandanavia indicate that nutrient concentrations [dissolved inorganic nitrogen (DIN) and total phosphorus (TP)] in northern nutrient poor lakes are declining (Eimers et al 2009;Canham et al 2012;Huser et al 2018;Isles et al 2018), while concentrations of dissolved organic carbon (DOC) are increasing (Monteith et al 2007;de Wit et al 2016;Kritzberg 2017). The effect of declining trends in nutrient concentrations is a general decline in DIN:TP ratios, implying that phytoplankton N limitation is becoming more widespread in nutrient poor lakes (Isles et al 2018). However, our knowledge is currently limited as to the extent that declining trends in nutrient concentrations and DIN:TP ratios in the face of increasing DOC trends affect nutrient limitation regimes and phytoplankton quantity and quality [i.e., carbon:nitrogen:phosphorus (C:N:P) stoichiometry and fatty acid (FA) composition] in mountain lakes.…”

“…Declines in lake DIN concentrations are associated with declining inputs of reactive N from the atmosphere (Simpson et al 2014;Isles et al 2018), whereas declines in lake TP concentrations are associated with climate change and recovery from acidification effects on catchment processes (Huser et al 2018;Isles et al 2018). Earlier studies have shown that elevated N deposition raised lake water N:P stoichiometry in many northern oligotrophic lakes, which in turn shifted phytoplankton from N-to P-limited conditions (Bergström et al 2005;Elser et al 2009a), increased biomass (Bergström and Jansson 2006) and C:P and N:P ratios in phytoplankton (Elser et al 2009b(Elser et al , 2010.…”