Three Decades of Changing Nutrient Stoichiometry from Source to Sea on the Swedish West Coast

Abstract

European ecosystems have been subject to extensive shifts in anthropogenic disturbance, primarily through atmospheric deposition, climate change, and land management. These changes have altered the macronutrient composition of aquatic systems, with widespread increases in organic carbon (C), and declines in nitrogen (N) and phosphorus (P). Less well known is how these disturbances have affected nutrient stoichiometry, which may be a more useful metric to evaluate the health of aquatic ecosystems than individual nutrient concentrations. The Swedish west coast has historically experienced moderate to high levels of atmospheric deposition of sulfate and N, and eutrophication. In addition, coastal waters have been darkening with damaging effects on marine flora and fauna. Here, we present three decades of macronutrient data from twenty lakes and watercourses along the Swedish west coast, extending from headwaters to river mouths, across a range of land covers, and with catchments ranging 0.037–40,000 km2. We find a high degree of consistency between these diverse sites, with widespread increasing trends in organic C, and declines in inorganic N and total P. These trends in individual macronutrients translate into large stoichiometric changes, with a doubling in C:P, and increases in C:N and N:P by 50% and 30%, showing that freshwaters are moving further away from the Redfield Ratio, and becoming even more C rich, and depleted in N and P. Although recovery from atmospheric deposition is linked to some of these changes, land cover also appears to have an effect; lakes buffer against C increases, and decreases in inorganic N have been greatest under arable land cover. Our analysis also detects coherently declining P concentrations in small forest lakes; so called (and unexplained) “oligotrophication.” Taken together, our findings show that freshwater macronutrient concentrations and stoichiometry have undergone substantial shifts during the last three decades, and these shifts can potentially explain some of the detrimental changes that adjacent coastal ecosystems are undergoing. Our findings are relevant for all European and North American waters that have experienced historically high levels of atmospheric deposition, and provide a starting point for understanding and mitigating against the trajectories of long-term change in aquatic systems.

Spatial and Seasonal Variations in Dissolved Methane Across a Large Lake

“…Li et al, 2020). This finding agrees with a growing body of work arguing that nutrient stoichiometry, and not just absolute concentration, is important in controlling aquatic biogeochemistry (Graeber et al, 2021;Peacock et al, 2022;Stutter et al, 2018;Taylor & Townsend, 2010), including GHG emissions (Peacock et al, 2017;Webb et al, 2021). Finally, a negative relationship between surface water CH 4 concentration and water depth (Figure 6g) is frequently reported Juutinen et al, 2009;Natchimuthu et al, 2016) and is likely due to a greater opportunity for methanotrophy within the water column of deeper lakes (M. Li et al, 2020), as well as colder sediments having lower rates of methane production (Schulz et al, 1997).…”

Review article: A systematic review of terrestrial dissolved organic carbon in northern permafrost

“…Arctic coasts are eroding at rates of up to 25 m yr -1 (Fritz, Vonk, & Lantuit, 2017) and exporting large quantities of terrestrial organic matter export directly to the ocean that is rapidly mineralized (Tanski et al, 2019). Enhanced DOC export from these coastal tundra ecosystems may disrupt aquatic food webs through altering nutrient and light supply, as has been shown for Swedish coastal systems (Peacock et al, 2022).…”

Decadal Changes in Anthropogenic Inputs and Precipitation Influence Riverine Exports of Carbon, Nitrogen, and Phosphorus, and Alter Ecosystem Level Stoichiometry

“…Natural or anthropogenic, the main drivers of change are expected to vary across watersheds and regions. For instance, information spanning three decades from rivers and lakes in southern Sweden showed an increase in C to nutrient ratios (Peacock et al., 2022). In this region, C increased due to a reduction in sulfate deposition, whereas N decreased from a combination of lower atmospheric deposition and better agricultural practices.…”