IVL Swedish Environmental Research Institute

European ecosystems have been subject to extensiveshifts in anthropogenic disturbance, primarilythrough atmospheric deposition, climate change,and land management. 

These changes have alteredthe 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 haveaffected nutrient stoichiometry, which may be amore useful metric to evaluate the health ofaquatic ecosystems than individual nutrient concentrations.

The Swedish west coast has historicallyexperienced moderate to high levels of atmosphericdeposition of sulfate and N, and eutrophication. Inaddition, coastal waters have been darkening withdamaging effects on marine flora and fauna.

Here,we present three decades of macronutrient datafrom twenty lakes and watercourses along the Swedish west coast, extending from headwaters toriver mouths, across a range of land covers, andwith catchments ranging 0.037–40,000 km2. Wefind a high degree of consistency between thesediverse sites, with widespread increasing trends inorganic C, and declines in inorganic N and total P.

These trends in individual macronutrients translateinto large stoichiometric changes, with a doublingin C:P, and increases in C:N and N:P by 50% and30%, showing that freshwaters are moving furtheraway from the Redfield Ratio, and becoming evenmore C rich, and depleted in N and P.

Althoughrecovery from atmospheric deposition is linked tosome of these changes, land cover also appears tohave an effect; lakes buffer against C increases, anddecreases in inorganic N have been greatest underarable land cover. Our analysis also detects coherentlydeclining P concentrations in small forestlakes; so called (and unexplained) ‘‘oligotrophication.’’

Taken together, our findings show thatfreshwater macronutrient concentrations and stoichiometryhave undergone substantial shifts duringthe last three decades, and these shifts can potentiallyexplain some of the detrimental changes thatadjacent coastal ecosystems are undergoing.

Ourfindings are relevant for all European and NorthAmerican waters that have experienced historicallyhigh levels of atmospheric deposition, and provide a starting point for understanding and mitigating against the trajectories of long-term change in aquatic systems.