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

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 km(2). 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

Consequences of intensive forest harvesting on the recovery of Swedish lakes from acidification and on critical load exceedances

Across much of the northern hemisphere, lakes are at risk of re-acidification due to incomplete recovery from historical acidification and pressures associated with more intensive forest biomass harvesting. Critical load (CL) calculations aimed at estimating the amount of pollutants an ecosystem can receive without suffering adverse consequences are dependent on these factors. Here, we present a modelling study of the potential effects of intensified forest harvesting on re-acidification of a set of 3239 Swedish lakes based on scenarios with varying intensities of forest biomass harvest and acid deposition. There is some evidence that forestry would have caused a certain level of acidification even if deposition remained at 1860 levels. We show that all plausible harvest scenarios delay recovery due to increased rates of base cation removal. Scenario results were used to estimate critical loads for the entire population of lakes in Sweden. The forestry intensity included in critical load calculations is a political decision. After scaling calculations to the national level, it was apparent that a high but plausible forest harvest intensity would lead to an increase in the area of CL exceedances and that even after significant reductions in forest harvest intensity, there would still be areas with CL exceedances. Our results show that forest harvest intensity and regional environmental change must be carefully considered in future CL calculations

Letter to editor regarding Kotta et al. 2020 : Cleaning up seas using blue growth initiatives: Mussel farming for eutrophication control in the Baltic Sea

Non peer reviewe

Biosilicification Drives a Decline of Dissolved Si in the Oceans through Geologic Time

Biosilicification has driven variation in the global Si cycle over geologic time. The evolution of different eukaryotic lineages that convert dissolved Si (DSi) into mineralized structures (higher plants, siliceous sponges, radiolarians, and diatoms) has driven a secular decrease in DSi in the global ocean leading to the low DSi concentrations seen today. Recent studies, however, have questioned the timing previously proposed for the DSi decreases and the concentration changes through deep time, which would have major implications for the cycling of carbon and other key nutrients in the ocean. Here, we combine relevant genomic data with geological data and present new hypotheses regarding the impact of the evolution of biosilicifying organisms on the DSi inventory of the oceans throughout deep time. Although there is no fossil evidence for true silica biomineralization until the late Precambrian, the timing of the evolution of silica transporter genes suggests that bacterial silicon-related metabolism has been present in the oceans since the Archean with eukaryotic silicon metabolism already occurring in the Neoproterozoic. We hypothesize that biological processes have influenced oceanic DSi concentrations since the beginning of oxygenic photosynthesis

Greenhouse gas production in nitrogen removal wetlands

Nitrogen removal wetlands are constructed in agricultural areas as a measure to reduce the amount of non-point source nitrogen that reaches coastal areas. Methane and nitrous oxide, two greenhouse gases, are produced under anaerobic conditions and an increased amount of waterlogged areas will therefore increase the potential emission of these gases. The question has arisen: Are we solving one environmental problem with measures that render another? In a study in pristine New Zealand wetlands, I found that plant phosphorus and nitrate in soil best describe the variation in anaerobic soil respiration to carbon dioxide and methane. Plant phosphorus, indicating wetland productivity, was positively correlated with methane production, whereas nitrate in soil was negatively correlated with methane production. Swedish constructed nitrogen removal wetlands are considered to be highly productive and they have high concentrations of nitrate in the water. Will the Swedish nitrogen removal wetlands produce a lot of methane or will the production be hampered? In a field study I found that temperature explains most of the seasonal variation in methane emission. When the temperature effect was taken into account I found a negative effect of nitrate on methane emission. In laboratory experiments I also found that temperature was important for methane production, and that this was due to low substrate availability at lower temperatures, i.e. temperature had an indirect effect on methane producers. At higher nitrate additions more nitrous oxide was produced and methane was to some extent restricted by high nitrate concentrations. Low carbon availability in a meadow soil compared to in sediment from a recently restored wetland resulted in lower nitrous oxide production from meadow soil than from wetland sediment. A faster conversion of nitrous oxide to nitrogen gas also suggests that nitrogen removal is more efficient in a permanently inundated wetland than in a meadow soil that is only occasionally wet. In another study I compared the benefit (nitrogen removal) with the risk (methane emission) in 36 nitrogen removal wetlands in southwestern Sweden. There was no positive relationship between nitrogen removal and methane emission, which means that a wetland can be optimized for nitrogen removal without an increased risk of methane emission. Nitrate concentration in the water was positively correlated to nitrogen removal and negatively correlated to methane concentration in the water (at constant temperature). The results therefore suggest that nitrogen removal wetlands should be located in highly nitrate-loaded areas. Overall I have shown that nitrate concentrations occurring in the constructed nitrogen removal ponds, can have a negative impact on methane development

Mussel farming as a nutrient reduction measure in the Baltic Sea: Consideration of nutrient biogeochemical cycles.

Nutrient loads from the land to the sea must be reduced to combat coastal eutrophication. It has been suggested that further mitigation efforts are needed in the brackish Baltic Sea to decrease nutrients, especially in eutrophic coastal areas. Mussel farming is a potential measure to remove nutrients directly from the sea. Mussels consume phytoplankton containing nitrogen (N) and phosphorus (P); when the mussels are harvested these nutrients are removed from the aquatic system. However, sedimentation of organic material in faeces and pseudo-faeces below a mussel farm consumes oxygen and can lead to hypoxic or even anoxic sediments causing an increased sediment release of ammonium and phosphate. Moreover, N losses from denitrification can be reduced due to low oxygen and reduced numbers of bioturbating organisms. To reveal if mussel farming is a cost-effective mitigation measure in the Baltic Sea the potential for enhanced sediment nutrient release must be assessed