Climate-change driven increases in temperature and precipitation are leading to increased discharge of freshwater and terrestrial material to Arctic coastal ecosystems. These inputs bring sediments, nutrients and organic matter (OM) across the land-ocean interface with a range of implications for coastal ecosystems and biogeochemical cycling. To investigate responses to terrestrial inputs, physicochemical conditions were characterized in a river- and glacier-influenced Arctic fjord system (Isfjorden, Svalbard) from May to August in 2018 and 2019. Our observations revealed a pervasive freshwater footprint in the inner fjord arms, the geochemical properties of which varied spatially and seasonally as the melt season progressed. In June, during the spring freshet, rivers were a source of dissolved organic carbon (DOC; with concentrations up to 1410 μmol L^–1). In August, permafrost and glacial-fed meltwater was a source of inorganic nutrients including NO₂ + NO₃, with concentrations 12-fold higher in the rivers than in the fjord. While marine OM dominated in May following the spring phytoplankton bloom, terrestrial OM was present throughout Isfjorden in June and August. Results suggest that enhanced land-ocean connectivity could lead to profound changes in the biogeochemistry and ecology of Svalbard fjords. Given the anticipated warming and associated increases in precipitation, permafrost thaw and freshwater discharge, our results highlight the need for more detailed seasonal field sampling in small Arctic catchments and receiving aquatic systems.

Despite the global prevalence of both mercury (Hg) contamination and anthropogenic eutrophication, relatively little is known about the behavior of Hg in eutrophic and hypereutrophic systems or the effects of lake trophic status on Hg uptake and trophodynamics. In the current study we explore Hg trophodynamics at 8 tropical East African study sites ranging from mesotrophic to hypereutrophic, in order to assess the influence of lake trophic status on Hg uptake and biomagnification. Comprehensive water, plankton and fish samples were collected for analysis of total mercury (THg) and stable carbon and nitrogen isotopic ratios. We found evidence that uptake of THg into phytoplankton tended to be lower in higher productivity systems. THg concentrations in fish were generally low, and THg trophic magnification factors (TMFs; representing the average increase in contaminant concentrations from one trophic level to the next) ranged from 1.9 to 5.6. Furthermore TMFs were significantly lower in hypereutrophic lakes than in meso- and eutrophic lakes, and were negatively related to chlorophyll a concentrations both across our study lakes, and across African lakes for which literature data were available. These observations suggest that THg concentrations were strongly influenced by trophic status, with year-round high phytoplankton and fish growth rates reducing the potential for high THg in fish in these productive tropical lakes.

We investigated the factors influencing cyanobacterial biomass and microcystin (MC) concentrations in several Ugandan lakes from September 2008 to February 2009. We characterized thermal structure, light availability, nutrient concentrations, chlorophyll a, phytoplankton δ¹³C (as an indicator of CO₂ limitation), and phytoplankton community composition and abundance as well as MC concentrations. We used these data to test several hypotheses based on previous research in temperate lakes regarding the factors that encourage high cyanobacterial biomass and MC concentrations. Site characteristics that appeared to favor high cyanobacterial biomass (especially Microcystis) included: high total phosphorus concentrations, low total nitrogen to total phosphorus (TN : TP) ratios, and possibly low CO₂ availability. Light availability, total nitrogen concentrations, and thermal structure of the water column were not related to cyanobacterial biomass. MC concentrations were strongly related to Microcystis biomass (and were not related to the biomass of any other cyanobacterial taxa), which was positively correlated with total phosphorus and chlorophyll a concentrations. MC cell content may be moderated by CO₂ availability, with MC cell quotas tending to be lower where the potential for C‐limitation of photosynthesis was higher. In these phosphorus‐rich tropical lakes, the shallowest study sites were most conducive to the development of large standing crops of Microcystis and high MC concentrations. The environmental conditions that appear to favor high cyanobacterial biomass and MC concentrations in our Ugandan study lakes are similar to what has been observed for temperate lakes.

Increases in terrestrial organic matter (tOM) transport from catchments to boreal lakes can affect methylmercury (MeHg) accumulation in aquatic biota both directly by increasing concentrations of aqueous MeHg, and indirectly through effects on MeHg bioavailability and on energy pathways in the lower food web. We carried out a detailed seasonal study of water chemistry, zooplankton diet, and MeHg accumulation in zooplankton in two lakes with contrasting tOM concentrations. Between-lake differences explained 51% of the variability in our water chemistry data, with no observed effect of season or sampling depth, contrary to our expectations. Higher tOM was correlated with higher aqueous Hg concentrations, lower areal pelagic primary productivity, and an increased contribution of terrestrial particles to pelagic particulate organic matter. Based on dietary marker analysis (δ¹³C, δ¹⁵N, and fatty acid [FA] composition), zooplankton diet was strongly linked to feeding mechanism, with dietary reliance on phytoplankton highest in the selective-feeding calanoid copepods, and lowest in filter feeding cladocerans. Zooplankton dietary reliance on phytoplankton and their concentrations of high-quality lipids, including polyunsaturated fatty acids, were higher in the clear-water lake than in the brown-water lake, where bacterial and terrestrial food sources were more prevalent. MeHg was highest in zooplankton from the brown-water lake, with highest concentrations in the 200-500 μm zooplankton size fraction for both lakes. Contrary to our expectations, there was no effect of season on zooplankton dietary markers or MeHg. Our results suggest that, overall, higher tOM results in higher MeHg concentrations in water and zooplankton, and reduces zooplankton dietary reliance on phytoplankton. Increased tOM thus leads to a decrease in the nutritional quality of zooplankton (i.e. higher MeHg concentrations, and lower concentrations of essential fatty acids), which may cascade up the food web with negative implications for higher trophic levels.