Theses

The strongly lake-type-specific balance between primary production and respiration determines whether a lake acts regionally as a net sink or source of CO2. In the global change context even small changes in photosynthesis and respiration rates and/or in the loadings of organic matter or nutrients (e.g. phosphorus) may change the metabolic type of the lake and turn it from autotrophic to heterotrophic system or vice versa. Present thesis summarizes for Lake Võrtsjärv: the first data on seasonal changes of the ecosystem metabolism based on high frequency measurements; the reconstruction of long term series of phytoplankton primary production by bio-optical modelling; the incoming and outgoing fluxes of carbon, nitrogen, phosphorus, and silicon; and gives an overview of phosphorus retention in Lake Võrtsjärv compared with 54 lakes and reservoirs in different climate regions around the world.

Mercury (Hg) and bioaccumulation of its organic form methylmercury (MeHg) has long been regarded as a global issue for human and wildlife health. This is of particular concern for Sweden, where forest harvests and wetlands remain hotspots of MeHg export to boreal freshwaters. The aim of the study was to better understand how environmental change impacts MeHg bioavailability and bioaccumulation with a focus on the base of the food web in Swedish freshwaters. In the thesis, I first examined the impacts of forest harvest on Hg bioaccumulation in fish. From analysis of more than a thousand fish samples from six lakes, it was clear that there was a large variation of fish Hg levels over time and between lakes. The study emphasizes the need for long-term studies to assess the influences on Hg levels in biota. Beaver ponds were also studied to understand the processes influencing MeHg concentrations in water. Increased MeHg production in the sediment was stimulated shortly after flooding by new beaver dams, and that stimulation was related to the quality of the organic carbon. To explore the basis for large discrepancies between aqueous MeHg exposure and the ultimate degree of MeHg bioaccumulation in different aquatic ecosystems, a literature review was conducted on MeHg bioaccumulation from water into the base of the food web. The bioconcentration factor for MeHg, that is the ratio of MeHg concentrations in water and seston or zooplankton, was found to be critical for subsequent MeHg bioaccumulation. To better identify linkages between MeHg uptake and different nutrient sources at the base of the food web, fatty acids were used to distinguish specific diet sources in aquatic primary consumers of plankton and macroinvertebrates. Correlation of Hg bioaccumulation with specific fatty acids confirmed that Hg content increased simultaneously with the retention of polyunsaturated fatty acids in primary consumers. These findings contribute to a better focus on the role of the base of aquatic food webs in transferring MeHg upwards along trophic cascades, as well as landscape influences on elevated MeHg bioaccumulation. Further advances in understanding how the structures at the base of aquatic food webs function with respect to mercury cycling are needed to better predict how changes in the landscape, climate and water chemistry will alter Hg bioaccumulation in fish.

Assessing the importance of macroinvertebrate trophic dead ends in the lower transfer of methylmercury in littoral food webs

Seasonal phytoplankton community patterns and affecting factors in fish-mussel systems

Planktonic community; Thermal stratification; Vertical mixing

Compositional change of FGs is expected to follow longitudinal processes in rivers, and also better indicate ecological status than other systems based on taxa composition or species richness. In order to prove the reliability of FGs in rivers, their compositional changes are related to natural gradients like geography, as well as to physical and chemical anthropogenic pressures, occurring along the Loire. Major findings of the Thesis are the followings: (i) Different lentic phytoplankton functional approaches are able to identify reliable river zones along the Loire based on FG composition of potamoplankton. The level of understanding provided, however, depends on the taxonomical and ecological resolution of approaches. In rivers, no satisfactory water quality management can be built without fine resolution of benthic and planktonic diatoms, as well as of cyanobacterial taxa. (ii) The FG classification is able to delimit natural and human-mediated changes in the potamoplankton composition along the River Loire. These changes can be described by the Q(r) composition index, which successfully indicates local morphological alternations like damming, or regional scale differences in nutrient availability according to land use practices. (iii) Both the German PhytoFluss index based on taxa-level resolution and the Hungarian HRP index based on FGs provide comparable and reliable water quality indications along the River Loire. In case of adequate implementation of indices according to site-specific river typology, these national potamoplankton assessments can be successfully compared or implemented at international level, as also required by the European Water Framework Directive. (iv) FG classification of potamoplankton displays similar river zonation in each year along the River Loire. Besides quantitative differences, major FGs and the subdominant accessory FGs all follow similar longitudinal trends, while further FGs were identified to reflect specific annual hydrological regimes. (v) In the River Loire, ecosystem functioning (as the biomass:TP ratio) displayed different relationships with taxa and FG richness. The highest potamoplankton richness was identified as a consequence of physically mixed habitats from either natural or humanmediated sources, while the best functioning occurred at low taxa number and at medium FG richness. Accordingly, high richness/diversity in large rivers might not provide automatic evidence for better ecosystem functioning, and cannot be a general objective without understanding its functional properties and controlling factors at different scales.