| Summary: | Shallow-water sediment systems are continuously exposed to a range of anthropogenic stressors, including increased nutrient loading, physical disturbance and toxicants. Superimposed on these local stressors are stressors related to the on-going climate change. Interacting stressors may strengthen or weaken effects of individual stressors, resulting in – often unexpected – non-additive effects. Thus, the combined effect from several simultaneous stressors in shallow-water sediment systems is the main topic of this thesis. Individual and combined stressor effects were studied on intact sediment cores placed in an outdoor flow-through facility and in the laboratory. Both functional (primary and bacterial production, community respiration, meiofaunal grazing, denitrification, nitrogen mineralization and sediment-water fluxes of oxygen and nutrients) and structural (biomass and composition of microphytobenthos and meiofauna) were studied in order to assess effects of multiple stressors. It has been suggested that global warming can shift the trophic status of ecosystems from net autotrophy to net heterotrophy. A spring experiment (Paper I) showed, however, that the presence and activity of a well-developed microphytobenthic mat already in early spring sustained net autotrophy during simulated warming. The effects of increased temperature on the structure and function of the sediment systems were generally rather moderate. Consequently, it is possible that the presence and activity of microphytobenthos can buffer changes induced by seawater temperature. An autumn experiment (Paper II) suggested that warming combined with nutrient enrichment can induce shorter, intensive, heterotrophic periods that can be followed by longer autotrophic periods because initially increased mineralization induces a shortage of labile organic matter. Even though warming increased nutrient availability through increased mineralization, warming did not exacerbate the stimulatory effects of nutrient enrichment. The lack of interactive ...
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