Effects of warming on the ecology of algal-dominated phytobenthic communities in the Baltic Sea
Abstract: Through climate change, the global average air and surface water temperature has risen 0.85°C during the last 100 years. The Baltic Sea experienced one of the most rapid increase in temperature recorded of marine ecosystems. During the last two decades of the 20th century, the surface water temperature of the Baltic Sea has increased seven times faster than the global average.This thesis is an investigation of how community traits, trophic interactions and ecophysiological processes in the filamentous algal belt in the northern Baltic Sea are affected by warming. The majority of the studies were conducted in or in the vicinity of the Forsmark Biotest basin, an artificial heated enclosure of the southern Bothnian Sea (northern Baltic Sea). One study also included sampling along a natural salinity gradient - the Swedish east coast.In the benthic diatom community, we found that cell size decreases with decreasing seasonal temperature, and increased with warming during the cold season. Warming also selected for motile and colonial traits. Along the salinity gradient, cell size decreased with decreasing salinity, apparently mediated by changes in the nitrogen to phosphorus ratio.In the filamentous algal community, warming increased algal cover and photosynthetic capacity, and affected the ratio of carotene to chlorophyll a ratio. Warming also desensitized the photosynthetic response and growth of algal communities exposed to anthropogenic stressors: increased nitrogen and phosphor concentrations as well as copper additions. In connection to one of the field studies, the first finding of the non-native bivalve Mytilopsis leucophaeata in Sweden was also done.Using a fish exclusion experiment in heated and non-heated areas, we found that warming decreases the number of trophic levels in the lower parts of the food web, which in turn lead to increased top-down control and higher algal biomass at heated than unheated sites.In summary, warming has here been shown to have major impacts on the phytobenthic community due to a combination of direct effects on physiological processes, as well as indirect effects mediated by interactions among species and trophic levels.
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