Seaweed and herbivory in acidified ocean

Rising CO2 mediated ocean acidification (OA) together with climate change and global warming alters the availability of carbon to the marine environment and lead to large variations in carbon-nutrient-ratios in marine macrophytes. This high atmospheric CO2 partly sequestered by global ocean will probably enrich coastal seaweeds with carbon but suppress nutrient availability to herbivores. Nutritional quality of food for mesograzers will alter in response to the changes of high carbon-nutrient ratio in marine seaweeds. These processes potentially influence marine benthic ecosystems on all hierarchical levels from cells to communities. Intensive research has been made on the impact of OA on marine biota in order to predict future implications for food web dynamics and other ecosystem processes. My M.Sc-thesis addresses (1) the direct effects of OA on the growth and metabolism of the coastal seaweed, Fucus vesiculosus and (2) functional responses of the dominant herbivore, Idotea baltica, to the potentially altered nutritional quality of the seaweed. In laboratory perturbation experiments, fragments of F. vesiculosus were exposed for 4 weeks to two different CO2 scenarios. Experimental CO2 levels of 280 ppm (pre-industrial atmospheric level) and 700 ppm (IPCC projection for the year 2100) respectively, were adjusted by incubation of seawater with pre-mixed gases. Biomass of F. vesiculosus was monitored throughout the experiments. Nutritional quality was measured by analyzing C, N, P and ash content of the F. vesiculosus tissues. Herbivore responses were measured in terms of ingestion rates, assimilation efficiency and respiration. Results indicate that growth of F. vesiculosus is negatively affected under acidified conditions. Elemental composition of algae remains similar in both pCO2 manipulations. Consumption rates, assimilation efficiency and respiration did not differ significantly between I. baltica that has been feeding on algal fragments from different CO2 treatments. From the experimental evidence it can be conclude that, reduced growth of primary producers along with unchanged grazing rates under higher CO2 condition might negatively affect seaweed populations with potential implications for entire benthic communities. The direct effects of OA on marine macroalgae might not propagate through higher trophic levels.

ISCED Categories

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0511 - Biology", "0521 - Ecology