PhD ceremony Ms. K. Reiss: Marine trophic cascades. Synergistic effects of fishing and eutrophication

PhD ceremony: Ms. K. Reiss, 11.00 uur, Academiegebouw, Broerstraat 5, Groningen

Dissertation: Marine trophic cascades. Synergistic effects of fishing and eutrophication

Promotor(s): prof. J.L. Olsen

Faculty: Mathematics and Natural Sciences

With respect to marine ecosystem management, the results of Katrin Reiss imply that changes in the food web through fishing and from eutrophication cannot be managed separately but have to be considered in conjunction with each other. A decline of top-predators (e.g. through overexploitation) can cascade down the food web and induce an excessive algal growth. Nutrient enrichment is an important factor for enabling and enhancing such cascading food web effects. Specifically, the composition of herbivores seems to play a key role for the transmission of top-down (from decline of predators) and bottom-up effects (through nutrient enrichment).

Humans make use of ecosystem goods and services in various ways. Nowadays, particularly the seas are being exploited and polluted with largely unknown consequences. However, an understanding of the human imprint on marine ecosystems is necessary in order to preserve their goods and services, such as for nutrient cycling, food production, and recreational use. Specifically, two anthropogenic pressures, fishing and eutrophication, have large direct but also indirect effects, e.g. through the food web. While fishing usually removes large predatory fish from the top-end of the food web, high nutrient loads fuel the primary production from the bottom-end of the food web.

The central aim of the thesis of Reiss was to investigate the interrelationship between the removal of top-predators and nutrient enrichment on a coastal benthic food web. She shows that the removal of top-predators induced a trophic cascade, which eventually led to an increased algal growth. However, she also shows that omnivores can potentially buffer such cascading effects. When multiple predators were assembled together in a community, their combined food web effects strongly depended on their identity and density, and the emerging interference. Furthermore, her experiments showed that ecosystem productivity plays a crucial role for the propagation of food web effects.

Thus, the thesis of Reiss shows that anthropogenic impacts on single components of an ecosystem (e.g. through fishing or nutrient pollution) can have complex and far-reaching consequences for the entire system. Specifically, the herbivore composition is hypothesised as a key link for propagating food web effects.