Minor cause, major effect: Interactions in ecosystems can intensify impact of climate change

The study is published in the current issue of the journal Advances in Marine Biology.

Animals and plants living in coastal areas are tough organisms. For example, the inhabitants of the shallow Baltic Sea have to cope with fluctuating temperatures, with variable salinity and even with short term pH value changes. "In a few weeks, these natural fluctuations can exceed the average shifts that are predicted for the next century due to (global) climate change," Professor Martin Wahl, Marine Biologist at the Leibniz Institute of Marine Sciences (IFM-GEOMAR) in Kiel (Germany) explains. Is global change therefore negligible for coastal ecosystems? Professor Wahl, Dr. Inken Kruse and Dr. Mark Lenz from IFM-GEOMAR, together with 14 colleagues from European and American scientific institutions, answer this question with a definite "No." "Even the smallest variations in an ecosystem can have major consequences due to ecological intensification, says Professor Wahl." "On the other hand, stressors can even buffer each other in certain cases."

The authors from Germany, Finland, the Netherlands, the USA, Portugal, and Sweden explain the often surprising interactions between pressures due to abiotic stresses (e.g. warming) on the one hand and biotioc stresses (e.g. herbivores, parasites) on the other hand with the help of the stress ecology of macro algae. One of these macroalgae, the seaweed Fucus vesiculosus, commonly known as bladderwrack, inhabits the coasts of the North Sea and the Baltic Sea as well as the coast of the Atlantic and Pacific Oceans. "There it plays a key role in the ecosystems of shallow waters," Martin Wahl explains. "Even though the seaweed of the Baltic Sea should be accustomed to difficult (harsh) living conditions, its populations have diminished considerably in the last decades. „Actually, Fucus vesiculosus can live in depths of up to six meters. However, we find it now only in depths of up to two meters at the Western Baltic Sea," says Professor Wahl.

This shift cannot be explained solely by the direct effects of global change, which include eutrophication and bioinvasions. To understand this shift nevertheless, the authors of the recent study collected all available information about the bladderwrack from earlier studies, which e.g. included data on its distribution, data on light availability and nutrients, grazers, defense strategies, reactions to environmental pollution or the genetic variety of algae populations. "Thus we've been able to show a cascade of effects and interactions, which influence single algae or whole populations," Wahl says. To give an example: Shading weakens energy reserves leading to weakened antifouling and anti-grazing defenses, which in turn increases fouling and grazing pressure. Both reduce the photosynthetic area of the thallus, amplifying the energy shortage under low light conditions. Low light and temperature-stress reduce growth, jeopardizing the alga's ability to compensate for tissue loss to grazers, the activity of which is augmented by temperature stress. "The list of possible intensifications is long and complex," Martin Wahl explains. To enhance the understanding of it the stress ecology of macroalgaeis currently being modelled.

The results of the new study are exemplary for ecosystems in coastal areas and shelf seas in general. "Hardly any species will perish because of a single effect of climate change, but we can't ignore it either," Wahl says summing up the results of the new study. He hopes for a new perception of the problem "because we still know far too little about the snowball effect which ecological intensification can cause."