Rabea Diekmann

Marine Ecosystem Regime Shifts Induced by Climate and Overfishing: A Review for the Northern Hemisphere

Abstract Abrupt and rapid shifts in food web and community structure, commonly termed regime shifts, have been increasingly reported for exploited marine ecosystems around the world. Here, we present a review on regime shifts in Northern hemisphere marine ecosystems, most of them using a multivariate approach to statistically analyse time series. We show that rapid shifts occurred in synchrony during the late 1980s/early 1990s, suggesting a common large-scale climate driver and essentially matching times of change in the North Atlantic Oscillation and other atmospheric indices, which modified, for example, the local temperature regimes. We further show that trophic cascades triggered by overfishing and causing a switch of trophic regulation are regularly involved in ecosystem reorganizations. Eutrophication and the introduction of alien species can be important as well, potentially affecting tipping points or the food web structure. Our review highlights how multiple drivers potentially interact in a way that one driver undermines resilience (e.g. overfishing) and the other (e.g. climate change) gives the final impulse for an abrupt change. Further, ecosystem regime shifts can be particularly difficult to reverse when alternative stable states are involved. Understanding the drivers and mechanisms leading to regime shifts is crucial for developing ecosystem-based management strategies and establishing early-warning systems to avoid catastrophic ecosystem changes and achieve a sustainable exploitation of ecosystem services.

Habitat Heterogeneity Determines Climate Impact on Zooplankton Community Structure and Dynamics

Understanding and predicting species distribution in space and time and consequently community structure and dynamics is an important issue in ecology, and particularly in climate change research. A crucial factor determining the composition and dynamics of animal populations is habitat heterogeneity, i.e., the number of structural elements in a given locality. In the marine pelagic environment habitat heterogeneity is represented by the distribution of physical oceanographic parameters such as temperature, salinity and oxygen that are closely linked to atmospheric conditions. Little attention has been given, however, to the role of habitat heterogeneity in modulating the response of animal communities to external climate forcing. Here we investigate the long-term dynamics of Acartia spp., Temora longicornis, and Pseudocalanus acuspes, three dominant zooplankton species inhabiting different pelagic habitats in the Central Baltic Sea (CBS). We use the three copepods as indicator species for changes in the CBS zooplankton community and apply non-linear statistical modeling techniques to compare spatial population trends and to identify their drivers. We demonstrate that effects of climate variability and change depend strongly on species-specific habitat utilization, being more direct and pronounced at the upper water layer. We propose that the differential functional response to climate-related drivers in relation to strong habitat segregation is due to alterations of the species’ environmental niches. We stress the importance of understanding how anticipated climate change will affect ecological niches and habitats in order to project spatio-temporal changes in species abundance and distribution.

Towards Integrated Ecosystem Assessments (IEAs) of the Baltic Sea: Investigating Ecosystem State and Historical Development

Integrated Ecosystem Assessments are an integral part of ecosystem-based management approaches and aim at describing the ecosystem and its temporal development as an entity, including environmental and anthropogenic drivers and ecosystem responses. Here, we present ecological principles and some theoretical background how ecosystems respond to changing environmental conditions and provide definitions for regime shifts. Further, general guidelines how ecosystems can be investigated in a holistic way are given and the necessary statistical background is explained. In the appendix, hands-on support is provided and the reader can reproduce some of the analytical methods by codes using R and other freely available software. Corresponding to the proposed methods, the ecosystem state and historical development of the Central Baltic Sea covering the period from 1974 to 2007 was analysed. We identified regime shifts in the pelagic ecosystem affecting multiple trophic levels during the late 1980s and early 1990s. The two regimes encompassing the years 1974–1987 and 1994–2007 were characterised by the opposite dominance of key fish and zooplankton species.