Saskia A. Otto

Early Detection of Ecosystem Regime Shifts: A Multiple Method Evaluation for Management Application

Critical transitions between alternative stable states have been shown to occur across an array of complex systems. While our ability to identify abrupt regime shifts in natural ecosystems has improved, detection of potential early-warning signals previous to such shifts is still very limited. Using real monitoring data of a key ecosystem component, we here apply multiple early-warning indicators in order to assess their ability to forewarn a major ecosystem regime shift in the Central Baltic Sea. We show that some indicators and methods can result in clear early-warning signals, while other methods may have limited utility in ecosystem-based management as they show no or weak potential for early-warning. We therefore propose a multiple method approach for early detection of ecosystem regime shifts in monitoring data that may be useful in informing timely management actions in the face of ecosystem change.

Ecological Network Indicators of Ecosystem Status and Change in the Baltic Sea

Several marine ecosystems under anthropogenic pressure have experienced shifts from one ecological state to another. In the central Baltic Sea, the regime shift of the 1980s has been associated with food-web reorganization and redirection of energy flow pathways. These long-term dynamics from 1974 to 2006 have been simulated here using a food-web model forced by climate and fishing. Ecological network analysis was performed to calculate indices of ecosystem change. The model replicated the regime shift. The analyses of indicators suggested that the system’s resilience was higher prior to 1988 and lower thereafter. The ecosystem topology also changed from a web-like structure to a linearized food-web.

Synchronous marine pelagic regime shifts in the Northern Hemisphere

Regime shifts are characterized by sudden, substantial and temporally persistent changes in the state of an ecosystem. They involve major biological modifications and often have important implications for exploited living resources. In this study, we examine whether regime shifts observed in 11 marine systems from two oceans and three regional seas in the Northern Hemisphere (NH) are synchronous, applying the same methodology to all. We primarily infer marine pelagic regime shifts from abrupt shifts in zooplankton assemblages, with the exception of the East Pacific where ecosystem changes are inferred from fish. Our analyses provide evidence for quasi-synchronicity of marine pelagic regime shifts both within and between ocean basins, although these shifts lie embedded within considerable regional variability at both year-to-year and lower-frequency time scales. In particular, a regime shift was detected in the late 1980s in many studied marine regions, although the exact year of the observed shift varied somewhat from one basin to another. Another regime shift was also identified in the mid- to late 1970s but concerned less marine regions. We subsequently analyse the main biological signals in relation to changes in NH temperature and pressure anomalies. The results suggest that the main factor synchronizing regime shifts on large scales is NH temperature; however, changes in atmospheric circulation also appear important. We propose that this quasi-synchronous shift could represent the variably lagged biological response in each ecosystem to a large-scale, NH change of the climatic system, involving both an increase in NH temperature and a strongly positive phase of the Arctic Oscillation. Further investigation is needed to determine the relative roles of changes in temperature and atmospheric pressure patterns and their resultant teleconnections in synchronizing regime shifts at large scales.

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.

Zooming in on size distribution patterns underlying species coexistence in Baltic Sea phytoplankton.

Scale is a key to determining which processes drive community structure. We analyse size distributions of phytoplankton to determine time scales at which we can observe either fixed environmental characteristics underlying communities structure or competition-driven size distributions. Using multiple statistical tests, we characterise size distributions of phytoplankton from 20-year time series in two sites of the Baltic Sea. At large temporal scales (5-20 years), size distributions are unimodal, indicating that fundamental barriers to existence are here subtler than in other systems. Frequency distributions of the average size of the species weighted by biovolume are multimodal over large time scales, although this is the product of often unimodal short-term (<1 year) patterns. Our study represents a much-needed structured, high-resolution analysis of phytoplankton size distributions, revealing that short-term analyses are necessary to determine if, and how, competition shapes them. Our results provide a stepping-stone on which to further investigate the intricacies of competition and coexistence.

Community ecology in 3D: Tensor decomposition reveals spatio-temporal dynamics of large ecological communities

Understanding spatio-temporal dynamics of biotic communities containing large numbers of species is crucial to guide ecosystem management and conservation efforts. However, traditional approaches usually focus on studying community dynamics either in space or in time, often failing to fully account for interlinked spatio-temporal changes. In this study, we demonstrate and promote the use of tensor decomposition for disentangling spatio-temporal community dynamics in long-term monitoring data. Tensor decomposition builds on traditional multivariate statistics (e.g. Principal Component Analysis) but extends it to multiple dimensions. This extension allows for the synchronized study of multiple ecological variables measured repeatedly in time and space. We applied this comprehensive approach to explore the spatio-temporal dynamics of 65 demersal fish species in the North Sea, a marine ecosystem strongly altered by human activities and climate change. Our case study demonstrates how tensor decomposition can successfully (i) characterize the main spatio-temporal patterns and trends in species abundances, (ii) identify sub-communities of species that share similar spatial distribution and temporal dynamics, and (iii) reveal external drivers of change. Our results revealed a strong spatial structure in fish assemblages persistent over time and linked to differences in depth, primary production and seasonality. Furthermore, we simultaneously characterized important temporal distribution changes related to the low frequency temperature variability inherent in the Atlantic Multidecadal Oscillation. Finally, we identified six major sub-communities composed of species sharing similar spatial distribution patterns and temporal dynamics. Our case study demonstrates the application and benefits of using tensor decomposition for studying complex community data sets usually derived from large-scale monitoring programs.

Winter–spring climate effects on small-sized copepods in the coastal Baltic Sea

The general positive effect of warmer winters on the abundance of small-sized zooplankton in the following spring and early summer has been reported from different parts of the Baltic Sea, but the mechanism of this link is not clear. Although causal links cannot be deduced with confidence from observational data, sufficiently detailed analyses can nevertheless provide insights to the potential mechanisms. We present an example of such an analysis, scrutinizing the effects of winter and spring hydroclimate on the abundance of small-sized dominant calanoid copepods (Eurytemora affinis and Acartia spp.), using data from 2080 zooplankton samples collected over 55 years (1957–2012) from a shallow coastal habitat (Parnu Bay, Gulf of Riga) in the Baltic Sea. Our results indicated that the milder winters brought about higher abundances, and reduced seasonality of small-sized copepods, whereas ambient sea surface temperature (SST) mostly affected the relative abundance of adult stages. The sliding window correlation tests revealed temporal shifts in the effects of controlling variables: with the continuous increase in SST, the effect of winter temperature on the abundance of Acartia spp. weakened. In contrast, E. affinis was consistently affected by SST, but the effect of winter temperature was more pronounced during the period of on average colder winters.

Pelagic habitat: exploring the concept of good environmental status

Acknowledgements This food for thought contribution was initiated by theme session J “What is a good pelagic habitat?” at the 2016 ICES annual science conference http://www.ices.dk/news-and-events/asc/ASC2016/Pages/Theme-session-J.aspx. All who took part in the session are thanked for their contributions. All participants were welcome to contribute to this article. Funding J. F. Tweddle was supported by the Natural Environment Research Council [NERC grant reference number NE/P005756/1]. E. Bresnan was supported by the Scottish Government’s schedules of service ST02a and ST03r. A. McQuatters-Gollop was supported by the Natural Environment Research Council [NERC grant reference number NE/L002663/1].

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.