Retrospective analysis of changes to biodiversity and ecosystem functioning: Report on responses of biodiversity indicators (species, communities, traits) to past abiotic variables and relationships between biodiversity and ecosystem functioning in the Baltic Sea. BIOSC3 Deliverable, D4.1. EU BONUS BIOSC3

Executive summary of BIO-C3 WP4, Task 4.1 

The overall purpose of Task 4.1 was to assess how biodiversity (species, communities, traits) respond to (past) abiotic variables and how the relationships between biodiversity and ecosystem functioning in the Baltic Sea may have changed over time and/or in response to environmental change over time (from species to their functions to the community-level functioning).

We have accomplished our goals by showing that long-term changes can be quantified already at the lowest level, the primary producers (work 1, 2, 3 and 9 in iv)) throughout the trophic network up to fish (work 7,8,9,10 in iv)). However, the signals may vary depending on season, and for example we show that phytoplankton blooms earlier in spring, by 1-2 weeks over the last 20 years, but total summer phytoplankton biomass variation over time, across the Baltic Sea, could not be well-explained by a common temporal trend among stations with little additional explanatory power from regional climate variables. We have targeted both the pelagic and the benthic food chain and its functioning by investigating temporal changes in pelagically feeding fish (work 7 in iv)) and long-term changes in community and populations of zooplankton (work 4, 5 in iv)) as well as functional changes in coastal benthic macrofauna and fish (work 6, 9 in iv)) and the offshore benthic feeding fish community (work 8 in iv)). We have related these changes to the natural salinity gradient (work 8, 14 in iv)) as well as large scale anthropogenic and climatic drivers (work 2,5,9 in in iv)). Concerning the temporal aspects, many studies have been able to capture the suggested regime shift periods in the Baltic Sea (1980s-90s) or investigate the resulting products of it.

Our intention within this task, as outlined in the plan, was to bring in functional traits and trait-based approaches as a novel tool to study and understand changes in ecosystem structure and function. We have done this successfully by evaluating long-term trends and changes in benthic macrofauna (work 6 in iv)) where we show that functional identity of communities is as essential to investigate as more common functional measures such as functional richness. We also include functional trait measures to elucidate that long-term functional changes of coastal macorfauna and fish, is showing trends rather than abrupt shifts despite turnover of taxa (work 10 in iv)). Trait-based measures were also applied to investigate the community assembly of demersal fish, and highlighted an east-west difference in drivers of fish community composition (work 8 in iv)). In addition, we also linked characteristics of species to their functioning, for example in order to predict primary production based on macrophyte functional traits (work 4 in iv)). Using physiological, life-history, behavioral and morphological information about individuals and species have thus proven to be an essential way forward for understanding functional changes in the Baltic Sea ecosystem and we recommend bringing such knowledge into future advice and management actions for the region. Nevertheless, we have also evaluated the performance of traditional benthic indices over time (work 11, 12, 13 in iv)). We show that these measures provide a good understanding of the long-term progression of one of the most macrofaunal data-rich areas in the Baltic Sea, the Polish coast (work 11 in iv)), and a fair response to both eutrophication and sediment dumping pressure (work 12, 13 in iv)), althoguh the assessment (particularly, set-up of the sensitivity values) should be adjusted for the ecosystem in question and be habitat- specific. Furthermore we compared the performance of six alternative modelling approachesfor obtaining reliable spatial information about fish diversity indices (work 14 in iv)). Within Task 4.1 we also evaluated the functional response of non-indigenous species on the benthic community over time (work 6 in iv)) and for the performance of benthic indices (12 in iv)).

Accurate assessments of ecological changes that have occurred in the past is a prerequisite for predictive analysis and rely generally, as for our studies in Task 4.1, on national and/or regionally gathered monitoring data. Therefore, we further recommend to stakeholders such as HELCOM, ICES, as well as national and regional environmental bodies to make sure that coordination of sampling programs across trophic levels are in place. The difficulty in finding comparable data sets with several trophic or organismal groups sampled within an ecologically relevant area (i.e. where interactions can take place) was evident in many activities within this task.

Our results and knowledge gained so far directly link to other tasks within BIO-C3, especially Task 4.2 and 4.3 (changes in food web and future predictions) as well as WP5 (the role of indicators). Future steps include the realizations of these links and transfer of our gained knowledge, but also a joint publication summarizing the outcomes within this task, planned to be ready for submission within the final year of the project.