Sirpa Lehtinen

Climate Change and Eutrophication Induced Shifts in Northern Summer Plankton Communities

Marine ecosystems are undergoing substantial changes due to human-induced pressures. Analysis of long-term data series is a valuable tool for understanding naturally and anthropogenically induced changes in plankton communities. In the present study, seasonal monitoring data were collected in three sub-basins of the northern Baltic Sea between 1979 and 2011 and statistically analysed for trends and interactions between surface water hydrography, inorganic nutrient concentrations and phyto- and zooplankton community composition. The most conspicuous hydrographic change was a significant increase in late summer surface water temperatures over the study period. In addition, salinity decreased and dissolved inorganic nutrient concentrations increased in some basins. Based on redundancy analysis (RDA), warming was the key environmental factor explaining the observed changes in plankton communities: the general increase in total phytoplankton biomass, Cyanophyceae, Prymnesiophyceae and Chrysophyceae, and decrease in Cryptophyceae throughout the study area, as well as increase in rotifers and decrease in total zooplankton, cladoceran and copepod abundances in some basins. We conclude that the plankton communities in the Baltic Sea have shifted towards a food web structure with smaller sized organisms, leading to decreased energy available for grazing zooplankton and planktivorous fish. The shift is most probably due to complex interactions between warming, eutrophication and increased top-down pressure due to overexploitation of resources, and the resulting trophic cascades.

Sources of uncertainty in assessment of marine phytoplankton communities

Characterisation of phytoplankton communities is important for classification of the ecological status of marine waters. In order to design a monitoring programme, it is important to know what degree of variation in the measurements occur at each level (water body, station and sample), so that resources can be spent in a way that maximise the precision of the measured parameters. Seven European water bodies were sampled to assess the variation in pigment concentrations and population densities attributed to water body, station and sample levels. It was found that the main proportion of the variation between pigment measurements was explained by the variation between stations (12–91% of variation) followed by the variation between water bodies (0–89% of variation). For measurements of population density, the main proportion of the variation between densities of cells recorded was explained by the variation between the taxonomists counting the samples (61%), whilst the main proportion of the variation between numbers of taxa recorded was explained by the variation between water bodies (83%). When the cell density of the nine dominant classes were analysed separately, the main proportion of variation was explained at the water body level for all but two class.

The Diatom/Dinoflagellate Index as an Indicator of Ecosystem Changes in the Baltic Sea 1. Principle and Handling Instruction

An ecosystem shift in the Baltic Proper at the end of the 1980s could be identified by a decreasing springtime diatom/dinoflagellate ratio. Therefore the suggested diatom/dinoflagellate index (Dia/Dino index) was developed as a descriptive indicator of ecosystem changes for applications pertaining to the Marine Strategy Framework Directive (MSFD). Changes in the dominance of these two phytoplankton classes impact the food web because both their quality as a food source for grazers and their periods of occurrence differ, which may lead to a mismatch in zooplankton growth. The rapid sinking of diatoms results in a high Dia/Dino index, indicative of low-level food availability for zooplankton but high-level food delivery to the zoobenthos. Consequently, the Dia/Dino index can be used to follow the food pathway (Descriptor 4 of MSFD: “food web”). Moreover, a low Dia/Dino index may indicate silicate limitation caused by eutrophication, whereas a high Dia/Dino index supports mitigated eutrophication, as the strong sedimentation of nutrients removes them from the water and deposits them in the sediment (Descriptor 5 of MSFD: “eutrophication”). Diatom dominance, and thus a high Dia/Dino index, is typical in historical data and is therefore assumed to reflect good environmental status (GES). The principles of the Dia/Dino index are explained herein using examples from two very different regimes, the Eastern Gotland Basin and Kiel Bay. In the assessment of the environmental status of these water bodies, GES boundaries of 0.5 and 0.75, respectively, are suggested. The conditions for calculating the Dia/Dino index are described and the limitations and advantages of this indicator are discussed.

The pelagic food web

1. Environmental drivers and food web structure in the pelagic zone vary from south to north in the Baltic Sea. 2. While nitrogen is generally the limiting nutrient for primary production in the Baltic Sea, phosphorus is the limiting nutrient in the Bothnian Bay. 3. In the Gulf of Bothnia the food web is to a large extent driven by terrestrial allochthonous material, while autochthonous production dominates in the other parts of the Baltic Sea. 4. Changes in bacterioplankton, protist and zooplankton community composition from south to north are mainly driven by salinity. 5. Bacteria are crucial constituents of the pelagic food web (microbial loop) and in oxygen-poor and anoxic bottom waters where they mediate element transformations. 6. Diatoms and dinoflagellates are the major primary producers in the pelagic zone. Summer blooms of diazotrophic (nitrogen-fixing) filamentous cyanobacteria are typical of the Baltic Sea, especially in the Baltic Sea proper and the Gulf of Finland. 7. The mesozooplankton (mainly copepods and cladocerans) channel energy from primary producers and the microbial food web to fish and finally to the top predators in the pelagic system (waterbirds and mammals). 8. Herring and sprat populations are affected by the foraging intensity of their main predator (cod), and therefore the environmental conditions that affect cod may also influence mesozooplankton due to food web effects “cascading down the food web”. 9. Anthropogenic pressures, such as overexploitation of fish stocks, eutrophication, climate change, introduction of non-indigenous species and contamination of top predators by hazardous substances, cause changes in the pelagic food web that may have consequences for the balance and stability of the whole ecosystem.

Approach for Supporting Food Web Assessments with Multi-Decadal Phytoplankton Community Analyses—Case Baltic Sea

Combining the existing knowledge on links between functional characteristics of phytoplankton taxa and food web functioning with the methods from long-term data analysis, we present an approach for using phytoplankton monitoring data to draw conclusions on potential effects of phytoplankton taxonomic composition on the next trophic level. This information can be used as a part of marine food web assessments required by the Marine Strategy Framework Directive of the European Union. In this approach, both contemporary taxonomic composition and recent trends of changes are used to assess their potential consequences for food web functioning. The approach consists of four steps: (1) long-term trend analysis of class-level and total phytoplankton biomass using generalized additive models (GAMs) and calculating average biomass share of each phytoplankton class from the total phytoplankton biomass, (2) comparing the current phytoplankton community composition and its long-term changes with non-metric ordination analysis (NMDS) of genus-level biomass, (3) describing which taxa (the most accurate taxonomic level) are primarily responsible for forming the biomass and for causing the possible changes, and (4) interpretation of the phytoplankton results to assess the potential effects on the next trophic level. Within step 4, special attention is given to the following characteristic of taxa: potential suitability or quality as food for grazers, harmfulness, size, and trophy. These characteristics are selected based on existing scientific knowledge on their relevance to the higher trophic levels. In this article, we present the concept of the suggested approach and demonstrate the phytoplankton analyses with multi-decadal monitoring data from the northern Baltic Sea. We also discuss the future development of the approach towards a food web index by combining or replacing the taxonomic analyses with functional trait-based approaches.