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Since water quality monitoring can be rather costly, it is important to properly design the monitoring network so that maximum information can be received with moderate effort. One component of monitoring is measuring the phytoplankton community composition and detecting whether that has changed. From April to October 2012, by using ships-of-opportunity (SOOP), we collected simultaneous samples from 15 stations in the Gulf of Finland and the Baltic Proper to study the spatial variability in the phytoplankton species composition and biomass throughout the growing season. The analysis was performed with 10 dominant taxa constituting 83–97% of the total phytoplankton biomass. We set the criterion of the statistically significant (p

Estonian, Finnish and Swedish institutes carrying out national HELCOM Baltic Sea monitoring programmes participated in a field inter-calibration for parameters describing eutrophication. The main o ...

The response of nutrient and chlorophyll fields to climate change by the end of the twenty-first century was evaluated in the Gulf of Finland (Baltic Sea) using comparison of a hindcast simulation for 1997–2006 and future climate forcing, assuming an A1B greenhouse gas emission scenario and business-as-usual riverine nutrient load for 2090–2099. The comparison of simulated oxygen, phosphate and nitrate levels from the hindcast model with the measurements indicated a good performance of the 3D ecosystem model, except for overestimated near-bottom layer nitrates. The mean chlorophyll level was slightly overestimated by the model, whereas the variability in the surface layer chlorophyll level was well reproduced. Future projection simulations indicate no considerable changes in the upper layer oxygen concentrations compared with the hindcast simulation and observations, but deeper near-bottom layers were projected to become anoxic, causing an increase in phosphate and a decrease in nitrate concentrations in these layers. The increase in surface layer phosphate and the decrease in nitrate concentrations lead to an increase in summer cyanobacteria blooms and an increase in nitrogen fixation, which therefore led to an increase in the annual mean chlorophyll content in the upper layer.

Assessments of the environmental status of the Baltic Sea as called for by the Marine Strategy Framework Directive (MSFD) must be based on a set of indicators. A pre-core indicator is the diatom/dinoflagellate index (Dia/Dino index), which reflects the dominance of diatoms or dinoflagellates during the phytoplankton spring bloom. Here we explain the principles of the Dia/Dino index and the conditions for its calculation using examples from two very different water bodies, the Eastern Gotland Basin and Kiel Bay. The index is based on seasonal mean diatom and dinoflagellate biomass values. A precondition for its applicability is the coverage of the bloom. As a criterion, the maximum value of diatom or dinoflagellate biomass has to exceed a predefined threshold, e.g., 1000 μg/L in the investigated areas. If this condition is not fulfilled, an alternative Dia/Dino index can be calculated based on silicate consumption data. 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. If diatoms are dominant, their rapid sinking reduces the food stock for zooplankton but delivers plenty of food 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 (Descriptor 5 of MSFD: “eutrophication”). The Dia/Dino index was able to identify the regime shift that occurred at the end of the 1980s in the Baltic Proper. Diatom dominance, and thus a high Dia/Dino index, are typical in historical data and are therefore assumed to reflect good environmental status (GES). In assessments of the environmental status of the Eastern Gotland Basin and Kiel Bay, Dia/Dino index GES thresholds of 0.5 and 0.75, respectively, are suggested. The GES thresholds as calculated by the alternative Dia/Dino index are 0.84 and 0.94, respectively.

There are very few time series documenting clear trends of change in the biomass of total phytoplankton or single taxa that coincide with trends of increasing nutrient concentrations. Weekly or biweekly monitoring since 1997 on a cross section of the central Gulf of Finland (NE Baltic Sea) with similar climatic and hydrographic conditions, but different nutrient levels, provided a uniform dataset. In order to evaluate seasonal (June–September) patterns of phytoplankton succession, more than 1,200 samples were statistically analyzed by selecting 12 dominant taxa using wet weight biomass values. In addition, the continuously measured hydrographic parameters on board the ships of opportunity, and simultaneous nutrient analyses gave high frequency information on the water masses. The objective of this study was to identify the taxa that may prove indicative in the assessment of eutrophication in the appropriate monitoring time periods. None of the most common bloom-forming species (Aphanizomenon sp., Nodularia spumigena, and Heterocapsa triquetra) showed reliable correlations with enhanced nutrient concentrations. The species we suggest as reliable eutrophication indicators—oscillatorialean cyanobacteria and the diatoms Cyclotella choctawhatcheeana and Cylindrotheca closterium—showed the best relationships with total phosphorus concentrations. Their maxima appear toward the end of July or in August–September when phytoplankton community structure is more stable, and less frequent observations may give adequate results. Another diatom, Skeletonema costatum, exhibited stronger correlations with dissolved inorganic and total nitrogen in June, during the period of the summer phytoplankton minimum.

The spatially heterogeneous character of aquatic life and rapid changes in pelagic communities make it difficult to assess the status of the marine environment using traditional monitoring methods. In order to distinguish between human induced and natural changes in the ecosystem the environmental parameters and the factors affecting them need to be monitored at a wide range of temporal and spatial scales. The capacity of any single monitoring method or strategy should not be overestimated. Instead, different methods complement each other. In the Baltic Sea high frequency recordings of phytoplankton biomass and related environmental parameters (temperature, salinity, nutrients, chlorophyll a) in the near surface layer (4–5m) have been conducted with unattended water sampling onboard several commercial ferries over the last 14 years. The collected data have been used for delivery of almost on-line information for environmental authorities and public, for developing HELCOM indicator fact-sheets and indexes/maps characterising inter-annual and short-term changes of productivity and phytoplankton biomass in different sea areas, and for forecasts of algae blooms. The combination of high frequency automated sampling onboard merchant ships with satellite imagery, traditional sampling and meteorological information has increased the understanding of ecological processes in the Baltic Sea.