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The coastal zone of the Baltic Sea is diverse with strong regional differences in the physico-chemical setting. This diversity is also reflected in the importance of different biogeochemical processes altering nutrient and organic matter fluxes on the passage from land to sea. This review investigates the most important processes for removal of nutrients and organic matter, and the factors that regulate the efficiency of the coastal filter. Nitrogen removal through denitrification is high in lagoons receiving large inputs of nitrate and organic matter. Phosphorus burial is high in archipelagos with substantial sedimentation, but the stability of different burial forms varies across the Baltic Sea. Organic matter processes are tightly linked to the nitrogen and phosphorus cycles. Moreover, these processes are strongly modulated depending on composition of vegetation and fauna. Managing coastal ecosystems to improve the effectiveness of the coastal filter can reduce eutrophication in the open Baltic Sea.

Much of the Baltic Sea is currently classified as 'affected by eutrophication'. The causes for this are twofold. First, current levels of nutrient inputs (nitrogen and phosphorus) from human activities exceed the natural processing capacity with an accumulation of nutrients in the Baltic Sea over the last 50-100 years. Secondly, the Baltic Sea is naturally susceptible to nutrient enrichment due to a combination of long retention times and stratification restricting ventilation of deep waters. Here, based on a unique data set collated from research activities and long-term monitoring programs, we report on the temporal and spatial trends of eutrophication status for the open Baltic Sea over a 112-year period using the HELCOM Eutrophication Assessment Tool (HEAT 3.0). Further, we analyse variation in the confidence of the eutrophication status assessment based on a systematic quantitative approach using coefficients of variation in the observations. The classifications in our assessment indicate that the first signs of eutrophication emerged in the mid-1950s and the central parts of the Baltic Sea changed from being unaffected by eutrophication to being affected. We document improvements in eutrophication status that are direct consequences of long-term efforts to reduce the inputs of nutrients. The reductions in both nitrogen and phosphorus loads have led to large-scale alleviation of eutrophication and to a healthier Baltic Sea. Reduced confidence in our assessment is seen more recently due to reductions in the scope of monitoring programs. Our study sets a baseline for implementation of the ecosystem-based management strategies and policies currently in place including the EU Marine Strategy Framework Directives and the HELCOM Baltic Sea Action Plan., (© 2015 The Authors. Biological Reviews published by John Wiley & Sons Ltd on behalf of Cambridge Philosophical Society.)

We analysed long-term monitoring data from 1979 to 2012 in the Gulf of Bothnia, the northernmost extension of the Baltic Sea, to examine changes in the summer food web structure, and to reveal the factors causing the observed changes. Of the two sub-basins in the Gulf of Bothnia, the Bothnian Sea is more dynamic in its hydrography and food web structure than the Bothnian Bay, due to the variable influence of the more eutrophic and more saline Baltic Proper. Variation in deep-water intrusion from the main Baltic Proper, and its effect on salinity and stratification, had a clear effect on the phyto- and zooplankton communities in the Bothnian Sea. Probably due to this intrusion, the nutrient status in this basin has also changed in terms of nitrogen limitation, with subsequent class- and genus-level changes in phytoplankton community composition. The migration of cod to the Bothnian Sea during the 1980s had profound effects on the herring population, but cascading effects affecting the basis of the food web were not obvious. In contrast to a more pronounced interplay of both top-down control and bottom-up nutrient limitation of the Bothnian Sea, the Bothnian Bay food web was mostly driven by hydrography and climate, and major changes were observed in the basis of the food web. Community changes were observed in both basins in the Gulf of Bothnia throughout the entire time period. Human influence considerably affects both the basis of the food web and its very top, where man has replaced the natural top predators. Though the eutrophication status of the Gulf has been rather stable, the results indicate that the potential symptoms should be surveyed specifically in the Bothnian Sea, and that management of Baltic herring stocks requires an understanding, and thus monitoring, of the entire food web. [ABSTRACT FROM AUTHOR]

This study follows up on a previous assessment of eutrophication status in the Baltic Sea, which covered the period 2001–2006. The updated assessment is based on new eutrophication targets, an improved eutrophication assessment tool (HEAT 3.0) as well as monitoring data for the period 2007–2011. Based on classifications of eutrophication status in all Baltic Sea sub-basins, we reveal that during the assessment period 2007–2011, the entire open Baltic Sea was affected by eutrophication. This is a different conclusion compared to earlier assessments and studies. Whilst the confidence of the assessment was high or moderate in most basins, there were indications of declining confidence in some assessment units and improved confidence in others. The problems in confidence were mainly related to scarcity of in situ monitoring data on chlorophyll- a and Secchi depth. The potential implications of our results, e.g. the expansion of the eutrophic zone and declining confidence in the classifications of eutrophication status, are discussed in relation to the existing Baltic Sea-wide nutrient management strategy as well as future assessment activities. [ABSTRACT FROM AUTHOR]

Uusitalo, L., Fleming-Lehtinen, V., Hällfors, H., Jaanus A., Hällfors, S., and London, L. 2013. A novel approach for estimating phytoplankton biodiversity – ICES Journal of Marine Science, 70: 408–417.Maintaining biodiversity is one of the main priorities in environmental protection. The biodiversity of phytoplankton, the key primary producers in the marine ecosystem, is, however, often very difficult to estimate, since the phytoplankton assemblage includes a vast number of taxa, many of which occur in such small quantities that they may not be recorded in routine sampling. Moreover, many taxa cannot be identified to species level by routine methods such as light microscopy of preserved samples, even by a skilled taxonomist. This means that, in general, we cannot assume to have a complete list of species in the ecosystem at any given point in time. In this paper, we present an approach for evaluating phytoplankton biodiversity in spite of this challenge. Since eutrophication, which increases phytoplankton biomass, has been identified as the most important factor causing degradation of the Baltic Sea ecosystem, the proposed approach was evaluated against total phytoplankton biomass. When phytoplankton biomass was low, both low and high biodiversity values were observed, and, as the phytoplankton biomass increased, the high biodiversity values disappeared. These results were consistent both using data based on individual samples and using yearly sampling station averages. [ABSTRACT FROM PUBLISHER]

Abstract: Secchi depth, as a proxy of water transparency, provides valuable information on the availability of light to the underwater ecosystems. Changes in water transparency have also been widely linked to eutrophication and phytoplankton biomass. This study aimed to describe the development of water transparency in the Baltic Sea through a unique century-long set of Secchi depth observations. Furthermore, the aim was to explain the role of phytoplankton in determining water transparency in these optically complex waters. Water transparency in the open Baltic Sea has decreased during the last one hundred years. The development differs between the sub-basins of the Baltic Sea. The decrease has been most profound in the north-eastern Baltic sub-basins, but apparent also in parts of the Southern and Central Baltic. In many of the northern areas the decrease has accelerated during the last decades, whereas in the Southern Baltic a recent increase was observed. The analysis of simultaneous chlorophyll a observations during the period from 1972 to 2006 revealed that during summer time, 13–17% of the light attenuation can be attributed to phytoplankton. In spring, the average proportion is between 31 and 42%, with great variation between observations. We find Secchi depth a suitable indicator of eutrophication, integrating various organic matter related features. It should always be applied with sufficient background information of the optical properties of the water mass, and complemented by other indicators. [Copyright &y& Elsevier]