Long-term information on possible changes in cyanobacterial blooms in the Baltic Sea, formed mainly by Nodularia spumigena and Aphanizomenon sp., was sought in published records in historical (years 1887–1938) and modern (years 1974–1998) phytoplankton data sets. Old and new sampling methods and fixatives were tested to improve the comparison of data that had been collected and analyzed in different ways. A hundred years ago, plankton was mainly of interest as a source of fish food; eutrophication problems were only locally reported from the coast, mainly in southern haffs and the receiving waters of larger cities. There were few recordings of open-sea blooms before World War II. Abundances of Nodularia spumigena and Aphanizomenon sp. were low in the old material, and 137 summer samples from 1887–1938 showed no peak abundance. High abundances are common in the new material, and the range of the numbers of both taxa has increased markedly relative to the old material. Since the 1960s, cyanobacterial blooms have been common in the open sea in both the Baltic proper and the Gulf of Finland, indicating high availability of nutrients.

The distributions of carotenoid pigments were studied in recent and postglacial sediments collected in the Gotland Basin, in the northern part of the Baltic proper, and in the eastern Gulf of Finland in May 1999. The aim was to provide a systematic, historical review of the occurrence and intensity of cyanobacterial blooms in the Baltic Sea in recent centuries. The presence of pigments was determined using high-pressure liquid chromatography (HPLC), with attention mainly focused on pigments considered as markers for cyanobacteria. The pigment concentrations in sediments from the 3 sampling locations were found to differ markedly. In general, concentrations were highest in sediments from the Gotland Basin, and lowest in sediments from the eastern Gulf of Finland. In all 3 cores echinenone was the dominant carotenoid in the topmost layer of the sediment (0–1 cm). In the deeper sections of the cores, myxoxanthophyll and zeaxanthin dominated. To our knowledge, this is first time that myxoxanthophyll and echinenone have been detected in Baltic Sea sediments from the early Litorina stage. The decrease in the pigment content with sediment depth coincides with a decrease in carbon content, and also fits in with general historical records of the occurrence and intensity of cyanobacterial blooms in the Baltic Sea, which show that cyanobacterial blooms were seldom recorded before World War II.

Understanding the mechanisms that govern biological diversity in various environments is one of the greatest challenges for the scientific community of today. Compared to terrestrial and benthic habitats, mechanisms regulating species diversity in planktonic ecosystems have been dealt with relatively little. This is mainly due to the scarcity of the experimental evidence from field studies where the multitude of spatiotemporal scales have been covered with sufficient resolution. This paper discusses the pecularities of the aquatic system as a growth environment for phyto-plankton in comparison to terrestrial/connected systems. Examples of the regulation of species diversity in the Baltic Sea are presented.

During the 1990s nitrogen reserves have shown rising trends in the northern Baltic Sea. Changes in denitrification explain some of the fluctuations observed in nitrogen reserves. Although denitrification is an anaerobic process, the most efficient removal of nitrogen by denitrification occurs where the sediment is moderately well oxidized. A dramatic decrease in the ratio of SiO₄ to DIN (DIN = NO₃ NH₄) in the northern Baltic proper during the period 1973–1999 was recorded. If dissolved silica limits phytoplankton spring bloom, diatom blooms fade and become replaced by flagellates leading to changes in summer phosphate reserves and sedimentation. Seven years have elapsed from the previous strong saltwater inflow and anoxia has spread over large areas. Deep bottom phosphorus levels have started to increase and denitrification capacity is weakening. Thus, all efforts to minimize N- and P-loading will, in the long run, help the Baltic Sea to recover from the unacceptable status of eutrophication.

Despite a decrease of about 30% in the external nutrient inputs, no corresponding improvement has been observed in the state of the open Gulf of Finland. At the end of the 1990s the external nutrient load to the Gulf totalled 120 000 tonnes (t) yr⁻¹ of nitrogen (N) and 7000 t yr⁻¹ of phosphorus (P). Relative to its surface area, the nutrient load of the Gulf is 2 to 3 times the average of the Baltic Sea. Despite the decrease in loading, an increase in the phosphate-P concentration was observed both in the surface and near-bottom layers around the mid-1990s. The reason for this development was most probably the acceleration of internal loading, triggered by poor oxygen conditions at the sediment–water interface of the eastern Gulf, where the oxygen conditions weakened during the 1990s, after being relatively good in the 1980s and the early 1990s. On the basis of experimental data from the coastal Gulf of Finland, the phosphate-P flux from the reduced surface sediment to water averaged 13 kg km⁻² d⁻¹. This corresponds to total amounts which can explain the observed trends of P in the open Gulf. The low N:P ratio of the sediment efflux can partly explain the N limitation of primary production in the Gulf.

The biogeographical peculiarities of the Baltic Sea have developed since the last glacial period. The characteristic mixture of marine, brackish water, and freshwater species, and relicts from previous periods in the Baltic, is threatened by ongoing environmental changes. This review focuses on the recent impacts of nonindigenous species, eutrophication, and a temporary oxygen deficit in the deep basins, on the biogeographical integrity of the Baltic on different spatial and time scales. Today the biota of brackish waterbodies are exposed to each other because of the breakdown in geographical barriers due to shipping traffic, leading to an exchange of species and further homogenization of aquatic animal and plant life worldwide.

In the Baltic Sea ecosystem reproductive disorders have occurred in top consumers such as seals and some fish-eating birds, due to biomagnification of toxic substances, e.g. DDT and PCBs. Reproductive disturbances have also affected fish during the last 25 years. However, there is no strong evidence that toxic substances have caused these problems. Rather, the disorders seem to result from a combination of two or more biotic or abiotic factors. The M74 syndrome, which kills fry of salmon and sea trout, is characterized by a deficiency of thiamine (vitamin B₁). Several factors may contribute to the thiamine deficiency, including the diet of salmon in the sea and halogenated organic compounds. Cod do not develop M74, and poor cod recruitment is mainly due to poor oxygen conditions in the spawning areas in combination with overfishing. Toxic compounds in pulp-mill effluents retard gonadal development in perch, but the mechanisms and the active substances have not been identified. Recruitment problems in perch in the coastal waters outside some pulp mills may also be explained by a lack of food items for juvenile fish, rather than reproductive failure. There are very limited data on reproductive disorders in crustaceans from the Baltic Sea. Most data come from studies of the benthic amphipod Monoporeia affinis, which has been used in monitoring programs. Several signs of reproductive disorder have been reported in this amphipod, e.g. malformation and death of embryos, and asynchronous maturation of males and females.

Human-induced environmental changes, especially eutrophication, and fluctuations of natural environmental conditions under the changes in climate and solar activity affect the reproduction and growth of various fish species and communities in the Baltic Sea. The importance of human impacts has increased considerably during the last decades, overshadowing in many cases natural factors. High exploitation rates have depressed some valuable fish species and affected the species dominance hierarchy. Oxygen depletion in deeper layers progressively deteriorates the living conditions of certain marine species. Mass immigration of nonindigenous species with ballast waters can seriously affect ecosystems and fish stocks. The summary effect of these variables upon marine, relict and freshwater species can yield unexpected results. Fish resources should be properly assessed and managed by their natural units (populations). Based on cyclic fluctuations of global climatic processes, composition of long-term forecasts on changes in the structure and abundance of fish fauna should be started.

Grave environmental problems, including contamination of biota by organochlorines and heavy metals, and increasing deep-water oxygen deficiency, were discovered in the Baltic Sea in the late 1960s. Toxic pollutants, including the newly discovered PCB, were initially seen as the main threat to the Baltic ecosystem, and the impaired reproduction found in Baltic seals and white-tailed eagles implied a threat also to human fish eaters. Countermeasures gradually gave results, and today the struggle to limit toxic pollution of the Baltic is an international environmental success story. Calculations showed that Baltic deep-water oxygen consumption must have increased, and that the Baltic nutrient load had grown about fourfold for nitrogen and 8 times for phosphorus. Evidence of increased organic production at all trophic levels in the ecosystem gradually accumulated. Phosphorus was first thought to limit Baltic primary production, but measurements soon showed that nitrogen is generally limiting in the open Baltic proper, except for nitrogen-fixing cyanobacteria. Today, the debate is concerned with whether phosphorus, by limiting nitrogen-fixers, can control open-sea ecosystem production, even where phytoplankton is clearly nitrogen limited. The Baltic lesson teaches us that our views of newly discovered environmental problems undergo repeated changes, and that it may take decades for scientists to agree on their causes. Once society decides on countermeasures, it may take decades for them to become effective, and for nature to recover. Thus, environmental management decisions can hardly wait for scientific certainty. We should therefore view environmental management decisions as experiments, to be monitored, learned from, and then modified as needed.

The dynamics of marine ecosystems, i.e. the changes of observable chemical-biological quantities in space and time, are driven by biological and physical processes. Predictions of future developments of marine systems need a theoretical framework, i.e. models, solidly based on research and understanding of the different processes involved. The natural way to describe marine systems theoretically seems to be the embedding of chemical-biological models into circulation models. However, while circulation models are relatively advanced the quantitative theoretical description of chemical-biological processes lags behind. This paper discusses some of the approaches and problems in the development of consistent theories and indicates the beneficial potential of the coupling of marine biology and oceanography in models.

Global climate change is expected to have an effect on the physical and ecological characteristics of the Baltic Sea. Estimates of future climate on the regional scale can be obtained by using either statistical or dynamical downscaling methods of global AOGCM scenario results. In this paper, we use 2 different coupled ice-ocean models of the Baltic Sea to simulate present and future ice conditions around 100 years from present. Two 10-year time slice experiments have been performed using the results of atmospheric climate model simulations as forcing, one representing pre-industrial climate conditions (control simulation), and the other global warming with a 150% increase in CO₂ greenhouse gas concentration (scenario simulation). Present-day climatological ice conditions and interannual variability are realistically reproduced by the models. The simulated range of the maximum annual ice extent in the Baltic in both models together is 180 to 420 · 10³ km² in the control simulation and 45 to 270 · 10³ km² in the scenario simulation.

The range of the maximum annual ice thickness is from 32 to 96 cm and from 11 to 60 cm in the control and scenario simulations, respectively. In contrast to earlier estimates, sea ice is still formed every winter in the Northern Bothnian Bay and in the most Eastern parts of the Gulf of Finland. Overall, the simulated changes of quantities such as ice extent and ice thickness, as well as their interannual variations are relatively similar in both models, which is remarkable, because the 2 coupled ice-ocean model systems have been developed independently. This increases the reliability of future projections of ice conditions in the Baltic Sea.

We study the Baltic Sea countries' declaration to reduce nutrient loads by 50% in each country in an ecological-economic model. The model consists of country-based abatement-cost functions, and transfer coefficients describing how phosphorus and nitrogen flow from one country to another, as estimated in a hydrological model of the Baltic Sea. We show that for nitrogen in particular the overall abatement costs of the current policy are much higher and that the benefits are more uneven than under a cost-efficient policy. Consequently, one can expect that countries with high marginal abatement costs have the least incentives to follow the agreement and to invest in nitrogen abatement. This is also confirmed by our data. Therefore, we suggest and outline a joint implementation policy to promote cost efficiency and to increase incentives for investments.

The Baltic Sea is one of the world's seas that is most severely affected by human activities. Although there is an international agreement that nutrient input to the Baltic should be reduced, the measures taken so far have not resulted in major reductions in nutrient inputs nor in environmental improvements. The reasons for this are partly due to lack of knowledge on large-scale relationships and couplings between physics, biogeochemistry and ecological properties. But there is also a lack of overall drainage basin-wide analyses on cost-effective measures. There is a danger in making the wrong decisions, e.g. implement reduction schemes that are at worst ineffective or at best, far from cost effective. Researchers from many disciplines are faced with a common challenge: To develop a decision-support system, which can be used as the scientific base for cost-effective measures for the entire Baltic Sea. Such an effort is now being made within the research program MARE ( http://www.mare.su.se).

The aim of this study is to present a historical continuum of nearly 100 years of environmental investigations and changes in trophic levels in the sea area of Helsinki. The survey is based on a re-examination of original planktological and meteorological data; the principle being that eutrophication can be detected in plankton assemblages despite changes in methodology. The bays around Helsinki were found to be moderately eutrophic to hypereutrophic at the beginning of the 20^th century. There were considerable differences in the development of the bays both in terms of degradation and recovery. The efficiency of wastewater treatment and especially the introduction of sea outfall have played an important role in decreasing eutrophication in the bay area of Helsinki.

Vaasa, located on the Gulf of Bothnia, is representative of medium-sized cities on the coast of the Baltic Sea. This article discusses Vaasa's impact on the surrounding sea area and the city's reactions to the pollution of the sea. The history of wastewater treatment in Vaasa strongly suggests that first-generation environmental problems, e.g. the pollution problems caused by municipal wastewater discharges, were solved only as a last resort after a prolonged development process. The first wastewater treatment plant was completed in 1953. In the long run, municipal policies became more costly for the environment and for the town itself than would have been the case if the option of constructing a central treatment plant had been accepted in the first instance. In Vaasa, the environment itself did not provide the incentive, the change was motivated by health risks, noxious odors and poor outdoor-bathing possibilities that resulted from municipal wastewater discharges. No action was taken until ultimately forced by necessity. This article also discusses social science approaches to environmental studies.

From the historical perspective, the increasing number of inhabitants in the city of Stockholm has had a negative effect on water quality in the surrounding lakes and coastal region. Government control and measures taken to improve water quality have been in progress since the mid-19^th century. Water conditions in the 19^th century, compared to later years is difficult to assess from the first chemical and biological investigations, due to infrequent sampling and the different parameters and methods used. However, a retrospective evaluation of water quality can be made on the basis of results from plankton investigations, which began in the early 20^th century. The occurrence of the cyanobacteria Planktothrix agardhii, which indicates nutrient-rich conditions, was surveyed during summers with similar temperature conditions throughout the 20^th century. The results show that eutrophied conditions probably prevailed in the Stockholm archipelago from the beginning of 20^th century until the early 1990s. In the last decade of the 20^th century, water quality appeared to be better than 100 years earlier. Today's better water conditions are most probably an effect of proper measures taken in wastewater treatment.

The main source of pollution in the inner Oslofjord in the 20^th century has been municipal sewage discharges from the city of Oslo. At the beginning of the 20^th century, pollution was limited to the coastal waters and the harbor area of Oslo, in the vicinity of sewer outlets. High bacteria content caused a health hazard that city authorities attempted to reduce by constructing a sewerage system, including intercepting sewers and wastewater-treatment plants. Due to population growth, the impact area of increasing wastewater loading expanded. The entire inner Oslofjord was found to be affected in the 1930s. Scientific studies linked municipal sewage discharges to an increase in the algal production. In the 1940s, the bottom layers were found to be anoxic. The Oslo sewerage authorities were aware of the fjord's pollution, but regarded organic matter as the major problem and the activated sludge method as the best solution. The role of nutrients was not commonly acknowledged until in the late 1960s. Phosphorus removal was taken into use in the 1970s, and nitrogen removal was introduced in the late 1990s. Removal of nutrients has resulted in the slow recovery of the fjord.

This article focuses on the emergence of water pollution problems in the city of Malmö and on how they were discussed and solved. Various countermeasures were proposed and considered, but finally the discharge of wastewater into the Sound of Öresund was regarded as the best solution. As a result, a local pollution problem was transformed into a regional issue. The solutions adopted in Malmö are briefly compared with decisions made in Gothenburg and Stockholm.

This study provides an overview on the phyto- and zoo-plankton studies made of the Neva Bay and of the changes that have taken place in the plankton community during the 20^th century. It is known that plankton respond to changes in water quality, especially to changes in the content of organic matter. Comparisons of the literature data reveal the value of species as indicators of different conditions. Many plankton species in the Neva Bay show great year-to-year fluctuations due to varying environmental conditions. According to the results of studies conducted in 1911–1990, freshwater plankton species have dominated in the Neva Bay. The plankton community has essentially consisted of species transported from Lake Ladoga. Brackish-water species occasionally occurred in the plankton samples of the Neva Bay. High turbidity and rapid throughflow of water limit algal production in the bay and, due to the high outflow, the europhication effects of the catchment area are mainly conducted into the Gulf of Finland.

The Lithuanian water-management system developed on the basis of Soviet regulations in 1950–1990. Surface-water quality monitoring started in the 1950s, and the system was improved in the 1960s. Today, 48 rivers are being monitored using up to 70 parameters. Statutory monitoring of discharges started in 1962, wastewater standards were issued in 1957 and 1966, and then revised in 1996. Wastewater-treatment plants were built first in rural areas, in factories since the 1950s, and later in towns. Since 1991, large capacity municipal plants have been constructed with foreign assistance. Water quality has improved in some rivers since 1970, but Lithuania's main river, Nemunas, remains moderately polluted. The lower Nemunas is especially affected by discharges of municipal and industrial wastewater from Sovietsk and Neman (Russia), which account for half of the total loading. Hydrobiological data of 1994–1998 indicated the eutrophication of the Curonian Lagoon, and bacteriological pollution and blue-green algae blooms in the Baltic Sea north of Klaipeda.

Long-term changes in the environmental quality of water in Latvia (chemical composition of inland waters, waste-water treatment, and drinking-water treatment practices and quality) as a response to socioeconomic changes have been studied. Water composition, the major factors influencing water chemistry, and human impacts (waste-water loading) were studied to determine changes that occurred after recent reductions in pollution emissions, particularly nutrient loading, to surface waters. After 1991, (Latvia regained independence in 1991) inland water quality has begun to improve mainly as a result of decreases in nutrient loads from point and nonpoint sources and substantial efforts in the area of environmental protection. The situation differs, however, for drinking-water treatment, where practices have also changed during the whole period from 1980 till 2000. More stringent drinking-water-quality standards and novel insights regarding changes in water quality in the distribution network, necessitate further improvements in public water supply, and place this particular water issue among Latvia's main priorities.

Local Agenda 21 (LA21) processes have 2 central goals. i) On the basis of some of the empirical evidence in this study, the primary goal is to improve democratic (environmental) policy-making processes in such a manner that a larger share of the population will be able to participate in planning and decision making and will also be able to understand the consequences of these decisions. ii) The LA21 processes seek to improve (at least indirectly) the broadly defined environmental situation locally in a manner that takes into account both the local and the global contexts. The first part of this article discusses the concept and methods of LA21 and sheds light on the different action areas that are central to the Baltic LA21 processes. In addition, the study will describe and display the LA21 situation within one network of cities, the Union of the Baltic Cities (UBC). Networking, including transfer of information, models and ideas, has been among the main tools for the diffusion of LA21 ideas especially into newly democratized societies. Finally, the article will conclude with an overall assessment of the LA21 situation on the Baltic rim.