Baltic Sea Ecosystem Research Articles

Quantifying Contemporary Organic Carbon Stocks of the Baltic Sea Ecosystem

The identification of carbon pools and the quantification of carbon stocks is necessary to (1) track changes in ecosystem dynamics, (2) inform science-based ecosystem and blue-carbon management, and (3) evaluate ecosystem and food web models. However, estimates of organic carbon stocks in marine ecosystems are incomplete or inconsistent. Therefore, we provide a first consistent estimate of relevant organic carbon stocks of a distinct marine ecosystem- the Baltic Sea. We estimate its contemporary standing stocks of 18 non-living and living organic carbon pools using data from literature and open-access databases. In contrast to existing data, our estimates are valid for the entire Baltic Sea, include necessary pools and are verifiable, as we describe data sources, methods and the associated uncertainties in detail to allow reproduction and critical evaluation. The total organic carbon (TOC) in the Baltic Sea ecosystem amounts to 1,050 ± 90 gC/m2 (440 ± 40 Mt). The non-living stocks account for about 98.8% and the living stocks for 1.2% of the TOC. Our estimates indicate that benthos has the highest living organic carbon stock and that the stock of particulate organic carbon (POC) has been underestimated in some previous studies. In addition, we find a partially inverted biomass distribution with a higher stock of primary consumers than primary producers. Our estimates provide a baseline of the size and distribution of the organic carbon in the Baltic Sea for the current period. Analyses of inorganic carbon stocks and the interplay between inorganic and organic stocks must follow to further define the baseline of total carbon stocks in the Baltic Sea.

On the uncertainty and confidence in decision support tools (DSTs) with insights from the Baltic Sea ecosystem.

Ecosystems around the world are increasingly exposed to multiple, often interacting human activities, leading to pressures and possibly environmental state changes. Decision support tools (DSTs) can assist environmental managers and policy makers to evaluate the current status of ecosystems (i.e. assessment tools) and the consequences of alternative policies or management scenarios (i.e. planning tools) to make the best possible decision based on prevailing knowledge and uncertainties. However, to be confident in DST outcomes it is imperative that known sources of uncertainty such as sampling and measurement error, model structure, and parameter use are quantified, documented, and addressed throughout the DST set-up, calibration, and validation processes. Here we provide a brief overview of the main sources of uncertainty and methods currently available to quantify uncertainty in DST input and output. We then review 42 existing DSTs that were designed to manage anthropogenic pressures in the Baltic Sea to summarise how and what sources of uncertainties were addressed within planning and assessment tools. Based on our findings, we recommend future DST development to adhere to good modelling practise principles, and to better document and communicate uncertainty among stakeholders.

Reconstructing N<sub>2</sub>-fixing cyanobacterial blooms in the Baltic Sea beyond observations using 6- and 7-methylheptadecane in sediments as specific biomarkers

Abstract. Summer cyanobacterial blooms represent a threat to the Baltic Sea ecosystem, causing deoxygenation of the bottom water and the spread of the so-called dead zones. The history of the Baltic Sea cyanobacterial blooms is known from in situ and satellite observations since the early 1980s but is still not well understood. By comparing both weekly resolved sediment trap material and a well-dated sediment core from the eastern Gotland Basin with monitoring and satellite cyanobacterial data of the last ca. 35 years, it is shown here that 6- and 7-methylheptadecane lipids (expressed as 6+7Me-C17 : 0) may be potentially considered semiquantitative biomarkers for diazotrophic cyanobacteria, and more specifically for Nodularia spumigena. Using this organic proxy, it was thus possible to reconstruct the history of cyanobacterial blooms beyond the observational period with a resolution of 2–4 years since 1860. Cyanobacteria were constantly present but in relatively low abundance until 1920, when they started to alternate between periods with high and low abundance. Interestingly, there seems to be no significant increase in cyanobacterial abundance in the 1950s, when eutrophication and deoxygenation of the Baltic Sea increased considerably. While the early increase in cyanobacteria may be related to a small increase in phosphorus loading, decadal to multi-decadal fluctuations are likely related to variability in the Baltic Sea surface temperature and, ultimately, to the Atlantic Multi-decadal Oscillation. A 7000-year 6+7Me-C17 : 0 record from the Bothnian Sea also suggests a relationship with the mean summer temperature in the Baltic Sea region but at a multi-centennial to multi-millennial timescale. The intensity of the cyanobacterial blooms in the Baltic Sea is thus likely mainly related to natural processes such as temperature variability, at least at a multi-decadal to multi-millennial timescale.

What evidence exists for the impact of Baltic Sea ecosystems on human health and well-being? A systematic map protocol

BackgroundThe Baltic Sea ecosystems supply many benefits to society, termed ecosystem services. These depend upon a healthy marine environment requiring marine and relevant land-based policies integrated with public health policies. Until recently marine environment protection policies have largely focussed on human impacts on the environment and have not taken into account impacts of ecosystems on human health beyond the direct impacts of hazardous substances, such as those present in seafood. Whilst endeavours have been made to integrate human health and well-being into marine policies, interviews with key stakeholders through a participatory process revealed that the linkages were not sufficiently strong to inform policymaking. The existing evidence base urgently needs to be identified and synthesised to support relevant policy updates of the Marine Strategy Framework Directive (MSFD) 2008/56/EC and the Baltic Sea Action Plan (BSAP) (2007) as well as to help direct future research priorities.MethodThe protocol is based on the primary question, “What linkages have been researched between Baltic Sea ecosystems and the positive and negative impacts to human health and well-being?” Using systematic mapping, this study will identify and map the state and the geographical distribution of the existing research evidence linking human health and well-being with the Baltic Sea ecosystems. The types of ecosystem services supplied by the Baltic Sea and the associated health and well-being impacts will be categorised and presented in a graphical matrix, illustrating ecosystem service type and the types of health and well-being outcomes. The systematic mapping procedure will result in a narrative report published with a searchable database, which will contain a descriptive summary of the information from all of the eligible studies. The systematic map and database will be displayed on the website of the Finnish Environment Institute (SYKE).

Deficiency syndromes in top predators associated with large-scale changes in the Baltic Sea ecosystem.

Vitamin B1 (thiamin) deficiency is an issue periodically affecting a wide range of taxa worldwide. In aquatic pelagic systems, thiamin is mainly produced by bacteria and phytoplankton and is transferred to fish and birds via zooplankton, but there is no general consensus on when or why this transfer is disrupted. We focus on the occurrence in salmon (Salmo salar) of a thiamin deficiency syndrome (M74), the incidence of which is highly correlated among populations derived from different spawning rivers. Here, we show that M74 in salmon is associated with certain large-scale abiotic changes in the main common feeding area of salmon in the southern Baltic Sea. Years with high M74 incidence were characterized by stagnant periods with relatively low salinity and phosphate and silicate concentrations but high total nitrogen. Consequently, there were major changes in phytoplankton and zooplankton, with, e.g., increased abundances of Cryptophyceae, Dinophyceae, Diatomophyceae and Euglenophyceae and Acartia spp. during high M74 incidence years. The prey fish communities also had increased stocks of both herring and sprat in these years. Overall, this suggests important changes in the entire food web structure and nutritional pathways in the common feeding period during high M74 incidence years. Previous research has emphasized the importance of the abundance of planktivorous fish for the occurrence of M74. By using this 27-year time series, we expand this analysis to the entire ecosystem and discuss potential mechanisms inducing thiamin deficiency in salmon.

The potential to foster governance learning in the Baltic Sea Region: Network governance of the European Union Strategy for the Baltic Sea Region

AbstractThe Baltic Sea ecosystem is displaying symptoms of extreme stress from multiple sources. These complex and “wicked” problems require cooperation by actors for multidisciplinary pooling of knowledge to completely understand the problem and adaptively learn in order to develop appropriate policy strategies. Inherent in the design of the European Union Strategy for the Baltic Sea Region (EUSBSR) is the promise to foster cooperation by key BSR network governance actors, connecting them to develop joint policy arrangements needed for governing the challenges of the Baltic Sea. The EUSBSR is the key knowledge broker in the region and has the capacity to augment BSR network governance of the Baltic Sea. Despite the importance of knowledge transmission and policy learning to the EUSBSR, there has been no formal study of it in the literature. This paper aims to make a theoretical contribution to the literature by evaluating the latent potential of the EUSBSR to foster policy learning across BSR governance networks. It does this through document analysis focussing on the interaction platforms of the EUSBSR that interconnects BSR governance networks across policy sectors.

Expedition studies of the Baltic sea ecosystem in the 34th and 36th cruises of the research vessel "Academic Nikolay Strakhov"

In the 34th and 36th cruises of the r/v Akademik N. Strakhov, performed in the summer and autumn of 2017, the structure of the waters was studied on transects located along the route of inflow of saline North Sea waters into the Baltic Sea. Vertical profiling was performed with intervals between stations no more than 2 miles, in the free-fall mode, from the sea surface to the bottom. Reliable data on a thin structure at all depths, including a thin bottom layer, were obtained. In addition to the sections, new data were obtained on the bottom topography, as well as on the influence of advection of saline aerated waters on the composition, structure and distribution of biological communities in the Baltic Sea.

Food web and fisheries in the future Baltic Sea

We developed numerical simulations of potential future ecological states of the Baltic Sea ecosystem at the end of century under five scenarios. We used a spatial food web (Ecospace) model, forced by a physical–biogeochemical model. The scenarios are built on consistent storylines that describe plausible developments of climatic and socioeconomic factors in the Baltic Sea region. Modelled species diversity and fish catches are driven by climate- and nutrient load-related changes in habitat quality and by fisheries management strategies. Our results suggest that a scenario including low greenhouse gas concentrations and nutrient pollution and ecologically focused fisheries management results in high biodiversity and catch value. On the other hand, scenarios envisioning increasing societal inequality or economic growth based on fossil fuels, high greenhouse gas emissions and high nutrient loads result in decreased habitat quality and diminished biodiversity. Under the latter scenarios catches are high but they predominantly consist of lower-valued fish.

Towards end-to-end (E2E) modelling in a consistent NPZD-F modelling framework (ECOSMO E2E_v1.0): application to the North Sea and Baltic Sea

Abstract. Coupled physical–biological models usually resolve only parts of the trophic food chain; hence, they run the risk of neglecting relevant ecosystem processes. Additionally, this imposes a closure term problem at the respective “ends” of the trophic levels considered. In this study, we aim to understand how the implementation of higher trophic levels in a nutrient–phytoplankton–zooplankton–detritus (NPZD) model affects the simulated response of the ecosystem using a consistent NPZD–fish modelling approach (ECOSMO E2E) in the combined North Sea–Baltic Sea system. Utilising this approach, we addressed the above-mentioned closure term problem in lower trophic ecosystem modelling at a very low computational cost; thus, we provide an efficient method that requires very little data to obtain spatially and temporally dynamic zooplankton mortality. On the basis of the ECOSMO II coupled ecosystem model we implemented one functional group that represented fish and one group that represented macrobenthos in the 3-D model formulation. Both groups were linked to the lower trophic levels and to each other via predator–prey relationships, which allowed for the investigation of both bottom-up processes and top-down mechanisms in the trophic chain of the North Sea–Baltic Sea ecosystem. Model results for a 10-year-long simulation period (1980–1989) were analysed and discussed with respect to the observed patterns. To understand the impact of the newly implemented functional groups for the simulated ecosystem response, we compared the performance of the ECOSMO E2E to that of a respective truncated NPZD model (ECOSMO II) applied to the same time period. Additionally, we performed scenario tests to analyse the new role of the zooplankton mortality closure term in the truncated NPZD and the fish mortality term in the end-to-end model, which summarises the pressure imposed on the system by fisheries and mortality imposed by apex predators. We found that the model-simulated macrobenthos and fish spatial and seasonal patterns agree well with current system understanding. Considering a dynamic fish component in the ecosystem model resulted in slightly improved model performance with respect to the representation of spatial and temporal variations in nutrients, changes in modelled plankton seasonality, and nutrient profiles. Model sensitivity scenarios showed that changes in the zooplankton mortality parameter are transferred up and down the trophic chain with little attenuation of the signal, whereas major changes in fish mortality and fish biomass cascade down the food chain.

Shipborne nutrient dynamics and impact on the eutrophication in the Baltic Sea

The Baltic Sea is a severely eutrophicated sea-area where intense shipping as an additional nutrient source is a potential contributor to changes in the ecosystem. The impact of the two most important shipborne nutrients, nitrogen and phosphorus, on the overall nutrient-phytoplankton-oxygen dynamics in the Baltic Sea was determined by using the coupled physical and biogeochemical model system General Estuarine Transport Model–Ecological Regional Ocean Model (GETM-ERGOM) in a cascade with the Ship Traffic Emission Assessment Model (STEAM) and the Community Multiscale Air Quality (CMAQ) model. We compared two nutrient scenarios in the Baltic Sea: with (SHIP) and without nutrient input from ships (NOSHIP). The model uses the combined nutrient input from shipping-related waste streams and atmospheric depositions originating from the ship emission and calculates the effect of excess nutrients on the overall biogeochemical cycle, primary production, detritus formation and nutrient flows. The shipping contribution is about 0.3% of the total phosphorus and 1.25–3.3% of the total nitrogen input to the Baltic Sea, but their impact to the different biogeochemical variables is up to 10%. Excess nitrogen entering the N-limited system of the Baltic Sea slightly alters certain pathways: cyanobacteria growth is compromised due to extra nitrogen available for other functional groups while the biomass of diatoms and especially flagellates increases due to the excess of the limiting nutrient. In terms of the Baltic Sea ecosystem functioning, continuous input of ship-borne nitrogen is compensated by steady decrease of nitrogen fixation and increase of denitrification, which results in stationary level of total nitrogen content in the water. Ship-borne phosphorus input results in a decrease of phosphate content in the water and increase of phosphorus binding to sediments. Oxygen content in the water decreases, but reaches stationary state eventually.

Implementation of environmental technology for sustainable development of rural areas in Poland

In Poland agriculture use 60% of the land area. There are about 2.5 millions farms and average each of them cover 7 hectare. The farming and weak solutions of sanitary problems contribute substantially to the eutrophication of the Baltic Sea ecosystem, through the river discharge and transport of nutrients. New knowledge should be transferred to the local authorities and organisations, and in the final end, to the users of land and farmers. It seems important to work for the understanding of how the local environment can improved, which in turn can cause substantial improvement of nutrient losses from land and from the discharge of sewage in small villages. Different examples of extension ideas of Agenda 21 from Poland and co-operation between Swedish and Polish Universities in implementation ideas of sustainable development which will be presented, shown many possibilities for imitate by others in the Baltic region.

The Wastewater Disposal System Modernization During Processing of Amber Deposit as a Way to Reduce the Anthropogenic Load on the Baltic Sea Ecosystem

The Wastewater Disposal System Modernization During Processing of Amber Deposit as a Way to Reduce the Anthropogenic Load on the Baltic Sea Ecosystem

Abiotic and biotic drivers of flatfish abundance within distinct demersal fish assemblages in a brackish ecosystem (western Baltic Sea)

The study explores the fine scale spatial and temporal distribution of the entire demersal fish and flatfish assemblages in a brackish ecosystem with a special focus on the abiotic and biotic drivers influencing the abundance of the three commercially and ecologically important flatfish species, namely flounder (Platichthys flesus), plaice (Pleuronectes platessa) and dab (Limanda limanda). Using a 12 year time series obtained by a summer survey in the western Baltic Sea, at least three distinct demersal fish species assemblages were identified. Their spatial distribution was relatively stable over time and could be linked to environmental gradients. The demersal fish assemblages were dominated by few species and differed primarily in the percentage of common species. Both, the demersal and the flatfish assemblage were structured according to different habitat preferences. While there were species-specific differences, boosted regression tree modelling (BRT) showed that interannual fluctuations explained a large percentage of the variance in flatfish abundance whereby salinity, water temperature and sediment type were identified as the most important abiotic drivers. Dab was mainly influenced by sediment type and high salinity, while for flounder the main driver was water temperature. Plaice was also impacted by salinity, but was primarily influenced by biotic variables. The availability of benthic prey organisms in the area was verified as biotic driver for flatfish, especially for plaice. The results underline the importance of accounting for fine scale spatial heterogeneity in marine spatial planning in the Baltic Sea and comparable brackish water ecosystems and highlight the importance of considering benthic prey availability and diet analyses in demersal fish monitoring programs.

Middle to Late Holocene Variations in Salinity and Primary Productivity in the Central Baltic Sea: A Multiproxy Study From the Landsort Deep

Anthropogenic forcing has led to an increased extent of hypoxic bottom areas in the Baltic Sea during recent decades. The Baltic Sea ecosystem is naturally prone to the development of hypoxic condi ...

A New Approach for Investigating the Impact of Pesticides and Nutrient Flux from Agricultural Holdings and Land-Use Structures on Baltic Sea Coastal Waters

Knowledge related to land-use management impacts on the Baltic Sea ecosystem is limited. The constant release of pollutants into water bodies has resulted in water quality degradation. Therefore, only the innovative approaches integrated with research will provide accurate solutions and methods for proper environment management and will enable understanding and prediction of the impacts of land-use in the Baltic Sea region. Modelling approaches have become essential to address water issues and to evaluate ecosystem management. There are many water quality models, but only a few work in the operational mode and only some of them can be used as an interactive tool for environmental management to assess the impact of pollution on water quality. This study presents a new approach for investigating the influence of pesticides and nutrient fluxes from agricultural holdings and land-use structures on coastal waters of the Baltic Sea. Called WaterPUCK, this method will enable calculation of the sufficient amount of fertilizers, investigation nutrients, and pesticide sources and model: the fate and distribution of nutrients and pesticides in the surface water and groundwater; loads of pollution to surface water and groundwater; fluxes of nutrients via submarine groundwater discharge (SGD) to the Baltic Sea coastal environment; the processes and mechanisms influencing the persistence of nutrients in the environment; and predict the changes in land use and climate change influence on the Bay of Puck ecosystem.

Uncertainties in Projections of the Baltic Sea Ecosystem Driven by an Ensemble of Global Climate Models

Many coastal seas worldwide are affected by human impacts such as eutrophication causing, inter alia, oxygen depletion and extensive areas of hypoxia. Depending on the region, global warming may reinforce these environmental changes by reducing air-sea oxygen fluxes, intensifying internal nutrient cycling and increasing river-borne nutrient loads. The development of appropriate management plans to effectively protect the marine environment requires projections of future marine ecosystem states. However, projections with regional climate models commonly suffer from shortcomings in the driving global General Circulation Models (GCMs). The differing sensitivities of GCMs to increased greenhouse gas concentrations affect regional projections considerably. In this study, we focused on one of the most threatened coastal seas, the Baltic Sea, and estimated uncertainties in projections due to climate model deficiencies and due to unknown future greenhouse gas concentration, nutrient load and sea level rise scenarios. To address the latter, simulations of the period 1975-2098 were performed using the initial conditions from an earlier reconstruction with the same Baltic Sea model (starting in 1850). To estimate the impacts of climate model uncertainties, dynamical downscaling experiments with four driving global models were carried out for two greenhouse gas concentration scenarios and for three nutrient load scenarios, covering the plausible range between low and high loads. The results of primary production, nitrogen fixation, and hypoxic areas show that uncertainties caused by the various nutrient load scenarios are greater than the uncertainties due to climate model uncertainties and future greenhouse gas concentrations. In all scenario simulations, a proposed nutrient load abatement strategy, i.e., the Baltic Sea Action Plan, will lead to a significant improvement in the overall environmental state. However, the projections cannot provide detailed information on the timing and the reductions of future hypoxic areas, due to uncertainties in salinity projections caused by uncertainties in projections of the regional water cycle and of the mean sea level outside the model domain.

Baltic Sea ecosystem response to various nutrient load scenarios in present and future climates

The Baltic Sea is a shallow, semi-enclosed brackish sea suffering like many other coastal seas from eutrophication caused by human impact. Hence, nutrient load abatement strategies are intensively discussed. With the help of a high-resolution, coupled physical-biogeochemical circulation model we investigate the combined impact of changing nutrient loads from land and changing climate during the 21st century as projected from a global climate model regionalized to the Baltic Sea region. Novel compared to previous studies are an extraordinary spin-up based upon historical reconstructions of atmospheric, nutrient load and runoff forcing, revised nutrient load scenarios and a comparison of nutrient load scenario simulations with and without changing climate. We found in almost all scenario simulations, with differing nutrient inputs, reduced eutrophication and improved ecological state compared to the reference period 1976–2005. This result is a long-lasting consequence of ongoing nutrient load reductions since the 1980s. Only in case of combined high-end nutrient load and climate scenarios, eutrophication is reinforced. Differences compared to earlier studies are explained by the experimental setup including nutrient loads during the historical period and by the projected nutrient loads. We found that the impact of warming climate may amplify the effects of eutrophication and primary production. However, effects of changing climate, within the range of considered greenhouse gas emission scenarios, are smaller than effects of considered nutrient load changes, in particular under low nutrient conditions. Hence, nutrient load reductions following the Baltic Sea Action Plan will lead to improved environmental conditions independently of future climate change.

Disentangling the impact of nutrient load and climate changes on Baltic Sea hypoxia and eutrophication since 1850

In the Baltic Sea hypoxia has been increased considerably since the first oxygen measurements became available in 1898. In 2016 the annual maximum extent of hypoxia covered an area of the sea bottom of about 70,000 km2, comparable with the size of Ireland, whereas 150 years ago hypoxia was presumably not existent or at least very small. The general view is that the increase in hypoxia was caused by eutrophication due to anthropogenic riverborne nutrient loads. However, the role of changing climate, e.g. warming, is less clear. In this study, different causes of expanding hypoxia were investigated. A reconstruction of the changing Baltic Sea ecosystem during the period 1850–2008 was performed using a coupled physical-biogeochemical ocean circulation model. To disentangle the drivers of eutrophication and hypoxia a series of sensitivity experiments was carried out. We found that the decadal to centennial changes in eutrophication and hypoxia were mainly caused by changing riverborne nutrient loads and atmospheric deposition. The impacts of other drivers like observed warming and eustatic sea level rise were comparatively smaller but still important depending on the selected ecosystem indicator. Further, (1) fictively combined changes in air temperature, cloudiness and mixed layer depth chosen from 1904, (2) exaggerated increases in nutrient concentrations in the North Sea and (3) high-end scenarios of future sea level rise may have an important impact. However, during the past 150 years hypoxia would not have been developed if nutrient conditions had remained at pristine levels.

Unexplained interannual oscillations of cyanobacterial blooms in the Baltic Sea

Population oscillations in multi-species or even single species systems are well-known but have rarely been detected at the lower trophic levels in marine systems. Nitrogen fixing cyanobacteria are a major component of the Baltic Sea ecosystem and sometimes form huge surface accumulations covering most of the sea surface. By analysing a satellite-derived 39-year (1979–2017) data archive of surface cyanobacteria concentrations we have found evidence of strikingly regular interannual oscillations in cyanobacteria concentrations in the northern Baltic Sea. These oscillations have a period of ~3 years with a high-concentration year generally followed by one or two low-concentration years. Changes in abiotic factors known to influence the growth and survival of cyanobacteria could not provide an explanation for the oscillations. We therefore assume that these oscillations are intrinsic to the marine system, caused by an unknown, probably mainly biological mechanism that may be triggered by a combination of environmental factors. Interactions between different life cycle stages of cyanobacteria as well as between predator-prey or host-parasite are possible candidates for causing the oscillations.

Changes in the inflow of saline water into the Bornholm Basin (SW Baltic Sea) during the past 7100 years – evidence from benthic foraminifera record

The Baltic Sea (~393 000 km2) is the largest brackish sea in the world and its hydrographic and environmental conditions are strongly dependent on the frequency of saline water inflows from the North Sea. To improve our understanding of the natural variability of the Baltic Sea ecosystem detailed reconstructions of past saline water inflow changes based on palaeoecological archives are needed. Here we present a high‐resolution study of benthic foraminiferal assemblages accompanied by sediment geochemistry (loss on ignition, total organic carbon) and other microfossil data (ostracods and cladocerans) from a well‐dated 8‐m‐long gravity core taken in the Bornholm Basin. The foraminiferal diversity in the core is low and dominated by species of Elphidium. The benthic foraminiferal faunas in the central Baltic require oxic bottom water conditions and salinities >11–12 PSU. Consequently, shell abundance peaks in the record reflect frequent saline water inflow phases. The first appearance of foraminiferal tests and ostracods in the investigated sediment core is dated to c. 6.9 cal. ka BP and attributed to the first inflows of saline and oxygenated bottom waters into the Bornholm Basin during the Littorina Sea transgression. The transgression terminated the Ancylus Lake phase, reflected in the studied record by abundant cladocerans. High absolute foraminiferal abundances are found within two time intervals: (i) c. 5.5–4.0 cal. ka BP (Holocene Thermal Maximum) and (ii) c. 1.3–0.75 cal. ka BP (Medieval Climate Anomaly). Our data also show three intervals of absent or low saline water inflows: (i) c. 6.5–6.0 cal. ka BP, (ii) c. 3.0–2.3 cal. ka BP and (iii) c. 0.5–0.1 cal. ka BP (Little Ice Age). Our study demonstrates a strong effect of saline and well‐oxygenated water inflows from the Atlantic Ocean on the Baltic Sea ecosystem over millennial time scales, which is linked to the major climate transitions over the last 7 ka.