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Impacts of Climate Change on the Ecosystem of the Baltic Sea Subject
... Climate change in coastal and pelagic areas is already apparent in the Baltic Sea, Kattegat and Skagerrak with increasing temperatures, shorter ice periods and extended bottoms with hypoxic conditions (Viitasalo 2019). These changes affect species distribution ranges, spawning behaviour and habitat selection and may have both negative and positive effects on populations (Härmä et al. 2008;Olsson et al. 2012a;Nissling and Wallin 2020). ...
... Similarly, habitat shrinkage due to increased temperatures has been observed for cod around Denmark resulting in increased fragmentation and decreased connectivity of viable habitats (Dinesen et al. 2019). Hypoxic or anoxic conditions may also occur in coastal areas with increased temperatures affecting spawning and nursery areas for a number of species (Viitasalo 2019). Due to many species living close to their physiological salinity limits, most species in the Baltic Sea will be affected and some species may even disappear. ...
Marine protected areas (MPAs) have become a key component of conservation and fisheries management to alleviate anthropogenic pressures. For MPA networks to efficiently promote persistence and recovery of populations, ecological connectivity, i.e. dispersal and movement of organisms and material across ecosystems, needs to be taken into account. To improve the ecological coherence of MPA networks, there is hence a need to evaluate the connectivity of species spreading through active migration and passive dispersal. We reviewed knowledge on ecological connectivity in the Baltic Sea, Kattegat and Skagerrak in the northeast Atlantic and present available information on species-specific dispersal and migration distances. Studies on genetic connectivity are summarised and discussed in relation to dispersal-based analyses. Threats to ecological connectivity, limiting dispersal of populations and lowering the resilience to environmental change, were examined. Additionally, a review of studies evaluating the ecological coherence of MPA networks in the Baltic Sea, Kattegat and Skagerrak was performed, and suggestions for future evaluations to meet management needs are presented.
Aim
The Baltic Sea forms a unique regional sea with its salinity gradient ranging from marine to nearly freshwater conditions. It is one of the most environmentally impacted brackish seas worldwide, and the low biodiversity makes it particularly sensitive to anthropogenic pressures including climate change. We applied a novel combination of models to predict the fate of one of the dominant foundation species in the Baltic Sea, the bladder wrack Fucus vesiculosus .
Location
The Baltic Sea.
Methods
We used a species distribution model to predict climate change‐induced displacement of F. vesiculosus and combined these projections with a biophysical model of dispersal and connectivity to explore whether the dispersal rate of locally adapted genotypes may match estimated climate velocities to recolonize the receding salinity gradient. In addition, we used a population dynamic model to assess possible effects of habitat fragmentation.
Results
The species distribution model showed that the habitat of F. vesiculosus is expected to dramatically shrink, mainly caused by the predicted reduction of salinity. In addition, the dispersal rate of locally adapted genotypes may not keep pace with estimated climate velocities rendering the recolonization of the receding salinity gradient more difficult. A simplistic model of population dynamics also indicated that the risk of local extinction may increase due to future habitat fragmentation.
Main conclusions
Results point to a significant risk of locally adapted genotypes being unable to shift their ranges sufficiently fast considering the restricted dispersal and long generation time. The worst scenario is that F. vesiculosus may disappear from large parts of the Baltic Sea before the end of this century with large effects on the biodiversity and ecosystem functioning. We finally discuss how to reduce this risk through conservation actions, including assisted colonization and assisted evolution.
Beneath the waves, oxygen disappears
As plastic waste pollutes the oceans and fish stocks decline, unseen below the surface another problem grows: deoxygenation. Breitburg et al. review the evidence for the downward trajectory of oxygen levels in increasing areas of the open ocean and coastal waters. Rising nutrient loads coupled with climate change—each resulting from human activities—are changing ocean biogeochemistry and increasing oxygen consumption. This results in destabilization of sediments and fundamental shifts in the availability of key nutrients. In the short term, some compensatory effects may result in improvements in local fisheries, such as in cases where stocks are squeezed between the surface and elevated oxygen minimum zones. In the longer term, these conditions are unsustainable and may result in ecosystem collapses, which ultimately will cause societal and economic harm.
Science , this issue p. eaam7240
Although the 2015 Paris Agreement seeks to hold global average temperature to 'well below 2 °C above pre-industrial levels and to pursue efforts to limit the temperature increase to 1.5 °C above pre-industrial levels', projections of global mean sea-level (GMSL) rise commonly focus on scenarios in which there is a high probability that warming exceeds 1.5 °C. Using a semi-empirical model, we project GMSL changes between now and 2150 CE under a suite of temperature scenarios that satisfy the Paris Agreement temperature targets. The projected magnitude and rate of GMSL rise varies among these low emissions scenarios. Stabilizing temperature at 1.5 °C instead of 2 °C above preindustrial reduces GMSL in 2150 CE by 17 cm (90% credible interval: 14–21 cm) and reduces peak rates of rise by 1.9 mm yr⁻¹ (90% credible interval: 1.4–2.6 mm yr⁻¹). Delaying the year of peak temperature has little long-term influence on GMSL, but does reduce the maximum rate of rise. Stabilizing at 2 °C in 2080 CE rather than 2030 CE reduces the peak rate by 2.7 mm yr⁻¹ (90% credible interval: 2.0–4.0 mm yr⁻¹).
Climate change has been identified as one of the biggest current drivers of environmental change. Climate model projections for the Baltic Sea forecast increased frequency and duration of extreme temperatures, together with declines in salinity, which are expected to have impacts on the biota. In this experimental study, the interacting effects of low salinity and short-term (8 days) extreme seawater temperatures, followed by an 11-day recovery period, on the foundational macroalga, Fucus vesiculosus, were investigated. To account for potential variation in the responses at local scale, individuals originating from two different local populations, a warm and a cold site were included.
Expanding hypoxia is today a major threat for many coastal seas around the world and disentangling its drivers is a large challenge for interdisciplinary research. Using a coupled physical-biogeochemical model we estimate the impact of past and accelerated future global mean sea level rise (GSLR) upon water exchange and oxygen conditions in a semi-enclosed, shallow sea. As a study site, the Baltic Sea was chosen that suffers today from eutrophication and from dead bottom zones due to (1) excessive nutrient loads from land, (2) limited water exchange with the world ocean and (3) perhaps other drivers like global warming. We show from model simulations for the period 1850–2008 that the impacts of past GSLR on the marine ecosystem were relatively small. If we assume for the end of the twenty-first century a GSLR of +0.5 m relative to today’s mean sea level, the impact on the marine ecosystem may still be small. Such a GSLR corresponds approximately to the projected ensemble-mean value reported by the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. However, we conclude that GSLR should be considered in future high-end projections (>+1 m) for the Baltic Sea and other coastal seas with similar hydrographical conditions as in the Baltic because GSLR may lead to reinforced saltwater inflows causing higher salinity and increased vertical stratification compared to present-day conditions. Contrary to intuition, reinforced ventilation of the deep water does not lead to overall improved oxygen conditions but causes instead expanded dead bottom areas accompanied with increased internal phosphorus loads from the sediments and increased risk for cyanobacteria blooms.
Biological invasions coupled with climate change drive changes in marine biodiversity. Warming climate and changes in hydrology may either enable or hinder the spread of non-indigenous species (NIS) and little is known about how climate change modifies the richness and impacts of NIS in specific sea areas. We calculated from climate change simulations (RCO-SCOBI model) the changes in summer time conditions which northern Baltic Sea may to go through by the end of the twenty-first century, e.g., 2–5 °C sea surface temperature rise and even up to 1.75 unit decrease in salinity. We reviewed the temperature and salinity tolerances (i.e., physiological tolerances and occurrence ranges in the field) of pelagic and benthic NIS established in—or with dispersal potential to—the northern Baltic Sea, and assessed how climate change will likely affect them. Our findings suggest a future decrease in barnacle larvae and an increase in Ponto-Caspian cladocerans in the pelagic community. In benthos, polychaetes, gastropods and decapods may become less abundant. By contrast, dreissenid bivalves, amphipods and mysids are expected to widen their distribution and increase in abundance in the coastal areas of the northern Baltic Sea. Potential salinity decrease acts as a major driver for NIS biogeography in the northern Baltic Sea, but temperature increase and extended summer season allow higher reproduction success in bivalves, zooplankton, amphipods and mysids. Successful NIS, i.e., coastal crustacean and bivalve species, pose a risk to native biota, as many of them have already demonstrated harmful effects in the Baltic Sea.
The Baltic Sea region is one of the largest brackish water areas in the world with a salinity declining from about 10 PSU in the south in the Baltic Proper, declining through the Bothnian Sea to 2 PSU in the northern Bothnian Bay. It is surrounded by 14 densely populated and industrialised countries, where 90 million people live within the drainage area. The narrow and shallow Danish straits limit the water exchange and result in a residence time of the water of 25-30 years. The north-south salinity gradient restricts the northward penetration of marine organisms, and the few which do occur in the north often show dwarfism. A permanent halocline at about 60-80 m depth prevents vertical circulation, and oxygenation of the deep water is limited to a few events of salt-water inflow. About one third of the bottom area of the Baltic Proper is devoid of higher life due to low oxygen. The Baltic Sea has no tides. The hard bottoms are dominated by communities of Fucus vesiculosus and Mytilus edulis, and some further 30 species. Soft-bottom communities are dominated by the bivalve Macoma balthica and three more species in the Baltic Proper, but both biomass and number of species decrease rapidly to the north. Production of pelagic fish like herring and sprat is high, but benthic species like cod and flounder are declining due to eutrophication. There is also a significant fishery of freshwater fish such as pike and perch in the coastal areas. The pollution load is high, affecting e.g. reproduction of top carnivores, although species such as seals and white-tailed sea eagle, which were threatened by DDT and PCB levels until recently, are now recovering. The main problem is eutrophication, favouring growth of nuisance algae, and a lowering of oxygen in the benthos. Since most organisms of this region are already living under severe physiological stress, they are sensitive to pollution, and the low number of species increases the risk that whole functional groups can be lost.
Climate change is likely to have large effects on the Baltic Sea ecosystem. Simulations indicate 2–4 °C warming and 50–80 % decrease in ice cover by 2100. Precipitation may increase ~30 % in the north, causing increased land runoff of allochthonous organic matter (AOM) and organic pollutants and decreased salinity. Coupled physical–biogeochemical models indicate that, in the south, bottom-water anoxia may spread, reducing cod recruitment and increasing sediment phosphorus release, thus promoting cyanobacterial blooms. In the north, heterotrophic bacteria will be favored by AOM, while phytoplankton production may be reduced. Extra trophic levels in the food web may increase energy losses and consequently reduce fish production. Future management of the Baltic Sea must consider the effects of climate change on the ecosystem dynamics and functions, as well as the effects of anthropogenic nutrient and pollutant load. Monitoring should have a holistic approach, encompassing both autotrophic (phytoplankton) and heterotrophic (e.g., bacterial) processes.
Electronic supplementary material
The online version of this article (doi:10.1007/s13280-015-0654-8) contains supplementary material, which is available to authorized users.
The combined effect of changing climate and changing nutrient loads from land due to altered land use, sewage water treatment and emissions was studied using a 3-dimensional high-resolution coupled physical-biogeochemical model for the Baltic Sea. Results suggest that global warming causes increased water temperatures and reduced sea ice cover, combined (eventually) with increased winter mean wind speeds and increased river runoff. The projected hydrographic changes could therefore have significant effects on the marine ecosystem. These changes may compete with nutrient load reductions-presently under discussion-that aim to improve the ecological status of the Baltic Sea. Targets that may be sufficient in the present climate might fail under future climate conditions. Using the model, we investigated 4 climate change scenarios and 3 nutrient load scenarios, ranging from a pessimistic 'business as usual' to the 'most optimistic' case (including the Baltic Sea Action Plan, BSAP). In addition, using cause-and-effect studies, we analyzed changing simulated nutrient cycles, oxygen concentrations, and phytoplankton concentrations. As model results for the northern part of the Baltic (Bothnian Bay and Bothnian Sea) are not reliable, we focus the analysis on the Baltic proper, including the Arkona, Bornholm and Gotland basins. The degree of nutrient reduction in nutrient-load reduction scenarios is likely to differ under a future climate, but actions of the BSAP will reduce phytoplankton concentrations also in the future climate. However, the sensitivity of non-linear responses to climate change depends on processes that are not well understood, with current understanding limited by modelling uncertainties (e.g. in the long-term functioning of Baltic Sea sediments as sources and sinks of nutrients).
Ecosystem-based management (EBM) has emerged as the generally agreed strategy for managing ecosystems, with humans as integral parts of the managed system. Human activities have substantial effects on marine ecosystems, through overfishing, eutrophication, toxic pollution, habitat destruction, and climate change. It is important to advance the scientific knowledge of the cumulative, integrative, and interacting effects of these diverse activities, to support effective implementation of EBM. Based on contributions to this special issue of AMBIO, we synthesize the scientific findings into four components: pollution and legal frameworks, ecosystem processes, scale-dependent effects, and innovative tools and methods. We conclude with challenges for the future, and identify the next steps needed for successful implementation of EBM in general and specifically for the Baltic Sea.
Changes to runoff due to climate change may influence management of nutrient loading to the sea. Assuming unchanged river nutrient concentrations, we evaluate the effects of changing runoff on commitments to nutrient reductions under the Baltic Sea Action Plan. For several countries, climate projections point to large variability in load changes in relation to reduction targets. These changes either increase loads, making the target more difficult to reach, or decrease them, leading instead to a full achievement of the target. The impact of variability in climate projections varies with the size of the reduction target and is larger for countries with more limited commitments. In the end, a number of focused actions are needed to manage the effects of climate change on nutrient loads: reducing uncertainty in climate projections, deciding on frameworks to identify best performing models with respect to land surface hydrology, and increasing efforts at sustained monitoring of water flow changes.
Electronic supplementary material
The online version of this article (doi:10.1007/s13280-015-0657-5) contains supplementary material, which is available to authorized users.
Substantial ecological changes occurred in the 1970s in the Northern Baltic during a temporary period of low salinity (S). This period was preceded by an episodic increase in the rainfall over the Baltic Sea watershed area. Several climate models, both global and regional, project an increase in the runoff of the Northern latitudes due to proceeding climate change. The aim of this study is to model, firstly, the effects on Baltic Sea salinity of increased runoff due to projected global change and, secondly, the effects of salinity change on the distribution of marine species. The results suggest a critical shift in the S range 5–7, which is a threshold for both freshwater and marine species distributions and diversity. We discuss several topics emphasizing future monitoring, modelling, and fisheries research. Environmental monitoring and modelling are investigated because the developing alternative ecosystems do not necessarily show the same relations to environment quality factors as the retiring ones. An important corollary is that the observed and modelled S changes considered together with species’ ranges indicate what may appear under a future climate. Consequences could include a shift in distribution areas of marine benthic foundation species and some 40–50 other species, affiliated to these. This change would extend over hundreds of kilometres, in the Baltic Sea and the adjacent North Sea areas. Potential cascading effects, in coastal ecology, fish ecology and fisheries would be extensive, and point out the necessity to develop further the “ecosystem approach in the environmental monitoring”.
Coastal waters are important in linking zoobenthos and fish, as many coastal fishes feed on benthic prey in these habitats. Major drivers, such as eutrophication and climate change, may alter this link, whereas shifts in the importance of these drivers may induce different responses in zoobenthos and fish, respectively, potentially changing productivity of coastal ecosystems. The aim of this study was to assess in which way abundance and distribution of benthic-feeding fish and biomass of zoobenthos have changed over time in response to eutrophication (Secchi depth) and climate change (temperature and salinity), respectively. This was done by analyzing gross changes in the responses over three decades (1983–2012) and across depth zones. Eutrophication and climate change caused different though specific impacts on fish and zoobenthos,
respectively. Throughout the 1980s, increasing benthicfeeding fish abundance in shallow waters (<6 m) was primarily attributed to eutrophication (decreasing Secchi depth), implying increased system productivity. During the 2000s, the effect of eutrophication levelled out, whereas temperature caused contrasting development of fish abundance at different depth zones. Shallow waters had lower fish abundances during warm years compared with colder ones, while the abundance increased in deeper, aphotic waters (6–20 m). In the deep waters, zoobenthos showed a contrasting, declining trend in biomass, coinciding with the decrease in salinity during the 2000s. This suggests altered ecosystem productivity and potential food shortage for benthic-feeding fish exploring the deep waters. An intensification of these trends is likely in the future, as climate change scenarios suggest further temperature increase and salinity decrease.
The influence of ambient temperature (2-7°C) and oxygen level (1.0-8.3 mi i i) on the development of Baltic cod eggs was investigated in laboratory experimenrs. The incubation period, i.e. the time from fer tilization to 50% hatching, decreased from 27.5 days at 2°C to 13.0 days at 7°C. Reduced oxygen levels did flot significantly affect the time of hatching. Throughout the incubation period highest mortality rates were found during gastrulation and immediately prior to hatching at ali tested oxs'gen levels. Egg survival decreased from around 30% at an oxygen level of 8 ml .11 to less than 10% at 2 m1 l oxygen content. At oxygen concentrations below 2 ml 021 the development ceased at a very early stage. Field observations revealed that in the past years Baltic cod eggs were most abundant below the halo dine, depth with unfavourable oxygen condition. Besides the effect on egg survival, low environmental oxygen may also affect the initial viability of larvae and consequently their ability to approach the feed ing areas close to the sea surface. Thus, the effective reproduction volume of water for cod m the central Baltic may have been smaller than expected and it is suggested that oxygen depletion was the limiting factor determining the reproductive success of cod in this area during the last decade.
Increased precipitation is one projected outcome of climate change that may enhance the discharge of freshwater to the coastal zone. The resulting lower salinity, and associated discharge of both nutrients and dissolved organic carbon, may influence food web functioning. The scope of this study was to determine the net outcome of increased freshwater discharge on the balance between auto‐ and heterotrophic processes in the coastal zone. By using long‐term ecological time series data covering 13 years, we show that increased river discharge suppresses phytoplankton biomass production and shifts the carbon flow towards microbial heterotrophy. A 76% increase in freshwater discharge resulted in a 2.2 times higher ratio of bacterio‐ to phytoplankton production (Pb:Pp). The level of Pb:Pp is a function of riverine total organic carbon supply to the coastal zone. This is mainly due to the negative effect of freshwater and total organic carbon discharge on phytoplankton growth, despite a concomitant increase in discharge of nitrogen and phosphorus. With a time lag of 2 years the bacterial production recovered after an initial decline, further synergistically elevating the microbial heterotrophy. Current climate change projections suggesting increased precipitation may therefore lead to increased microbial heterotrophy, thereby decreasing the transfer efficiency of biomass to higher trophic levels. This prognosis would suggest reduced fish production and lower sedimentation rates of phytoplankton, a factor of detriment to benthic fauna. Our findings show that discharge of freshwater and total organic carbon significantly contributes to the balance of coastal processes at large spatial and temporal scales, and that model's would be greatly augmented by the inclusion of these environmental drivers as regulators of coastal productivity.
The effect of different phosphorus loads (LP) on the phosphorus (P) and carbon (C) content (biomass) of algae and bacteria was assessed in continuous culture. We tested if a mixed freshwater microbial assemblage co-cultured with a phytoflagellate (Cryptomonas phaseolus) would comply with the ‘phytoplankton–bacteria paradox’ (sensu Bratbak & Thingstad 1985). This hypothesis states that the ratio of bacterial to algal abundance changes to the benefit of bacteria with decreasing LP. However, the phenomenon was originally investigated by simultaneously altering LP and microbial growth rates, and it is unclear to which extent it can be assigned to either parameter. Therefore, we set up 3 chemostat systems in triplicate at equal dilution rates, but with daily LP of 21, 41 or 62 µg l–1 d–1 (corresponding to 50, 100 and 150 µg P l–1). Higher LP led to a 5-fold increase in total algal abundance and biomass but to less than a doubling of these parameters in the bacterial assemblage. Total biomass ratios of bacteria to algae changed from 0.18 to 0.06 with increasing LP, while the bacteria–algae total phosphorus ratios decreased from 0.80 to 0.17. The cellular C:P ratio of algae remained similar at all P concentrations, whereas the molar C:P ratios of bacterial cells significantly increased at higher LP (from 44 to 73). An enrichment experiment with the 50 µg P l–1 treatment demonstrated that bacteria at the lowest LP were co-limited by P and C, and that increased P stimulated mainly the algal fraction. The phytoplankton–bacteria paradox at the level of a mixed microbial assemblage is thus characterised by the following aspects: (1) bacteria profit from their high affinity to P and are better competitors at lower LP; (2) although algae compete with bacteria for P, P-limited algae release extracellular C that stimulates growth of their bacterial competitors; (3) when bacteria depend on algae as their sole source of organic C, this provides a feedback mechanism by which algae limit the abundance of their competitors at higher LP; (4) large oscillations in the bacteria–algae ratios at the lowest LP point to a greater instability of this interaction with stronger P competition. However, bacteria were not able to outcompete C. phaseolus, as algae were their only C source.
Possible future changes in Baltic Sea acid-base (pH) and oxygen balances were studied using a catchment-sea coupled model system and numerical experiments based on meteorological and hydrological forcing datasets and scenarios. By using objective statistical methods, climate runs for present climate conditions were examined and evaluated using Baltic Sea modelling. The results indicate that increased nutrient loads will not inhibit future Baltic Sea acidification; instead, the seasonal pH cycle will be amplified by increased biological production and mineralization. All examined scenarios indicate future acidification of the whole Baltic Sea that is insensitive to the chosen global climate model. The main factor controlling the direction and magnitude of future pH changes is atmospheric CO2 concentration (i.e. emissions). Climate change and land-derived changes (e. g. nutrient loads) affect acidification mainly by altering the seasonal cycle and deep-water conditions. Apart from decreasing pH, we also project a decreased saturation state of calcium carbonate, decreased respiration index and increasing hypoxic area - all factors that will threaten the marine ecosystem. We demonstrate that substantial reductions in fossil-fuel burning are needed to minimise the coming pH decrease and that substantial reductions in nutrient loads are needed to reduce the coming increase in hypoxic and anoxic waters.
We present a multi-model ensemble study for the Baltic Sea, and investigate the combined impact of changing climate, external nutrient supply, and fisheries on the marine ecosystem. The applied regional climate system model contains state-of-the-art component models for the atmosphere, sea ice, ocean, land surface, terrestrial and marine biogeochemistry, and marine food-web. Time-dependent scenario simulations for the period 1960-2100 are performed and uncertainties of future projections are estimated. In addition, reconstructions since 1850 are carried out to evaluate the models sensitivity to external stressors on long time scales. Information from scenario simulations are used to support decision-makers and stakeholders and to raise awareness of climate change, environmental problems, and possible abatement strategies among the general public using geovisualization. It is concluded that the study results are relevant for the Baltic Sea Action Plan of the Helsinki Commission.
The combined future impacts of climate change and industrial and agricultural practices in the Baltic Sea catchment on the Baltic Sea ecosystem were assessed. For this purpose 16 transient simulations for 1961–2099 using a coupled physical-biogeochemical model of the Baltic Sea were performed. Four climate scenarios were combined with four nutrient load scenarios ranging from a pessimistic business-as-usual to a more optimistic case following the Baltic Sea Action Plan (BSAP). Annual and seasonal mean changes of climate parameters and ecological quality indicators describing the environmental status of the Baltic Sea like bottom oxygen, nutrient and phytoplankton concentrations and Secchi depths were studied. Assuming present-day nutrient concentrations in the rivers, nutrient loads from land increase during the twenty first century in all investigated scenario simulations due to increased volume flows caused by increased net precipitation in the Baltic catchment area. In addition, remineralization rates increase due to increased water temperatures causing enhanced nutrient flows from the sediments. Cause-and-effect studies suggest that both processes may play an important role for the biogeochemistry of eutrophicated seas in future climate partly counteracting nutrient load reduction efforts like the BSAP.
Fast ice and underlying water of the northern Baltic Sea were sampled at 1 wk intervals during winter 2000 in a coastal location in SW Finland. To investigate seasonal succession and interactions inside the food web and between the food web and physical and chemical properties of the ice, several biological (chlorophyll a, abundance of bacteria, flagellates and ciliates, bacterial cell volume and leucine incorporation, POC), chemical (total and dissolved nutrients) and physical (salinity, temperature) parameters were measured. The first ice-algae bloom with chlorophyll a (chl a) concentrations of up to 18.5 mug l(-1) occurred in mid-January, followed by the main ice-algae bloom in March and a heterotrophic postbloom situation shortly before the ice deteriorated in April. The ice-organism assemblage Was autotrophy-dominated; during the entire ice-covered period, ice algae formed on average 82 +/- 10% (mean +/- SD) of the organism biomass. The largest heterotrophic groups by biomass were bacteria (16.7 +/- 9.4%), ciliates and metazoans (4.4 +/- 2.7 and 4.4 +/- 4.8%, respectively). The ice food-web was characterized by the importance of ice bacteria and the presence of a potential microbial loop (bacterial secondary production accounting for a mean of 27.1 +/- 19.3 mumol C m(-2) d(-1)), grazing control on bacteria and flagellates, and also possible 'shortcuts' such as ciliate grazing on bacteria and metazoan herbivory. An ice-sheet warming event in February clearly affected the physical and chemical properties of the ice as well as the structure and function of ice-organism assemblages. Ice organisms probably controlled dissolved nutrient concentrations inside the ice by uptake and regeneration.
The paper describes an application of a NPZDF-model, which couples interactively a biogeochemical and a fish production model. The Baltic Sea is chosen as an example system, where the fish community is dominated by two prey species (sprat and herring) and one predator (cod). The linkage of the model components is established through feeding of prey fish on zooplankton and recycling of fish biomass to nutrients and detritus. The fish dynamics is driven by size dependent predator–prey interactions, reproduction and mortality. A challenge is the quantitative description of the multitude of processes involved. The model formulation is constrained by strict mass conservation.Examples of experimental runs over 40years are discussed: A baseline simulation gives an approximate hindcast scenario of stock variations and catches over the time period of the years from 1963 to 2003 with increasing and declining cod catches. As known from observations, the stock size of sprat shows a reverse trend, while the herring stock appears to be relatively stable. The modeled results display several observed features, e.g. stock sizes and magnitude of changes, but the response times and phases of the variations are not well reproduced. Moreover, scenarios are simulated to indicate how moderate adjustments of fishing rates of cod over different time periods would change the stock and catches.
The changes in phytoplankton community in water passing a large natural population of the blue mussels Mytilus edulis L.. in the Oresund strait between Malmo and Copenhagen were studied. Changes in both the biomass and composition of the phytoplankton community were detected and an explanation invoking intense grazing pressure from the mussels is proposed. The biomass, measured as biovolume, decreased from 0.42 to 0.11 mm(3) 1(-1) (74 % decrease) as the non-stratified turbulent water passed the mussel bed over a distance of 20 km. Beyond the mussel bed, biomass increased to 0.39 mm(3) 1(-1) within 27 km. A benthic filter-feeder model was adapted to correlate the theoretical maximum filtration with observations. The model was in accordance with observed values if a filtration efficiency of 25 % was applied. During the passage the phytoplankton community was shifted to smaller phytoplankton cells: phytoplankton with a diameter of 2 to 12 mu m increased from 65 to 89 % of the total phytoplankton community and all other plankton taxa declined. This observed shift in community was significant correlated with a predicted shift due to the mussel filtration.
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.
Dynamic model simulations of the future climate and projections of future lifestyles within the Baltic Sea Drainage Basin (BSDB) were considered in this study to estimate potential trends in future nutrient loads to the Baltic Sea. Total nitrogen and total phosphorus loads were estimated using a simple proxy based only on human population (to account for nutrient sources) and stream discharges (to account for nutrient transport). This population-discharge proxy provided a good estimate for nutrient loads across the seven sub-basins of the BSDB considered. All climate scenarios considered here produced increased nutrient loads to the Baltic Sea over the next 100 years. There was variation between the climate scenarios such that sub-basin and regional differences were seen in future nutrient runoff depending on the climate model and scenario considered. Regardless, the results of this study indicate that changes in lifestyle brought about through shifts in consumption and population potentially overshadow the climate effects on future nutrient runoff for the entire BSDB. Regionally, however, lifestyle changes appear relatively more important in the southern regions of the BSDB while climatic changes appear more important in the northern regions with regards to future increases in nutrient loads. From a whole-ecosystem management perspective of the BSDB, this implies that implementation of improved and targeted management practices can still bring about improved conditions in the Baltic Sea in the face of a warmer and wetter future climate.
The structure of many marine ecosystems has changed substantially during recent decades, as a result of overexploitation, climate change and eutrophication. Despite of the apparent ecological and economical importance of coastal areas and communities, this aspect has received relatively little attention in coastal systems. Here we assess the temporal development of zoobenthos communities in two areas on the Swedish Baltic Sea coast during 30 years, and relate their development to changes in climate, eutrophication and top-down regulation from fish. Both communities show substantial structural changes, with a decrease in marine polychaetes and species sensitive to increased water temperatures. Concurrently, opportunistic species tolerant to environmental perturbation have increased in abundance. Species composition show a similar temporal development in both communities and significant changes in species composition occurred in both data sets in the late 1980s and early 1990s. The change in species composition was associated with large scale changes in climate (salinity and water temperature) and to the structure of the local fish community, whereas we found no effects of nutrient loading or ambient nutrient concentrations. Our results suggest that these coastal zoobenthos communities have gone through substantial structural changes over the last 30 years, resulting in communities of different species composition with potentially different ecological functions. We hence suggest that the temporal development of coastal zoobenthos communities should be assessed in light of prevailing climatic conditions considering the potential for top-down effects exerted by local fish communities.
Suursaar, .; Tnisson, H; Alari, V.; Raudsepp, U.; Rstas, H., and Anderson, A., 2016. Projected changes in wave conditions in the Baltic Sea by the end of 21st century and the corresponding shoreline changes. In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research, Special Issue, No. 75, pp. 1012 - 1016. Coconut Creek (Florida), ISSN 0749-0208. The aim of the study is to analyse possible future changes in the Baltic Sea wave conditions and to project coastal changes in six differently exposed Estonian coastal sections resulting from changing wind climates. In the open parts of the Baltic Sea, the SWAN model with 3 NM spatial resolution was used for simulation of wave fields in 19662100. Regional climate projection EUR-11 assuming the RCP4.5 greenhouse gas scenario was used as wind forcing. In addition, using a site-dependently calibrated fetch-based wave model, a set of semi-realistic scenario calculations was obtained by modifying the baseline wind input data in order to investigate the reaction of wave climates and coastal developments. For coastal change, past developments in the shoreline and accumulation-erosion areas were tracked using repeated GPS measurements and GIS-overlaid cartographic and photographic material. The projections showed spatially and temporally varying wave fields and a slight overall increase, which corresponds to increased south-westerly winds. Depending on exposition, the wave climates would change differently even within a single semi-enclosed sea. Using the previously established empirical relationships between wave parameters and shoreline changes, we predict that erosion will probably increase in transitional zones while accumulation increases within bays. Sea-level rise and shortening of the sea-ice duration will probably have a remarkable contribution.
The Intergovernmental Panel on Climate Change (IPCC) is the leading international body for assessing the science related to climate change. It provides policymakers with regular assessments of the scientific basis of human-induced climate change, its impacts and future risks, and options for adaptation and mitigation. This IPCC Special Report on the Ocean and Cryosphere in a Changing Climate is the most comprehensive and up-to-date assessment of the observed and projected changes to the ocean and cryosphere and their associated impacts and risks, with a focus on resilience, risk management response options, and adaptation measures, considering both their potential and limitations. It brings together knowledge on physical and biogeochemical changes, the interplay with ecosystem changes, and the implications for human communities. It serves policymakers, decision makers, stakeholders, and all interested parties with unbiased, up-to-date, policy-relevant information. This title is also available as Open Access on Cambridge Core.
Anthropogenic climate change can alter the wind- and sea-ice climate and thus the wave conditions in the Baltic Sea. Here, transient simulations with the 3rd generation wave model WAM under two IPCC AR4 emission scenarios (A1B and B1) and two initial conditions of the forcing atmospheric fields are analyzed for the period 1961–2100. Future changes in the wave climate comprise higher significant wave height for most regions and simulations. Median waves show temporal and spatial consistent changes, whereas extreme waves (99th percentile and maximum) show much more variability in space and among the simulations. These changes in the wave fields result from not only higher wind speeds but also from a shift to more westerly winds, which leads to different fetch and thus to different significant wave height and direction. The multi-decadal and the inter-simulation variability illustrate the uncertainty in the estimation of the climate change signal.
The effects of climate changes on nutrient discharges within the Baltic Sea catchment were modelled, indicating increases in concentrations of phosphorus, but decreases in nitrogen for the southern Baltic Sea catchment. The process-based hydrological and nutrient flux model, HYPE, was set up for the entire Baltic Sea catchment area. The model was then used to examine how water and nutrient fluxes may change during four different climate scenarios. Changes to discharge varied regionally, with increases seen in the northeastern Baltic Sea catchment and decreases in the south and southwest. Changes to total nutrient loads did not necessarily follow the changes in discharge, indicating significant changes in nutrient concentrations. This indicates the importance of a process-based hydrological and nutrient model for analyses: it is the net result of several different nutrient sink and source processes that determine the predicted status of nutrients as a result of climate change.
To evaluate if climate influence zooplankton densities and dynamics in a coastal Baltic Sea area, we performed statistical analyses of two 12-13-year-long data series. The winter (December-March) North Atlantic Oscillation index (NAO) was used as the independent variable and monthly biomasses of seven groups of zooplankton as the dependent variables. Most of the statistically significant correlations were obtained for the spring-early-summer period and they all indicate higher zooplankton biomasses after winters with high NAO values (mild winters). This supports results from other Baltic Sea studies, indicating that winter/spring climate is important to the early summer zooplankton community.
Climate change is projected to increase air temperature, precipitation and river runoff in the Baltic Sea area. Consequently sea surface temperature will increase and salinity will gradually decline. Species' geographical ranges will shift and populations increase or decrease according to the temperature and salinity tolerances of each species. Warming up of the Baltic Sea also favours establishment of non-indigenous species and increases metabolism of organisms. Increasing metabolism speeds up production and growth rates of secondary producers, but it may also enhance the uptake of harmful substances. Other important processes include rising of water level, decreasing pH as well as diminishing of sea ice. These processes will immerse coastal areas, slow down calcification of bivalves and deteriorate living conditions of species associated with sea ice. Increasing runoff of nutrients from land during winter will increase primary production and sedimentation of organic matter, which may enhance anoxia and release of phosphorus from sediments. Increasing temperature and declining salinity will however have complex effects on water stratification that may either worsen or alleviate the oxygen deficiency. In the deepest basins anoxia may become more common, while at mid depths (70-100 m) oxygen conditions may improve. In the Gulf of Bothnia, in contrast, where the rivers carry a large load of dissolved organic carbon, increasing freshwater runoff may shift the system towards a more microbially mediated production, and hence decrease primary production.
The Baltic Sea is a marginal sea, located in a highly industrialized region in Central Northern Europe. Saltwater inflows from the North Sea and associated ventilation of the deep exert crucial control on the entire Baltic Sea ecosystem. This study explores the impact of anticipated sea level changes on the dynamics of those inflows. We use a numerical oceanic general circulation model covering both the Baltic and the North Sea. The model successfully retraces the essential ventilation dynamics throughout the period 1961–2007. A suite of idealized experiments suggests that rising sea level is associated with intensified ventilation as saltwater inflows become stronger, longer, and more frequent. Expressed quantitatively as a salinity increase in the deep central Baltic Sea, we find that a sea level rise of 1 m triggers a saltening of more than 1 PSU. This substantial increase in ventilation is the consequence of the increasing cross section in the Danish Straits amplified by a reduction of vertical mixing.
Changes in the macrofauna of the sub-halocline bottoms of the Eastern Gotland Basin and the Gulf of Finland during the 1965–1994 period have been studied in relation to the salinity and oxygen regimes. The study period covers several significant major inflows of North Sea water, as well as the longest stagnation period during the last hundred years, with semi-permanent anoxia affecting the distribution and community structure of the macrozoobenthos. At the beginning of the study period vast areas below the halocline were devoid of benthic macrofauna. The major inflows of North Sea water in 1975–1976 led to rapid benthic recolonisation down to approximately 150 m depth in the Eastern Gotland Basin, where oxygen conditions had improved, but had no effect on the macrozoobenthos communities in the Gulf of Finland. The subsequent, prolonged, stagnation period in 1977–1993 caused a deterioration of the macrozoobenthos as a consequence of anoxia and formation of hydrogen sulphide in sub-halocline (70–250 m) areas. Later, a recovery was observed in the 70–100 m depth zone when vertical stratification weakened and intensified vertical mixing resulted in improved oxygen conditions at these intermediate depths. The effect of this process was most pronounced in the Gulf of Finland. The 1993–1994 inflows led to the highest oxygen levels in the Gotland Deep since the 1930s. Consequently, in 1994 polychaetes were found at 243 m depth indicating a recolonisation of even the deepest part of the basin. The fluctuations in salinity have also affected the distribution of marine species. The biological significance of the hydrographical regime, potential impact of eutrophication on the oxygen balance of the deep waters, and the consistency of long-term data sets are discussed.
Large uncertainty surrounds projections of global sea-level rise, resulting from uncertainty about future warming and an incomplete understanding of the complex processes and feedback mechanisms that cause sea level to rise. Consequently, existing models produce widely differing predictions of sea-level rise even for the same temperature scenario. Here we present results of a broad survey of 90 experts who were amongst the most active scientific publishers on the topic of sea level in recent years. They provided a probabilistic assessment of sea-level rise by AD 2100 and AD 2300 under two contrasting temperature scenarios. For the low scenario, which limits warming to <2 °C above pre-industrial temperature and has slowly falling temperature after AD 2050, the median ‘likely’ range provided by the experts is 0.4–0.6 m by AD 2100 and 0.6–1.0 m by AD 2300, suggesting a good chance to limit future sea-level rise to <1.0 m if climate mitigation measures are successfully implemented. In contrast, for the high warming scenario (4.5 °C by AD 2100 and 8 °C in AD 2300) the median likely ranges are 0.7–1.2 m by AD 2100 and 2.0–3.0 m by AD 2300, calling into question the future survival of some coastal cities and low-lying island nations.
We evaluate experimentally the effect of carbonate saturation state at the sediment-water interface (SWI) on survivorship of various size classes of the juvenile bivalve Mercenaria mercenaria. Populations of 0.2-mm, 0.3-mm, 1-mm, and 2-mm M. mercenaria were introduced to sediments realistically undersaturated (experimental, saturation state with respect to aragonite = Ω aragonite = IMP/K′sp = ∼0.3) and saturated (control, Ωaragonite = ∼1.5) with respect to aragonite in order to evaluate the impact of saturation state and dissolution on survivorship. Linear regression analysis was used to examine mortality within each treatment over time and show significant mortality for each size class in experimental-undersaturated treatments only (P < 0.05). Mortality rates in experimental-undersaturated sediments were -11.8, -4.8, -1.9, and -1.1% d -1 for the 0.2-, 0.3-, 1.0-, and 2.0-mm bivalves, respectively. Analysis of covariance (ANCOVA) was used to examine differences in mortality between treatments over time and show significantly different mortality rates for the 0.2-, 0.3-, and 1-mm individuals (P < 0.05). Dissolution may represent a previously unrecognized yet significant source of mortality for "just-set" juvenile bivalves, particularly the very small individuals that have been largely ignored in recruitment studies to date. Dissolution-induced mortality may help explain the exponential losses of juvenile bivalves following their transition from the pelagic larval phase to the benthic juvenile phase.
Water from 24 Swedish and Finnish rivers running to the Gulf of Bothnia (comprising the Bothnian Sea and Bothnian Bay) was sampled on five occasions in 1991–93, and analysed for concentrations of total organic carbon (TOC), humic substances and metals (Fe, Al, Cu, Zn). In general, TOC is higher in the Finnish rivers, which drain forest and peatlands, than in the Swedish rivers, which drain vast mountainous and forested areas. The pH is slightly lower in the Finnish rivers. Humic substances comprise approximately 80% of the total amount of organic matter. Calculations of the specific transport of organic matter (i.e. the total transport per catchment area) showed that the average release from the Finnish catchments is more than twice the release from the Swedish catchments. Fe and Al exhibit the same concentration pattern as TOC. To estimate the humic fraction of some metals (Fe, Cu and Zn), a speciation study, utilizing a weak anion-exchange resin, was performed on water from rivers with different hydrochemical properties. The humic fractions of Fe, Cu and Zn were 20–40, 40–60 and
A regional ocean circulation model was used to project Baltic Sea climate at the end of the twenty-first century. A set of four scenario simulations was performed utilizing two global models and two forcing scenarios. To reduce model biases and to spin up future salinity the so-called Δ-change approach was applied. Using a regional coupled atmosphere–ocean model 30-year climatological monthly mean changes of atmospheric surface data and river discharge into the Baltic Sea were calculated from previously conducted time slice experiments. These changes were added to reconstructed atmospheric surface fields and runoff for the period 1903–1998. The total freshwater supply (runoff and net precipitation) is projected to increase between 0 and 21%. Due to increased westerlies in winter the annual mean wind speed will be between 2 and 13% larger compared to present climate. Both changes will cause a reduction of the average salinity of the Baltic Sea between 8 and 50%. Although salinity in the entire Baltic might be significantly lower at the end of the twenty-first century, deep water ventilation will very likely only slightly change. The largest change is projected for the secondary maximum of sea water age within the halocline. Further, the average temperature will increase between 1.9 and 3.2°C. The temperature response to atmospheric changes lags several months. Future annual maximum sea ice extent will decrease between 46 and 77% in accordance to earlier studies. However, in contrast to earlier results in the warmest scenario simulation one ice-free winter out of 96 seasons was found. Although wind speed changes are uniform, extreme sea levels may increase more than the mean sea level. In two out of four projections significant changes of 100-year surge heights were found.
Processes controlling,the production,of new,fish (recruitment) are poorly understood,and therefore challenge,population,ecologists and,resource,managers. Sprat in the Baltic Sea is no exception: recruitment,varies widely,between,years and,is virtually independent,of the biomass,of mature,sprat. Sprat is a key,prey and,predator,species in the Baltic ecosystem,and,is commercially,exploited,(1.86 3 10, kg/yr since 1974). The population,and,fishery,must,therefore,be managed,sustainably,and,if necessary,accom- modate,environmental,effects on population,dynamics.,We demonstrate,using 45 years of data that recruitment,depends,on temperature,conditions,during,the months,when,sprat gonads, eggs, and larvae are developing. We also show that recruitment can be predicted before adults spawn,(and fully 15 months,earlier than using present technology) by using linkages between recruitment, large-scale climate variability (North Atlantic Oscillation), Baltic Sea ice coverage, and water temperature. These relationships increase our under- standing of sprat population,dynamics,and enable a desirable integration of fisheries ecology and,management,with climatology,and,oceanography. Key words: Baltic Sea, food-web dynamics; climate change and fish populations; ecosystem management;,environmental,variability; fisheries ecology and management;,fisheries oceanography; population regulation; recruitment; sprat population dynamics.
Ice covers the archipelago area of the tideless northern Baltic Sea regularly in winter and it is considered to be an important factor affecting the upper limit of Fucus vesiculosus L., the only large perennial algal species of the area. The potential of F. vesiculosus for vegetative regeneration after simulated ice scraping was tested experimentally, and untreated F. vesiculosus stands growing in archipelago areas of different wave exposures were monitored in autumn and winter to record their behaviour during the period of formation of ice cover. The capacity of Fucus individuals for vegetative regeneration after simulated ice damage was considerable. Even small parts of residual holdfast tissue in minor irregularities of the rock surface were able to support regeneration after removal of the holdfast. The amount of regeneration decreased with increasing severity of the treatment. All basal parts of the thallus were equally capable of regeneration, and the most important factor affecting the amount of regenerated biomass was the amount of residual biomass. Independently of the severity of the treatment 1 g of residual biomass produced on average 1.3 g of new biomass during the summer period. In autumn, well before the formation of ice cover, the untreated Fucus thalli lost their buoyancy and sank to the bottom. In winter, the persistence of this behaviour was confirmed by SCUBA diving under the ice cover. The sinking phenomenon was most conspicuous in sheltered archipelago areas where the thalli grow large and often reach the water surface. Two different strategies for survival from scraping by sea ice are proposed: in exposed archipelago areas one third of the winters are totally ice free, but when a permanent ice cover is formed wind can easily break it to form pack ice, which can destroy the perennial vegetation to a depth of several meters. When the intensity of the disturbance varies from year to year, effective vegetative regeneration can be a useful strategy to keep the occasionally scraped zone occupied. In sheltered archipelago areas, ice cover is formed every year and no pack ice occurs. The ice-scraped zone extends to a depth of half a metre and varies much less than in exposed areas. Vegetative regeneration is ineffective in a zone which is regularly cleared, because the thalli never have enough time to reach the size needed for reproduction. However, the stability of the disturbed zone makes the attachment of even large surface-reaching Fucus individuals possible just below the ice cover if they can avoid freezing into it. Loss of buoyancy and sinking to the bottom seems to be an adaptation to the winter conditions of sheltered archipelago areas.