Bo G. Gustafsson

Internal ecosystem feedbacks enhance nitrogen-fixing cyanobacteria blooms and complicate management in the Baltic Sea

Abstract Eutrophication of the Baltic Sea has potentially increased the frequency and magnitude of cyanobacteria blooms. Eutrophication leads to increased sedimentation of organic material, increasing the extent of anoxic bottoms and subsequently increasing the internal phosphorus loading. In addition, the hypoxic water volume displays a negative relationship with the total dissolved inorganic nitrogen pool, suggesting greater overall nitrogen removal with increased hypoxia. Enhanced internal loading of phosphorus and the removal of dissolved inorganic nitrogen leads to lower nitrogen to phosphorus ratios, which are one of the main factors promoting nitrogen-fixing cyanobacteria blooms. Because cyanobacteria blooms in the open waters of the Baltic Sea seem to be strongly regulated by internal processes, the effects of external nutrient reductions are scale-dependent. During longer time scales, reductions in external phosphorus load may reduce cyanobacteria blooms; however, on shorter time scales the internal phosphorus loading can counteract external phosphorus reductions. The coupled processes inducing internal loading, nitrogen removal, and the prevalence of nitrogen-fixing cyanobacteria can qualitatively be described as a potentially self-sustaining “vicious circle.” To effectively reduce cyanobacteria blooms and overall signs of eutrophication, reductions in both nitrogen and phosphorus external loads appear essential.

Deoxygenation of the Baltic Sea during the last century

Significance Oxygen-deficient waters are expanding globally in response to warming and coastal eutrophication. Coastal ecosystems provide valuable services to humans, but these services are severely reduced with decreasing oxygen conditions. In the Baltic Sea, oxygen-deficient waters have expanded from 5,000 to over 60,000 km2 with large decadal fluctuations over the last century, reducing the potential fish yield and favoring noxious algal blooms. This increase is due to the imbalance between oxygen supply from physical processes and oxygen demand from consumption of organic material, enhanced by nutrient inputs and temperature increases. Further nutrient reductions will be necessary to restore a healthier Baltic Sea and counteract effects from warming. Deoxygenation is a global problem in coastal and open regions of the ocean, and has led to expanding areas of oxygen minimum zones and coastal hypoxia. The recent expansion of hypoxia in coastal ecosystems has been primarily attributed to global warming and enhanced nutrient input from land and atmosphere. The largest anthropogenically induced hypoxic area in the world is the Baltic Sea, where the relative importance of physical forcing versus eutrophication is still debated. We have analyzed water column oxygen and salinity profiles to reconstruct oxygen and stratification conditions over the last 115 y and compare the influence of both climate and anthropogenic forcing on hypoxia. We report a 10-fold increase of hypoxia in the Baltic Sea and show that this is primarily linked to increased inputs of nutrients from land, although increased respiration from higher temperatures during the last two decades has contributed to worsening oxygen conditions. Although shifts in climate and physical circulation are important factors modulating the extent of hypoxia, further nutrient reductions in the Baltic Sea will be necessary to reduce the ecosystems impacts of deoxygenation.

Reconstructing the Development of Baltic Sea Eutrophication 1850–2006

A comprehensive reconstruction of the Baltic Sea state from 1850 to 2006 is presented: driving forces are reconstructed and the evolution of the hydrography and biogeochemical cycles is simulated using the model BALTSEM. Driven by high resolution atmospheric forcing fields (HiResAFF), BALTSEM reproduces dynamics of salinity, temperature, and maximum ice extent. Nutrient loads have been increasing with a noteworthy acceleration from the 1950s until peak values around 1980 followed by a decrease continuing up to present. BALTSEM shows a delayed response to the massive load increase with most eutrophic conditions occurring only at the end of the simulation. This is accompanied by an intensification of the pelagic cycling driven by a shift from spring to summer primary production. The simulation indicates that no improvement in water quality of the Baltic Sea compared to its present state can be expected from the decrease in nutrient loads in recent decades.

modeling the combined impact of changing climate and changing nutrient loads on the baltic sea environment in an ensemble of transient simulations for 1961 2099

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.

Time-dependent modeling of the Baltic entrance area. 1. Quantification of circulation and residence times in the Kattegat and the Straits of the Baltic sill

A prognostic model for the estuarine circulation in the Baltic entrance area is described. The model is based on the work by Stigebrandt (1983) and is built of a number of flow-regulating physical processes and forced by oceanic sea level, local meteorological conditions, and freshwater supply to the Baltic. It resolves the horizontal variations by dividing the entrance area into 7 sub-basins. The vertical stratification in each sub-basin evolves with time due to mixing, diffusion, and water exchange with adjacent basins. Instead of using a fixed vertical grid, the stratification is described by a variable number of layers created by inflowing water and by a pycnocline retreat near the surface. The model is a highly cost-effective tool compared to high-resolution 3D-models since computations are 105–106 times faster. Still, the model reproduces the stratification on time-scales longer than a few days as verified by comparison with observed time-series. The magnitude of the simulated average estuarine circulation conforms well to independent estimates of the water exchange. The model is used to quantify the temporal and horizontal variability of circulation, mixing, and diffusion. Long-term average rate of work against the buoyancy forces by entrainment and diffusion is calculated for each sub-basin. The highest rates of work against the buoyancy forces by diffusion are found in the northern Kattegat and in the Fehmarn Belt while the lowest rate is found in the Öresund. The total mixing in terms of transformation of water masses is also quantified. The average residence times for surface and deep water are estimated for each subbasin. Residence times longer than 1 mo are found in Fehmarn Belt and in the deep water of southern Kattegat. In other parts of Kattegat and the Samsø Belt the residence times are 1 to 2 wk for surface water and 2 to 3 wk for deep water. The age of the water is defined as the time spent since a particular water mass was in contact with either the sea surface or with the vertical model boundaries has been estimated. By comparing the age distribution in the Kattegat with observations of oxygen concentration, it is concluded that the variability of the ventilation of deeper layers is of less importance to the occurrence of low oxygen concentrations compared to other factors such as interannual variability in primary production.

Response of the Baltic Sea to climate change—theory and observations

The dynamics controlling the response of the Baltic Sea to changed atmospheric and hydrologic forcing are reviewed and demonstrated using simple models. The response time for salt is 30 times longer than for heat in the Baltic Sea. In the course of a year, the Baltic Sea renews most of its heat but only about 3% of its salt. On the seasonal scale, surface temperature and icecoverage are controlled by the atmospheric conditions over the Baltic Sea as demonstrated by e.g. the strong inter-annual variations in winter temperature and ice-coverage due to variations in dominating wind directions causing alternating mild and cold winters. The response of surface temperature and ice-coverage in the Baltic Sea to modest climate change may therefore be predicted using existing statistics. Due to the long response time in combination with complicated dynamics, the response of the salinity of the Baltic Sea cannot be predicted using existing statistics but has to be computed from mechanistic models. Salinity changes primarily through changes in the two major forcing factors: the supply of freshwater and the low-frequency sea level fluctuations in the Kattegat. The sensitivity of Baltic Sea salinity to changed freshwater supply is investigated using a simple mechanistic steady-state model that includes baroclinic geostrophic outflow from the Kattegat, the major dynamical factor controlling the freshwater content in the Kattegat and thereby the salinity of water flowing into the Baltic Sea. The computed sensitivity of Baltic Sea surface salinity to changes of freshwater supply is similar to earlier published estimates from timedependent dynamical models with higher resolution. According to the model, the Baltic Sea would become fresh at a mean freshwater supply of about 60000 m 3 s � 1 , i.e. a 300% increase of the contemporary supply. If the freshwater supply in the different basins increased in proportion to the present-day supply, the Bothnian Bay would become fresh already at a freshwater supply of about 37000 m 3 s � 1 and the Bothnian Sea at a supply of about 45000 m 3 s � 1 . The assumption of baroclinic geostrophic outflow from the Kattegat, crucial for the salinity response of the Baltic Sea to changed freshwater supply, is

Hypoxia in future climates: A model ensemble study for the Baltic Sea

Using an ensemble of coupled physical-biogeochemical models driven with regionalized data from global climate simulations we are able to quantify the influence of changing climate upon oxygen condi ...

Comparing reconstructed past variations and future projections of the Baltic sea ecosystem first results from multi model ensemble simulations

Multi-model ensemble simulations for the marine biogeochemistry and food web of the Baltic Sea were performed for the period 1850‐2098, and projected changes in the future climate were compared with the past climate environment. For the past period 1850‐2006, atmospheric, hydrological and nutrient forcings were reconstructed, based on historical measurements. For the future period 1961‐2098, scenario simulations were driven by

Dynamics of the freshwater-influenced surface layers in the Skagerrak

Abstract The Skagerrak receives large amounts of freshwater, both in the form of rather low-saline waters from the Baltic Sea and the southern North Sea and in pure form from local rivers. These waters, mixed with the underlying Atlantic Water, participate in a variable, mainly wind-driven, cyclonic surface circulation in the Skagerrak. This paper presents a thorough analysis of hydrographic data to provide insight into the distribution and circulation of freshwater in the Skagerrak. From measured salinity and temperature profiles, we compute vertically integrated variables such as the freshwater content, the potential energy and the steric sea level. Furthermore, the density variance is calculated from the density fields. The calculations show that freshwater is accumulated mainly along the Swedish and Norwegian coasts. The computed topography of the steric sea level agrees well with published results for the coast as determined by geodetic methods. This shows that the steric effect gives the dominating contribution to the topography of the mean sea level in the Skagerrak. The computed distribution of freshwater along the coasts is thus largely verified by the geodetically determined mean sea level. We find strong indications of an extensive recirculation of freshwater within the Skagerrak, due to advection/dispersion from the storage along the Norwegian coast. The horizontal distribution of density variance in the upper 10 m mirrors the spreading of low-saline water from the Kattegat. Deeper down there are maxima in density variance along the coasts, reflecting occasional downwelling and strengthening of the baroclinic coastal currents due to time-varying winds. The mean potential energy along the coasts increases monotonously in the cyclonic direction, from the inflow region in the southwest to the outflow region off the western part of the Norwegian Skagerrak coast. The corresponding flow increase is mainly due to incorporation of Atlantic Water into the surface layer. Using a rough energy budget for the Skagerrak, we find that the potential energy of the coastal current system cannot be maintained by wind-driven diapycnal mixing. We suggest that isopycnal downwelling along the coasts provides the necessary energy. The considerable annual variation found in potential energy in the coastal currents covaries with the variation of the wind stress amplitude.

Impact of Climate Change on Ecological Quality Indicators and Biogeochemical Fluxes in the Baltic Sea: A Multi-Model Ensemble Study

Multi-model ensemble simulations using three coupled physical–biogeochemical models were performed to calculate the combined impact of projected future climate change and plausible nutrient load changes on biogeochemical cycles in the Baltic Sea. Climate projections for 1961–2099 were combined with four nutrient load scenarios ranging from a pessimistic business-as-usual to a more optimistic case following the Helsinki Commission′s (HELCOM) Baltic Sea Action Plan (BSAP). The model results suggest that in a future climate, water quality, characterized by ecological quality indicators like winter nutrient, summer bottom oxygen, and annual mean phytoplankton concentrations as well as annual mean Secchi depth (water transparency), will be deteriorated compared to present conditions. In case of nutrient load reductions required by the BSAP, water quality is only slightly improved. Based on the analysis of biogeochemical fluxes, we find that in warmer and more anoxic waters, internal feedbacks could be reinforced. Increased phosphorus fluxes out of the sediments, reduced denitrification efficiency and increased nitrogen fixation may partly counteract nutrient load abatement strategies.