Fredrik Wulff

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.

Silicon Retention in River Basins: Far-reaching Effects on Biogeochemistry and Aquatic Food Webs in Coastal Marine Environments

Abstract Regulation of rivers by damming as well as eutrophication in river basins has substantially reduced dissolved silicon (DSi) loads to the Black Sea and the Baltic Sea. Whereas removal of N and P in lakes and reservoirs can be compensated for by anthropogenic inputs in the drainage basins, no such compensation occurs for DSi. The resulting changes in the nutrient composition (DSi:N:P ratio) of river discharges seem to be responsible for dramatic shifts in phytoplankton species composition in the Black Sea. In the Baltic Sea, DSi concentrations and the DSi:N ratio have been decreasing since the end of the 1960s, and there are indications that the proportion of diatoms in the spring bloom has decreased while flagellates have increased. The effects on coastal biogeochemical cycles and food web structure observed in the Black Sea and the Baltic Sea may be far reaching, because it appears that the reductions in DSi delivery by rivers are probably occurring worldwide with the ever increasing construction of dams for flow regulation.

Humans, Hydrology, and the Distribution of Inorganic Nutrient Loading to the Ocean

Abstract Most modern estimates of dissolved nitrogen and phosphorus delivery to the ocean use Meybecks estimates from approximately 30 large rivers. We have derived an extended database of approximately 165 sites with nutrient loads. For both dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP), the logarithmic yields (log [load/area]) can be parameterized as functions of log (population density) and log (runoff/area) (R2 for DIN and DIP approximately 0.6). Landscape production of DIN and DIP is largely assimilated. Even though DIN and DIP follow substantially different biogeochemical cycles, loading for DIN and DIP is tightly coupled (R2 for log DIN versus log DIP approximately 0.8), with a constant loading ratio of about 18:1. Estimates of DIN and DIP fluxes are distributed globally around the world coastlines by using basin population density and runoff at 0.5° increments of latitude and longitude. We estimate that total loads for the 1990s are about three times Meybecks estimates for the 1970s.

Human-induced trophic cascades and ecological regime shifts in the Baltic Sea

A bstractThe ecosystems of coastal and enclosed seas are under increasing anthropogenic pressure worldwide, with Chesapeake Bay, the Gulf of Mexico and the Black and Baltic Seas as well known examples. We use an ecosystem model (Ecopath with Ecosim, EwE) to show that reduced top-down control (seal predation) and increased bottom-up forcing (eutrophication) can largely explain the historical dynamics of the main fish stocks (cod, herring and sprat) in the Baltic Sea between 1900 and 1980. Based on these results and the historical fish stock development we identify two major ecological transitions. A shift from seal to cod domination was caused by a virtual elimination of marine mammals followed by a shift from an oligotrophic to a eutrophic state. A third shift from cod to clupeid domination in the late 1980s has previously been explained by overfishing of cod and climatic changes. We propose that the shift from an oligotrophic to a eutrophic state represents a true regime shift with a stabilizing mechanism for a hysteresis phenomenon. There are also mechanisms that could stabilize the shift from a cod to clupeid dominated ecosystem, but there are no indications that the ecosystem has been pushed that far yet. We argue that the shifts in the Baltic Sea are a consequence of human impacts, although variations in climate may have influenced their timing, magnitude and persistence.

A system analysis of the Baltic Sea

1 Introduction.- 2 Physical Oceanography of the Baltic Sea.- 3 Climate and Hydrology of the Baltic Basin.- 4 Riverine Inputs of Nutrients to the Baltic Sea.- 5 Deposition of Nitrate and Ammonium from the Atmosphere to the Baltic Sea.- 6 Atmospheric and River Input of PCBs, DDTs and HCHs to the Baltic Sea.- 7 Pelagic Plankton Growth and Resource Limitations in the Baltic Sea.- 8 The Challenge of Sedimentation in the Baltic Sea.- 9 Role of Sediments in the Nutrient Dynamics of the Baltic.- 10 Dynamics and Distribution of Hydrophobic Organic Compounds in the Baltic Sea.- 11 Sediments and Macrofauna in the Baltic Sea - Characteristics, Nutrient Contents and Distribution.- 12 Statistical Analysis of Spatial and Temporal Variations in the Baltic Sea.- 13 A Nutrient Budget Model of the Baltic Sea.- 14 A Model of the Biogeochemical Cycles of Nitrogen and Phosphorus in the Baltic.- 15 A Multicompartment, Multi-Basin Fugacity Model Describing the Fate of PCBs in the Baltic Sea.- References.

Management Options and Effects on a Marine Ecosystem: Assessing the Future of the Baltic

Abstract We are using the coupled models in a decision support system, Nest, to evaluate the response of the marine ecosystem to changes in external loads through various management options. The models address all the seven major marine basins and the entire drainage basin of the Baltic Sea. A series of future scenarios have been developed, in close collaboration with the Helsinki Commission, to see the possible effects of improved wastewater treatment and manure handling, phosphorus-free detergents, and less intensive land use and live stocks. Improved wastewater treatment and the use of phosphorus-free detergents in the entire region would drastically decrease phosphorus loads and improve the marine environment, particularly the occurrence of cyanobacterial blooms. However, the Baltic Sea will remain eutrophic, and to reduce other effects, a substantial reduction of nitrogen emissions must be implemented. This can only be obtained in these scenarios by drastically changing land use. In a final scenario, we have turned 50% of all agricultural lands into grasslands, together with efficient wastewater treatments and a ban of phosphorus in detergents. This scenario will substantially reduce primary production and the extension of hypoxic bottoms, increase water transparency in the most eutrophied basins, and virtually eliminate extensive cyanobacterial blooms.

Modeling the Baltic Sea Eutrophication in a Decision Support System

Abstract SANBALTS (Simple As Necessary Baltic Long-Term Large-Scale) is a model of the coupled nitrogen and phosphorus cycles. This model has been developed as an integral part of the decision support system Marine Research on Eutrophications Nest with the overall aim to evaluate management options for reducing Baltic Sea eutrophication. Simulated nutrient and oxygen concentrations as well as transport flows and major biogeochemical fluxes can be analyzed in many different ways, including construction of detailed nutrient budgets and tracing the fate of nutrient inputs. The large amounts of data that exist for this sea makes it possible to validate model results with observations. Major biogeochemical properties of the Baltic Sea are discussed through an analyses of model sensitivity to external forcing and internal parameterizations. Model results emphasize two features that are especially important for ecosystem management: i) impacts of local measures would always be modified by the long-range transports from other regions and ii) the response to significant changes in loads would only be seen after several decades.

Modelling regional and large-scale response of Baltic Sea ecosystems to nutrient load reductions

The entire Baltic Sea, as well as many of its different sub-regions, are subject to eutrophication due to high nutrient inputs. To plan expensive water management measures one needs a tool to quantify effects of different water management policy decisions. The tools implemented here are simulation models based on similar descriptions of biochemical interactions in the water and sediments but coupled to different hydrodynamical models. For the Baltic Proper a 1D physical model with high vertical resolution but horizontally integrated was used. Simulations for 20 years made with 50% load reduction each 5 year show that for this domain and at these scales the recovery would take decades. The most effective is reduction of phosphorus, while reduction of only nitrogen leads to a dramatic increase in cyanobacteria blooms. For the Gulf of Finland a high-resolution 3D hydrodynamic model was coupled to a more crude 3D-box biogeochemical model describing concrete conditions during August and November 1991. In the Eastern Gulf of Finland the effects of a 50% load reduction from the St. Petersburg region are pronounced even after two weeks. Here, nitrogen reduction would be more beneficial than that of phosphorus, both locally and at a larger scale. The conclusion from these simulations is that the difference in effects of nitrogen versus phosphorus reduction is dependent on scales and locations of management.

Carbon flows in Baltic Sea food webs : a re-evaluation using a mass balance approach

Abstract The brackish Baltic Sea has been seen as particularly suitable for studies of food webs. Compared to fully marine ecosystems, it has low species diversity, which means fewer trophic linkages to analyse. The Baltic Sea is also one of the best-studied areas of the world, suggesting that most data requirements for food web models should be fulfilled. Nevertheless, the influence of physical and biological factors on trophic interactions and biogeochemical patterns varies spatially in the Baltic Sea, adding considerable complexity to food web studies. Food web structure and processes can be described and compared quantitatively between areas by estimating the flow of matter or energy through the organisms. Most such models have been based on carbon, though studies of complementary flows of other elements limiting production, such as nitrogen and phosphorus would be desirable. However, since ratios between carbon and other elements are used in calculating these flows, it is crucial, as a first step, to quantify the flows of carbon as accurately as possible. In this study, we used the EcopathII software (ver 3.1) to analyse models of carbon flow through the food webs in the three main areas of the Baltic Sea; the Baltic proper, Bothnian Sea and Bothnian Bay. A previously published study on carbon flow in the Baltic Sea [Elmgren, R. 1984. Trophic dynamics in the enclosed, brackish Baltic Sea. Rapp. P.-V. Reun. — Cons. Int. Explor. Mer. (183) 152–169.] was complemented with the data on respiration and flow to detritus [Wulff, F., Ulanowicz, R. 1989. A comparative anatomy of the Baltic Sea and Chesapeeake Bay ecosystems. In: F. Wulff, J.G. Field, K.H. Mann (Eds.), Flow Analysis of Marine Ecosystems: Theory and Practice. New York: Springer-Verlag.] in order to present complete mass balance models of carbon. The purpose of re-evaluating previous models with new analytic tools was to check how well their carbon flows balance, and to provide a basis for improved mass balance models using more recent data, including nutrients other than carbon. The resulting mass balance networks for the Baltic proper, Bothnian Sea and the Bothnian Bay were shown to deviate from steady state. There was an organic carbon surplus of 45, 25 and 18 g C m−2 year−1 in the pelagic zones of the Baltic proper, Bothnian Sea and Bothnian Bay, respectively. The Ecopath network analysis confirmed that the overall carbon flow was highest in the Baltic proper, somewhat lower in the Bothnian Sea and much lower in the Bothnian Bay. The only clear differences in food web structure between the basins was that the average trophic level was lower for demersal fish in the Bothnian Sea and higher for macrofauna in the Bothnian Bay, compared to the other basins. The analysis showed weakness in our current understanding in Baltic Sea food webs and highlighted areas where improvements could be made with more recent data.

LONG-TERM, SEASONAL AND SPATIAL VARIATIONS OF NITROGEN, PHOSPHORUS AND SILICATE IN THE BALTIC - AN OVERVIEW

Abstract A census for standing stocks of nitrate, ammonia, phosphate, total nitrogen, phosphorus and silicate in the Baltic Sea is given for two 5-year periods (1972–1976 and 1977–1981). It is based on monthly means calculated fromall observations in the ICES databank resolved into a one-degree longitude-latitude grid for 22 depth intervals. The spatial and temporal variations in monthly surface concentrations are described. Features such as the nitrogen limitation of the Baltic versus the phosphorus limitation of the Bothnian Bay are readily found. The total amount of nutrients in the different subareas of the Baltic are calculated, based on a corresponding volume database. The amounts of inorganic and total nitrogen have increased in the latter period, while silicate has decreased during the same period. Though both phosphate and total phosphorus have increased during some seasons in some basins, the total amounts in the entire Baltic Sea have not changed significantly. Although the inputs are, to a large extent, counteracted by internal sinks, the system is far from a steady state. Increasing winter concentrations of nitrogen and phosphorus and the depletion of silicate, indicate an increase of primary production.