Hypoxia-Related Processes in the Baltic Sea

Abstract

Hypoxia, a growing worldwide problem, has been intermittently present in the modern Baltic Sea since its formation ca. 8000 cal. yr BP. However, both the spatial extent and intensity of hypoxia have increased with anthropogenic eutrophication due to nutrient inputs. Physical processes, which control stratification and the renewal of oxygen in bottom waters, are important constraints on the formation and maintenance of hypoxia. Climate controlled inflows of saline water from the North Sea through the Danish Straits is a critical controlling factor governing the spatial extent and duration of hypoxia. Hypoxia regulates the biogeochemical cycles of both phosphorus (P) and nitrogen (N) in the water column and sediments. Significant amounts of P are currently released from sediments, an order of magnitude larger than anthropogenic inputs. The Baltic Sea is unique for coastal marine ecosystems experiencing N losses in hypoxic waters below the halocline. Although benthic communities in the Baltic Sea are naturally constrained by salinity gradients, hypoxia has resulted in habitat loss over vast areas and the elimination of benthic fauna, and has severely disrupted benthic food webs. Nutrient load reductions are needed to reduce the extent, severity, and effects of hypoxia.

Has eutrophication promoted forage fish production in the Baltic Sea?

“…After that, nutrient concentrations stabilized (Fig. 1), while the hydrographic status of the Baltic Sea changed due to lack of frequent major inflows since the late 1980s, which in combination with high nutrient concentrations led to increased hypoxic areas (Meier 2007;Conley et al 2009). Thus, from this period onwards the negative effects of high nutrient concentrations likely dominate.…”

“…Furthermore, future nutrient levels and availability for biological production are difficult to predict due to long response times to reduced nutrient loads (e.g. Conley et al 2009), combined effects of changing climate and nutrient loads (Hägg et al 2014) and the uncertainty of whether the nutrient loading objectives themselves can be achieved by all Baltic countries.…”

The Future of Baltic Sea Populations: Local Extinction or Evolutionary Rescue?

“…In addition, deep benthic habitats are fragmented owing to increasing occurrence of large oxygen-depleted areas (Conley et al 2009). Finally, stress in the form of decreased salinity and increased temperature of the Baltic ecosystem, is expected as an effect of global warming (Meier 2006;Neumann 2010).…”

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

“…As a consequence of eutrophication, environmental problems like spreading of hypoxia and increased frequency and intensity of cyanobacteria blooms have been observed (Vahtera et al 2007). Hypoxia increased both in the deep offshore waters (Conley et al 2009;Savchuk 2010) and in the coastal zone (Conley et al 2011). …”