Abstract
Excessive nutrient inputs over the last century have altered the subtle balance between oxygen supply and oxygen consumption and changed the Baltic Sea from a state with hypoxia confined to the deepest bottom waters to widespread hypoxia in most bottom waters. The Baltic Sea is naturally susceptible to hypoxia because the pronounced vertical stratification in the water column prevents the resupply of oxygen below the permanent halocline, and the salt water inputs from the adjacent North Sea through shallow sills influences both horizontal and vertical water exchange. Analysis of the extensive data available from different countries, their monitoring program and research cruises, has allowed for the computation of basin-wide trends of oxygen conditions over more than a century. The low oxygen zone has increased by a factor of 10 over the last 115 years and has grown from about 5,000 km2 around 1900 to more than 60,000 km2 in recent years. Anthropogenic nutrient inputs are the primary cause of the hypoxia, however, global warming has exacerbated low oxygen conditions. In the estuarine and coastal systems of the Baltic hypoxia is much more variable and strongly dependent on processes controlling vertical mixing. The low oxygen conditions have altered many biogeochemical cycles (P, N, Fe, Mn, S, etc.) and influenced many processes including the nutrients limiting phytoplankton production, altering microbial communities and changing the burial of elements in sediments. In addition, the enhanced accumulation of organic-rich sediments with hypoxia, e.g. the legacy of eutrophication, has increased benthic oxygen demand. Although reductions in nutrient loads have reduced overall eutrophication, especially local conditions, a response is not yet evident in the dynamics of hypoxia. The time lag in responses to current efforts to reduce nutrients is slow, which also challenges management efforts to reduce eutrophication. Additional efforts to achieve nutrient reductions from catchments will be necessary to improve oxygen conditions in the Baltic Sea.
Bio
Daniel Conley is a Professor in Biogeochemistry in the Department of Geology at Lund University, Sweden.
A native of the USA, Conley grew up on the shores of the Atlantic Ocean in Ft. Lauderdale, Florida, obtaining a B.S. at Tulane University, a M.Sc. at the University of Wisconsin – Green Bay and a Ph.D. at The University of Michigan in 1987. After working on the Chesapeake Bay from 1988-1994 at the University of Maryland Center for Environmental Studies, Conley moved to Denmark and worked at the National Environmental Research Institute, part of the Danish Ministry of the Environment, in Roskilde. He held a European Union Marie Curie Chair at Lund University from 2007-2009, where he is currently a Professor.
His research focuses on perturbation of nutrient cycles by human activities and the responses of marine ecosystems to changes in human impact and climate. He is engaged in research on the spread of dead zones in estuaries and coasts, and the impact of low oxygen upon biogeochemical cycles. He carries out research on the global biogeochemical silica cycle and has shown that the Si cycle is dominated by biological processes along the land-sea continuum. Most recently, Daniel has suggested that the first biological impacts on the global Si cycle were likely by prokaryotes during the Archean with further decreases in oceanic DSi with the evolution of widespread, large-scale skeletal biosilicification significantly earlier than the current paradigm.
