BG Biogeosciences BG Biogeosciences 1726-4189 Copernicus Publications Göttingen, Germany 10.5194/bg-11-6323-2014 Long-term trends at the Boknis Eck time series station (Baltic Sea), 1957–2013: does climate change counteract the decline in eutrophication? Lennartz S. T. https://orcid.org/0000-0001-7040-149X 1 Lehmann A. https://orcid.org/0000-0001-5618-6105 1 Herrford J. 1 Malien F. 1 Hansen H.-P. 1 Biester H. 2 Bange H. W. https://orcid.org/0000-0003-4053-1394 1 GEOMAR Helmholtz Centre for Ocean Research Kiel, Düsternbrooker Weg 20, 24105 Kiel, Germany TU Braunschweig, AG Umweltgeochemie, Institute of Geoecology, Langer Kamp 19c, 38106 Braunschweig, Germany 24 11 2014 11 22 6323 6339 Copyright: © 2014 S. T. Lennartz et al. 2014 This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/ This article is available from https://bg.copernicus.org/articles/11/6323/2014/bg-11-6323-2014.html The full text article is available as a PDF file from https://bg.copernicus.org/articles/11/6323/2014/bg-11-6323-2014.pdf

The Boknis Eck (BE) time series station, initiated in 1957, is one of the longest-operated time series stations worldwide. We present the first statistical evaluation of a data set of nine physical, chemical and biological parameters in the period of 1957–2013. In the past three to five decades, all of the measured parameters underwent significant long-term changes. Most striking is an ongoing decline in bottom water oxygen concentration, despite a significant decrease of nutrient and chlorophyll <i>a</i> concentrations. Temperature-enhanced oxygen consumption in the bottom water and a prolongation of the stratification period are discussed as possible reasons for the ongoing oxygen decline despite declining eutrophication. Observations at the BE station were compared with model output of the Kiel Baltic Sea Ice Ocean Model (BSIOM). Reproduced trends were in good agreement with observed trends for temperature and oxygen, but generally the oxygen concentration at the bottom has been overestimated.

 References Babenerd, B.: Increasing oxygen deficiency in Kiel Bay (Western Baltic) – a paradigm of progressing coastal eutrophication, Meeresforschung, 30, 121–140, 1991. BACC: Assessment of Climate Change for the Baltic Sea Basin, Springer, 2008. Bange, H. W., Bergmann, K., Hansen, H. P., Kock, A., Koppe, R., Malien, F., and Ostrau, C.: Dissolved methane during hypoxic events at the Boknis Eck time series station (Eckernförde Bay, SW Baltic Sea), Biogeosciences, 7, 1279–1284, <a href="http://dx.doi.org/10.5194/bg-7-1279-2010">https://doi.org/10.5194/bg-7-1279-2010</a>, 2010. Bange, H. W., Hansen, H.-P., Malien, F., Lass, K., Dale, A., Karstensen, J., Petereit, C., and Friedrichs, G.: Boknis Eck Time Series Station (SW Baltic Sea): Measurements from 1957 to 2010, LOICZ-Affiliated Activities, 2011. Bendtsen, J. and Hansen, J. L. S.: Effects of global warming on hypoxia in the Baltic Sea–North Sea transition zone, Ecol. Model., 264, 17–26, 2013. Bumke, K., Karger, U., Hasse, L., and Niekamp, K.: Evaporation over the Baltic Sea as an example of a semi-enclosed sea, Contr. Atmos. Phy., 71, 249–261, 1998. Burkey, J.: Matlab Function: Seasonal Kendall Trend Test for Data with and without Serial Dependance, MatLab fileexchange, 2012. Carstensen, J., Conley, D. J., Andersen, J. H., and Aertebjerg, G.: Coastal eutrophication and trend reversal: a Danish case study, Limnol. Oceanogr., 5, 398–408, 2006. Conley, D. J., Humborg, C., Rahm, L., Savchuk, O. P., and Wulff, F.: Hypoxia in the Baltic Sea and basin-scale changes in phosphorus biogeochemistry, Env. Sci. Tech., 36, 5315–5320, 2002. Ducklow, H. W., Doney, S. C., and Steinberg, D. K.: Contributions of long-term research and time-series observations to marine ecology and biogeochemistry, Annu. Rev. Mar. Sci., 1, 279–302, 2009. Diaz, R. and Rosenberg, R.: Spreading dead zones and consequences for marine ecosystems, Science, 321, 926–930, 2008. Feistel, R., Nausch, G., and Wasmund, N.: State and Evolution of the Baltic Sea, 1952–2005: a Detailed 50-Year Survey of Meteorology and Climate, Physics, Chemistry, Biology and Marine Environment, Wiley-Blackwell (an imprint of John Wiley & Sons Ltd), 2008. Franzke, C.: A novel method to test for significant trends in extreme values in serially dependent time series, Geophys. Res. Lett., 40, 1391–1395, 2013. Garcia, H. E. and Gordon, L. I.: Oxygen solubility in seawater: better fitting equations, Limnol. Oceanogr., 37, 1307–1312, 1992. Grasshoff, K., Kremling, K., and Ehrhardt, M.: Methods of Seawater Analysis, Wiley-VCH, 3rd Edn., 1999. Hanninen, J., Vuorinen, I., and Hjelt, P.: Climatic factors in the Atlantic control the oceanographic and ecological changes in the Baltic Sea, Limnol. Oceanogr., 45, 703–710, 2000. Hansen, H. P., Giesenhagen, H. C., and Behrends, G.: Seasonal and long-term control of bottom-water oxygen deficiency in a stratified shallow-water coastal system, ICES J. Mar. Sci., 56, 65–71, 1999. Hansen, J. L. S. and Bendtsen, J.: Effects of climate change on hypoxia in the North Sea – Baltic transition zone, IOPC Ser. Earth Environ. Sci., 6, Copenhagen, Denmark, 2009. HELCOM: Convention on the protection of the marine environment of the Baltic Sea area, Helsinki Convention, 1–27, 1974. HELCOM: Eutrophication in the Baltic Sea – an integrated assessment of the effects of nutrient enrichment in the Baltic Sea region, Baltic Sea Environ. Proc., 115, 1–145, 2009. Hirsch, R. M. and Slack, J. R.: A nonparametric trend test for seasonal data with serial dependence, Water Resour. Res., 20, 727–732, 1984. Hoppe, H.-G., Giesenhagen, H. C., Koppe, R., Hansen, H.-P., and Gocke, K.: Impact of change in climate and policy from 1988 to 2007 on environmental and microbial variables at the time series station Boknis Eck, Baltic Sea, Biogeosciences, 10, 4529–4546, <a href="http://dx.doi.org/10.5194/bg-10-4529-2013">https://doi.org/10.5194/bg-10-4529-2013</a>, 2013. Jonasson, L., Hansen, J. L. S., Wan, Z., and She, J.: The impacts of physical processes on oxygen variations in the North Sea-Baltic Sea transition zone, Ocean Sci., 8, 37–48, <a href="http://dx.doi.org/10.5194/os-8-37-2012">https://doi.org/10.5194/os-8-37-2012</a>, 2012. Koenker, R. and Hallock, K.: Quantile regression, Jour. Econ. Persp., 15, 143–156, 2001. Krey, J.: Die Bestimmung des Chlorophyll a in Meerwasser-Schöpfproben, Meereskundliche Arbeiten der Univeristät zu Kiel, 65, 201–209, 1939. Krey, J., Babenerd, B., and Lenz, J.: Beobachtungen zur Produktionsbiologie des Planktons in der Kieler Bucht 1957–1975, Berichte aus dem Institut für Meereskunde, 1, 1–20, 1980. Kronsell, J. and Andersson, P.: Total regional runoff to the Baltic Sea, HELCOM Baltic Sea Environment Fact Sheets 2012, available at: <a href="http://helcom.fi/baltic-sea-trends/environment-fact-sheets/hydrography/total-and-regional-runoff-to-the-baltic-sea/">http://helcom.fi/baltic-sea-trends/environment-fact-sheets/hydrography/total-and-regional-runoff-to-the-baltic-sea/</a>, 2012. Lass, H. U. and Matthäus, W.: On temporal wind variations forcing salt water inflows into the Baltic Sea, Tellus A, 48, 663–671, 1996. Lehmann, A.: A three-dimensional baroclinic eddy-resolving model of the Baltic Sea, Tellus A, 47, 1013–1031, 1995. Lehmann, A. and Hinrichsen, H.-H.: On the thermohaline variability of the Baltic Sea, J. Mar. Syst., 25, 333–357, 2000. Lehmann, A., Krauss, W., and Hinrichsen, H.-H.: Effects of remote and local atmospheric forcing on circulation and upwelling in the Baltic Sea, Tellus A, 54, 299–316, 2002. Lehmann, A., Myrberg, K., and Getzlaff, K.: Salinity dynamics of the Baltic Sea, Baltic Earth newsletter, 1, 3–5, 2013. Lehmann, A., Hinrichsen, H.-H., Getzlaff, K., and Myrberg, K.: Quantifying the heterogeneity of hypoxic and anoxic areas in the Baltic Sea by a simplified coupled hydrodynamic-oxygen consumption model approach, J. Mar. Syst., 134, 20–28, 2014. Novotny, K., Liebsch, G., Lehmann, A., and Dietrich, R.: Variability of sea surface heights in the Baltic Sea: an intercomparison of observations and model simulations, Mar. Geod., 29, 113–134, 2006. Pitkänen, H., Lehtoranta, J., and Räike, A.: Internal nutrient fluxes counteract decreases in external load: the case of the Estuarial Eastern Gulf of Finland, Baltic Sea, Ambio, 30, 194–301, 2001. Rabalais, N. N., Turner, R. E., Diaz, R. J., and Justic, D.: Global change and eutrophication of coastal waters, ICES J. Mar. Sci., 66, 1528–1537, 2009. Rheinheimer, G. and Nehring, D.: Meereskunde der Ostsee, Springer, 2nd Edn., 1995. Rosenberg, R.: Negative oxygen trends in Swedish coastal bottom waters, Mar. Pollut. Bull., 21, 335–339, 1990. Rudolph, C. and Lehmann, A.: A model-measurements comparison of atmospheric forcing and surface fluxes of the Baltic Sea, Oceanologia, 48, 333–380, 2006. Sanden, P. and Hakansson, B.: Long-term trends in Secchi depth in the Baltic Sea, Limnol. Oceanogr., 41, 346–351, 1996. Schreiber, T. and Schmitz, A.: Improved surrogate data for nonlinearity tests, Phys. Rev. Lett., 77, 635–638, 1996. Sen, P. K.: Estimates of the regression coefficient based on Kendall's Tau, J. Am. Stat. Assoc., 63, 1379–1389, 1968. Smetacek, V., Bodungen, B.v., Knoppers, B., Peinert, R., Pollehne, F., Stegmann, P., and Zeitzschel, B.: Seasonal stages characterizing the annual cycle of an inshore pelagic system, Rapp. P.v. Reun. Cons. int. Explor. Mer, 183, 126–135, 1984. Smetacek, V.: The annual cycle of Kiel Bight plankton: a long-term analysis, Estuaries, 8, 145–157, 1985. Tyler, J. E.: The Secchi disc, Limnol. Oceanogr., 13, 1–6, 1968. UNESCO.: Background Papers and Supporting Data on the International Equation of State of Seawater 1980, UNESCO Technical Papers in Marine Science, 1981. Wasmund, N. and Uhlig, S.: Phytoplankton trends in the Baltic Sea, J. Mar. Sci., 60, 177–186, 2003. Wasmund, N., Andrushaitis, A., Lysiak-Pastuszak, E., Müller-Karulis, B., Nausch, G., Neumann, T., Ojaveer, H., Olenina, I., Postel, L., and Witek, Z.: Trophic status of the south-eastern Baltic Sea: a comparison of coastal and open areas, Estuar. Coast. Shelf S., 53, 849–864, 2001. Wasmund, N., Tuimala, J., Suikkanen, S., Vandepitte, L., and Kraberg, A.: Long-term trends in phytoplankton composition in the western and central Baltic Sea, J. Mar. Syst., 87, 145–159, 2011. Welschmeyer, W. A.: Fluorometric analysis of chlorophyll <i>a</i> in the presence of chlorophyll <i>b</i> and pheopigments, Limnol. Oceanogr., 39, 1985–1992, 1994.