The Baltic Sea natural long-term variability of salinity
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MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 622:1-16 (2019) - DOI: https://doi.org/10.3354/meps12994 FEATURE ARTICLE Extension of the growing season of phytoplankton in the western Baltic Sea in response to climate change Norbert Wasmund*, Günther Nausch, Monika Gerth, Susanne Busch, Christian Burmeister, Regina Hansen, Birgit Sadkowiak Leibniz Institute for Baltic Sea Research, Seestr. 15, 18119 Rostock-Warnemünde, Germany *Corresponding author: norbert.wasmund@io-warnemuende.de ABSTRACT: Phenology of phytoplankton was investigated at a coastal station in the western Baltic Sea from 1988 to 2017 by means of microscopically determined biomass and chlorophyll a (chl a) data. The prolongation of the growing season in this marine area is much stronger than that known from terrestrial areas. The growing season, defined by biomass or chl a thresholds, increased by 125 or 129 d, respectively, and extends recently from February to December. The spring bloom started earlier at a rate of 1.4 d yr-1 and the end of the autumn bloom was delayed by 3.1 d yr-1. The duration of the growing season increased at a rate of 4.5 d yr-1. The earlier start of the growing season was correlated with a slight increase in sunshine duration during spring, whereas the later end of the growing season was correlated with a strong increase in water temperature in autumn. The period with sea surface temperature >10°C shifted towards the end of November. Correlations of the duration of the growing season with the phosphate and nitrate concentrations were probably not causative. The shifts in the spring and autumn blooms led to a prolongation of the summer biomass minimum. The earlier spring bloom was caused, among other factors, by a shift of the biomass maximum of the dominant diatom Skeletonema marinoi from May to February/March. The delay in the autumn bloom was induced by a retardation of dominant dinoflagellates and diatoms, such as Ceratium spp. and Dactyliosolen fragilissimus. KEY WORDS: Phenology · Seasonality · Long-term changes · Climate change · Phytoplankton · Chlorophyll · Temperature · Nutrients · Baltic Sea Full text in pdf format Information about this Feature Article Supplementary material NextCite this article as: Wasmund N, Nausch G, Gerth M, Busch S, Burmeister C, Hansen R, Sadkowiak B (2019) Extension of the growing season of phytoplankton in the western Baltic Sea in response to climate change. Mar Ecol Prog Ser 622:1-16. https://doi.org/10.3354/meps12994 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 622. Online publication date: July 18, 2019 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2019 Inter-Research.
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Salinity changes in the Baltic Sea have been attributed to a lack of balance between irregular pulses of North Sea water penetrating the Danish Straits and the freshwater runoff to the Baltic Sea. Both pulses and runoff are controlled by climatic factors in the Atlantic. The occurrence of major pulses of oceanic water has proved unpredictable, being non‐existent during the 1980s. We used dynamic regression models to relate the response of the Baltic Sea salinity to hypothetical controlling factors: westerly winds, freshwater runoff, and, ultimately, the Northern Atlantic oscillation (NAO). Our results provided evidence for general chain‐of‐events relationship between the NAO and a subsequent weather effect over the North Sea, which was finally extended to the Baltic Sea salinity. Westerly winds followed changes in the NAO with a lag of <1 month. Total freshwater runoff to the Baltic Sea followed changes in the NAO with a lag of <2 months. Furthermore, the salinity responded to freshwater runoff with a time lag of <1 yr, and a significant decreasing trend was found in the salinity series. On the basis of the existing lags, we foresee a possibility to predict not only oceanographic, but also biological interactions in the Baltic Sea.
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The semi-enclosed nature and estuarine characteristics, together with its strongly alternating bathymetry, make the Baltic Sea prone to much stronger interannual variations in the abiotic environment, than other spawning habitats of Atlantic cod (Gadus morhua). Processes determining salinity and oxygen conditions in the basins are influenced both by long term gradual climate change, e.g. global warming, but also by short-term meteorological variations and events. Specifically one main factor influencing cod spawning conditions, the advection of highly saline and well-oxygenated water masses from the North Sea, is observed in irregular frequencies and causes strong interannual variations in stock productivity. This study investigates the possibility to use the available hydrographic process knowledge to predict the annual spawning conditions for Eastern Baltic cod in its most important spawning ground, the Bornholm Basin, only by salinity measurements from a specific location in the western Baltic. Such a prediction could serve as an environmental early warning indicator to inform stock assessment and management. Here we used a hydrodynamic model to hindcast hydrographic property fields for the last 40+ years. High and significant correlations were found for months early in the year between the 33m salinity level in the Arkona Basin and the oxygen-dependent cod spawning environment in the Bornholm Basin. Direct prediction of the Eastern Baltic cod egg survival in the Bornholm Basin based on salinity values in the Arkona Basin at the 33 m depth level is shown to be possible for eggs spawned by mid-age and young females, which currently predominate the stock structure. We recommend to routinely perform short-term predictions of the Eastern Baltic cod spawning environment, in order to generate environmental information highly relevant for stock dynamics. Our statistical approach offers the opportunity to make best use of permanently existing infrastructure in the western Baltic to timely provide scientific knowledge on the spawning conditions of Eastern Baltic cod. Furthermore it could be a tool to assist ecosystem-based fisheries management with a cost-effective implementation by including the short term predictions as a simple indicator in the annual assessments.
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