Long-term changes in coastal zoobenthos in the northern Baltic Sea: the role of abiotic environmental factors

Heta Rousi 1 2 Ari O. Laine 0 Heikki Peltonen 2 Pentti Kangas 5 Ann-Britt Andersin 4 Jouko Rissanen 2 Eva Sandberg-Kilpi 3 Erik Bonsdorff 1 0 Metsa hallitus , Natural Heritage Services, Sapokankatu 2, FI-48100 Kotka , Finland 1 Environmental and Marine Biology, Department of Biosciences, A bo Akademi University , FI-20500 Turku , Finland 2 Finnish Environment Institute , PO Box 140, FI-00251 Helsinki , Finland 3 Tva rminne Zoological Station , JA Palm enin tie 260, FI-10900 Hanko , Finland 4 Kapellhamnsva gen , 156, FI-10960 Hango Norra , Finland 5 Aallonkohina , 6 E 54, FI-02320 Espoo , Finland We investigated site-specific changes in a coastal zoobenthic community during 1964 - 2007 in the northern Baltic Sea. Multivariate analysis indicated that the community structure had changed. The amphipods Monoporeia affinis and Pontoporeia femorata decreased drastically in the late 1970s - early 1980s, and by the early 1990s P. femorata vanished and M. affinis abundance was low. The decline of M. affinis and P. femorata was followed by an increase of the bivalve Macoma balthica and the arrival of the polychaete genus Marenzelleria in the 1990s. Trend analyses showed a rising trend for near-bottom temperature from the late 1960s. There was a significant decline in salinity during the early 1980s, which stabilized during the early 1990s. A negative trend was observed for oxygen concentration during the entire study period. There were interannual variations in the phosphorus and nitrogen concentrations, and a step-like increase in phosphorus concentration at the beginning of the 21st century. The correlations of temperature, salinity, oxygen, total phosphorus and nitrogen with zoobenthic communities were examined using Constrained Correspondence Analysis. Temperature was the most highly correlated explanatory variable for the benthic species. The study highlights the importance of long-term data sets in assessing the state and ecological processes of zoobenthic systems. - Knowledge about the nature of population fluctuations (natural or man-induced) is important for establishing the baseline for ecosystem changes (Beukema et al., 1996). The driving forces of marine coastal ecosystems that affect wide sea areas include eutrophication and climate change. Since the 1970s eutrophication has been one of the most serious threats to the Baltic Sea (Bonsdorff et al., 1997; Conley et al., 2009; HELCOM, 2009), which is the worlds largest brackish water basin (area 393 000 km2) (Lepparanta and Myrberg, 2009). Some zoobenthic species, such as the tellinid bivalve, Macoma balthica, respond to eutrophication primarily by population increase (Beukema et al., 1996; Bonsdorff et al., 1997), but if eutrophication proceeds further and results in prolonged hypoxia or anoxia the zoobenthic community will be seriously damaged. The effects of climate change on eutrophication of the Baltic Sea are yet to be unraveled (HELCOM, 2007; Conley et al., 2009). However, since many benthic species in the Baltic Sea live at the limit of their physiological tolerance (Segerstrale, 1957), even minor environmental changes are likely to induce large changes in zoobenthic communities. Abrupt changes in ecosystem structure and function through changes in trophic levels on a large geographic scale, are observed # 2013 International Council for the Exploration of the Sea. Published by Oxford University Press. All rights reserved. For Permissions, please email: today in various marine ecosystems of the world ocean (Mollman et al., 2008). It has been stated that there have been abrupt changes in the Baltic Sea due to climatic changes, eutrophication, and exploitation of marine mammals and fish (O sterblom et al., 2007; Mollman et al., 2008). Although ecosystems may resist the influences of external stress, and the ecosystem change might appear smooth at first, ecosystems can abruptly switch to an alternative state ( Osterblom et al., 2007), which is manifested among other things by changes in community structure. Hypoxia has become a growing world-wide problem in marine ecosystems during recent decades, and in the Baltic Sea the consequences of hypoxia have been severe (Diaz and Rosenberg, 2008; Conley et al., 2011). As temperatures increase, the effects of hypoxia worsen the physiological habitat of benthic organisms (Vaquer-Sunyer and Duarte, 2011; Kabel et al., 2012). Climate change is also likely to result in lowered salinity in the Baltic Sea, due to the predicted increase in precipitation and subsequent freshwater runoff (HELCOM, 2007; Lepparanta and Myrberg, 2009). The distribution of zoobenthos in the Baltic Sea is influenced by environmental gradients, e.g. in salinity and temperature, and there are patchy patterns caused e.g. by oxygen and sediment structure (Bonsdorff, 2006; Laine et al., 2007; Rousi et al., 2011). Species distributions might change because of species-specific environmental preferences and tolerance limits to these abiotic factors. The aim of this study is to analyse the changes in zoobenthos structures in the northern Baltic Sea, and to elucidate the role of environmental factors behind these changes. Species abundance is analysed in relation to hydrography, oxygen, and nutrients. Community characteristics and their structural factors through the study period are interpreted, mainly based on other studies from the Baltic and North Sea areas. We hypothesize that the longterm changes observed in the zoobenthic community of the area have been caused largely by changes in the environmental factors that have been observed in the Baltic Sea during recent decades: increasing temperature, increasing nutrients (eutrophication), and decreasing salinity. We show temporal changes in the zoobenthic community of the northern Baltic Sea and simultaneous changes in environmental factors. Material and methods Study area The area studied is located at the entrance to the Gulf of Finland, in the northern Baltic Sea (Figure 1). The study area consists of two long-term coastal monitoring stations, XLIV (35 m) and XXVI (20 m). The stations were established in 1926 by the legendary and pioneering Prof. Sven G. Segerstrale, who at that time studied changes and dynamics in the zoobenthos of the area (Segerstrale, 1937; Segerstrale, 1960; Segerstrale, 1973; Segerstrale, 1978). The stations are located in a semi-open basin (maximum depth ca. 35 m) in the archipelago zone, connected by a deeper furrow to the open sea. The area is characterized by water exchange and influenced both by outflow of low-salinity waters and inflow of more saline waters of open sea origin. Near-bottom salinity in this brackish water area varies from 6 to 7 psu (practical salinity unit). The physical and chemical factors in the sea area, as well as phytoplankton production and succession, are well known (e.g. Niemi, 1973; Hallfors et al., 1984; Kuparinen et al., 1984; Haapala, 1994). For zoobenthos, the seminal work by Segerstrale ( (...truncated)