Cold Winter Effects on Benthic Macrofauna Communities in Near- and Offshore Regions of the North Sea
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Helgol Mar Res (2006) 60: 224–238 DOI 10.1007/s10152-006-0038-3 ORIGINAL ARTICLE Henning Reiss Æ Katja Meybohm Æ Ingrid Kro¨ncke Cold winter effects on benthic macrofauna communities in near- and offshore regions of the North Sea Received: 1 August 2005 / Revised: 16 January 2006 / Accepted: 23 January 2006 / Published online: 14 February 2006 Ó Springer-Verlag and AWI 2006 Abstract In order to investigate the effect of severe tions, benthic macrofauna is regarded to be a good winters on the long-term variability of benthic macro- indicator of environmental changes in the marine envi- fauna communities four stations from the inner German ronment. The spatial distribution of benthic macrofa- Bight towards the Dogger Bank (North Sea) were unal communities in the southern North Sea is sampled in March 1990 and from May 1995 to 2002. significantly ruled by gradients of different environ- The stations were chosen to reflect a gradient in the mental factors such as temperature, food availability hydrographic regime, temperature and organic matter and hydrography (Heip et al. 1992; Kro¨ncke et al. 2004). supply. We initially hypothesized that the effect of severe Also long-term changes in benthic communities were winters on benthic communities will be more pro- mainly related to winter temperatures and storm fre- nounced in the offshore more stable environment than in quency (Kro¨ncke et al. 1998, 2001; Schroeder 2003). the nearshore regions. Our results clearly showed the However, episodic events such as extremely cold winters opposite pattern. The benthic communities at the near- can dramatically change the diversity and structure of shore stations in the German Bight changed dramati- benthic communities, which may make the detection of cally in species abundance and community structure long-term trends more difficult. For example, Wieking after the severe winter 1995/1996. But the community and Kro¨ncke (2001) observed a reverse development of structure in the German Bight returned to the previous long-term trends in different parts of the Dogger Bank stage within 2 years. In contrast, the offshore stations in due to the cold winter 1995/1996. However, such events the Oyster Ground and at the Dogger Bank remained are characteristic features of marine areas in temperate rather unaffected by the severe winter and changed regions. gradually during the study period. The results are dis- Several investigations in the Wadden Sea and cussed concerning the ecological importance of severe coastal areas have revealed that cold winters affect winters for benthic communities and the utility of our macrofaunal communities often dramatically, causing results for interpreting long-term changes. decreases in species number, abundance and biomass (Ziegelmeier 1964, 1970; Reichert and Do¨rjes 1980; Keywords North Sea Æ Severe winter Æ Long-term Buchanan and Moore 1986; Beukema 1990, 1992; study Æ German Bight Æ Oyster Ground Æ Dogger Bank Beukema et al. 1996;Kro¨ncke et al. 1998, 2001; Strasser and Pieloth 2001; Schroeder 2003; Strasser et al. 2003). Furthermore, highly successful recruitment Introduction after cold winters can result in increased biomass (Beukema 1990, 1992) indicating the importance of Because of its relatively stationary habit and, thus, its cold winters for the structure of littoral benthic com- incapability to avoid occasional unfavourable condi- munities. Although Ziegelmeier (1964, 1970) and Schroeder (2003) described a negative effect of cold winters in the German Bight, only little information is available Communicated by H.D. Franke for more offshore areas of the North Sea. Wieking H. Reiss (&) Æ K. Meybohm Æ I. Kro¨ncke and Kro¨ncke (2001) found a decrease in macrofaunal Department for Marine Research, Senckenberg Institute, abundance on the Dogger Bank in 1996 in compari- Su¨dstrand 40, 26382, Wilhelmshaven, Germany son with 1997 and 1998, which they discussed as an E-mail: [email protected] effect of the extreme winter 1995/1996. Several stud- Tel.: +49-4421-9475267 Fax: +49-4421-9475222 ies in the coastal areas of the southern North Sea 225 indicate that the effects of cold winters may differ between areas. In the Dutch Wadden Sea the effect of Materials and methods the cold winter 1978/1979 was more evident in the lower than at the higher tidal flats (Beukema 1990). Area of investigation In the benthic community at Norderney (East Frisian Wadden Sea) the sublitoral was found more affected In March 1990 and May 1995–2002, four stations by the severe winter of 1978/1979 than the littoral (Fig. 1) were sampled along a transect from the inner (Do¨rjes et al. 1986). In the German Bight Schroeder German Bight towards the Dogger Bank (Stebbing and (2003) found that the sublitoral macrofauna was Dethlefsen 1992). The depth at the four stations varied more affected by the cold winter 1995/1996 at an from 27 m in the inner German Bight to 43 m in the offshore station in the Oyster Ground than at the Oyster Ground (Table 1). The sediments consisted of nearshore stations. These results point to differently mud at station GB2, muddy sands at stations GB5 and severe effects of cold winters on macrofaunal com- OG7 and fine sands at station DG9. munities depending not only on abiotic parameters such as minimum temperature but also on the ben- Sampling and sample treatment thic community structure itself. A long-term study of macrofaunal communities From May 1995 to 2002 sampling was carried out with along a transect from the inner German Bight to the RV ‘‘Senckenberg’’. Samples were taken with a 0.1 m2 Dogger Bank, which was initiated during the ICES/IOC van Veen grab, except in 1995 when a small box corer workshop on ‘‘Biological effects of contaminants’’ in (122 cm2) was used. In March 1990, the samples were March 1990 (Stebbing and Dethlefsen 1992), was anal- taken with two different grabs (Kro¨ncke and Rachor ysed to evaluate the effect of the cold winter 1995/1996 1992). At stations GB2 and GB5 a 0.1 m2 van Veen on the interannual variability of the macrofaunal com- grab, at stations OG7 and DG9 a 0.2 m2 van Veen grab munities. The station transect followed gradients in was used. All samples were sieved over 1 mm screens temperature, organic matter supply and hydrographic and fixed in 4% buffered formalin. conditions from near- to offshore regions (Becker et al. 1992; Reiss and Kro¨ncke 2004, 2005). We hypothesize that (1) the effect of cold winters on Sediment data the benthic macrofauna was more pronounced in off- shore than in nearshore areas where the macrofauna is Sediment samples for the analyses of mud and sand adapted to a high natural variability of environmental contents as well as for total organic carbon (TOC) con- parameters, and (2) that the recovery of the macrofaunal tent analyses were taken from a separate grab. For mud communities will take longer in offshore areas. contents, sediments were sieved over 63 lm mesh size. Fig. 1 Study sites in the North Sea 226 Table 1 Geographical position and depth of the sample sites Station Latitude Longitude Depth (m) GB2 Inner German Bight 54°02¢N 008°03¢E27 GB5 West of Helgoland 54°06.5¢N 007°24¢E37 OG7 Oyster Ground 54°50¢N 005°35¢E43 DG9 Tail End, Dogger Bank 55°30¢N 004°10¢E30 TOC analyses were determined on freeze-dried total A decreasing gradient from the German Bight to- sediment that had been finely powdered and homogen- wards the Dogger Bank was also found for the absolute ised. An aliquot of 10–30 mg was combusted at 1010°C SST. SST in the coldest months (February and March) in a C/N analyser (Vario EL, elementar analysen) fol- were at the offshore stations OG7 and DG9 about 2°C lowing acidification with concentrated HCl in a desic- higher than at the nearshore stations GB2 and GB5. In cator to remove inorganic carbonates (Hedges and Stern contrast, for the warmest month (August) the situation 1984). was inverse, with SST 1–2°C higher in the nearshore than in the offshore region. Furthermore, the differences between the German Bight (GB2 and GB5) and the Sea-surface temperatures Oyster Ground (OG7) and Dogger Bank (DG9) were even more pronounced for bottom water temperatures The Federal Maritime and Hydrographic Agency of in summer, since stations OG7 and DG9 had a stratified Germany (BSH), Hamburg provided weekly data of the water column during summer while stations GB2 and water surface temperature. The sea surface temperature GB5 had a mixed water column throughout the year (SST) anomalies were calculated based on the long-term (Otto et al. 1990; Becker et al. 1992; Reiss and Kro¨ncke average (1973–1993) per month. 2004). Statistics Spatial distribution and temporal variability We used the PRIMER v5 program package to perform of sediments multidimensional scaling for abundance data to reveal similarities between stations (Clarke and Warwick Mud contents were highest at station GB2 (50%) and 1994). Similarities were calculated using the Bray-Curtis decreased towards the offshore stations with about 40% coefficient. Square-root transformation was used prior at station GB5, 20% at station OG7 and about 2% at to computation. Similarity percentage analysis (SIM- station DG9. The intra-annual variability in mud con- PER) was used to analyse dominance of species. To test tents was rather low at all stations, but in the Oyster for a similar multivariate pattern and, thus, a similar Ground (OG7) a thin fluffy layer of mud was occa- temporal development a Spearman rank correlation was sionally found at the top of the sediment, resulting in calculated using the RELATE routine of PRIMER. remarkable small-scale differences between samples in Non-parametric ANOVA (Kruskal–Wallis) was used to terms of surface mud content (H.