How predictable is high bivalve recruitment in the Wadden Sea after a severe winter? Matthias Strasser a, * , Rob Dekker b , Karel Essink c , Carmen-Pia Gu ¨nther d , Sandra Jaklin e , Ingrid Kro ¨ncke f , Poul Brinch Madsen g , Hermann Michaelis e , Grace Vedel h a Foundation Alfred Wegener Institute for Polar and Marine Research, Wadden Sea Station Sylt, Hafenstraße 43, 25992 List, Germany b Netherlands Institute for Sea Research, P.O. Box 59, 1790 AB Den Burg, The Netherlands c National Institute for Coastal Marine Management/RIKZ, P.O. Box 207, 9750 AE Haren, The Netherlands d Foundation Alfred Wegener Institute for Polar and Marine Research, Columbusstr., 27568 Bremerhaven, Germany e Niedersa ¨chsisches Landesamt fu ¨r O ¨ kologie, Forschungsstelle Ku ¨ste, An der Mu ¨hle 5, 26548 Norderney, Germany f Senckenberg Institute, Marine Science Division, Schleusenstr. 39a, 26382 Wilhelmshaven, Germany g Ribe Amtskommune, Sorsigvej 35, 6760 Ribe, Denmark h Sønderjyllands Amt, Jomfrustien 2, 6270 Tønder, Denmark Received 7 May 2002; accepted 18 July 2002 Abstract Higher than average recruitment among bivalves on the intertidal flats of the Wadden Sea was often observed after severe winters in the period 1940 – 1995. The occurrence of another severe winter in 1995/96 prompted us to test the hypothesis of severe winters leading to universally high bivalve recruitment on a large geographic scale (500 km coastline) in temperate shallow waters. We analysed data sets on bivalve abundance from seven areas in the Dutch, German and Danish Wadden Sea. The longer data sets showed generally higher bivalve recruitment in the 1970s and 1980s than in the 1990s which may be related to the near absence of severe winters since 1987. Considering the period 1988 onwards (the longest possible period in which 1995/96 was the only severe winter), recruitment of Cerastoderma edule was in 1996 - in agreement with our hypothesis - above average in all seven areas investigated. In contrast, recruitment of Macoma balthica and Mya arenaria was for the same period above average only in the southern Wadden Sea (south-west of Jade Bay) but not in the northern Wadden Sea (north of Eiderstedt peninsula). These regional differences may be related to (i) the different topography of the northern Wadden Sea (with barrier islands west of the mainland) compared to the southern Wadden Sea (with barrier islands north of the mainland) and subsequent differential effects of wind-induced currents on bivalve recruitment, (ii) differences in biotic factors such as standing stocks, larval supply or epibenthic predation or (iii) changes in environmental conditions. Our results demonstrate that large-scale comparisons along coasts are an indispensable addition to insights derived from local studies alone. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Winter effects; Recruitment; Intertidal; Cerastoderma edule; Macoma balthica; Mya arenaria 1385-1101/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII:S1385-1101(02)00198-3 * Corresponding author. E-mail address: mstrasser@awi-bremerhaven.de (M. Strasser). www.elsevier.com/locate/seares Journal of Sea Research 49 (2003) 47 – 57