Helgol Mar Res (1999) 53:45–55 © Springer-Verlag and AWI 1999 ORIGINAL ARTICLE M. Strasser · M. Walensky · K. Reise Juvenile-adult distribution of the bivalve Mya arenaria on intertidal flats in the Wadden Sea: why are there so few year classes? Received: 18 June 1998 / Accepted: 12 November 1998 Abstract Patchy distribution is frequently observed in ties >30 ind m–2 did not prevent relatively high M. aren- benthic marine invertebrates. In order to indentify fac- aria recruitment of >500 ind m–2. tors causing spatial patterns in the bivalve Mya arenaria, abundances of juveniles and adults, as well as death as- Key words Mya arenaria · Predation · Recruitment · semblages, were recorded on a 20-km scale in the inter- Population dynamics · Patchiness tidal zone of the Sylt-Rømø Bight. Both adults and juve- niles exhibited pronounced patchiness. Shell length of juveniles rarely exceeded 2 mm in 1995, which was most Introduction likely a consequence of epibenthic predators truncating the size spectrum. Only a few yearclasses dominated the Population structure and dynamics of Mya arenaria L. in adult population. While the northern part of the Bight the European Wadden Sea are characterized by the oc- was colonized mainly by a 1993-cohort, most M. aren- currence of adults in locally restricted dense beds (Kühl aria in the southern part were from the mid-1980s. It is 1955) and by a large annual variation in reproductive hypothesized that epibenthic predation is a major cause success (Beukema 1982, 1992). Patchy distribution of of the lack of dense M. arenaria beds from other years. adult M. arenaria is also reported from the Swedish west However, examination of the length–frequency distribu- coast (Möller and Rosenberg 1983) and the east coast of tion of death assemblages revealed that other unidenti- North America (Brousseau 1978; Emerson et al. 1988). fied causes of mortality exist. High abundances of adults Differential reproductive success in M. arenaria may in were found in the mid and lower intertidal zone but not part be explained by low recruitment after mild winters in the high intertidal zone. There was no indication that and heavy spatfall after severe winters (Kühl 1955; dispersal of M. arenaria spat in a landward or seaward Beukema 1982, 1992; Reise 1987). However, low re- direction contributed significantly to the observed distri- cruitment after the cold winter of 1986 shows that repro- bution pattern, since spat occurred abundantly at all tidal ductive success cannot be predicted by winter tempera- levels except in the high intertidal zone. There was no ture alone (Günther 1992). evidence of negative adult-juvenile interaction. M. aren- In general, distribution patterns of benthic organisms aria was not attracted by seagrass or projecting shell with pelagic larvae are shaped by pre- and post-settle- beds – the latter indicating erosion of the sediment – as ment processes. Concerning pre-settlement processes, abundances of adults and juveniles were generally low in Kühl (1981) stated that M. arenaria is capable of testing these habitats. The effects of sediment type and of the the substrate before settlement. This is supported by a bioturbating lugworm Arenicola marina were inconsis- large body of literature in which sediment preferences tent. While adults were more abundant on muddy sand of M. arenaria spat were found. However, these results than on sand, recruitment was independent of sediment are inconsistent. For example, silty sediment was pre- type. At all high density sites of adults (>50 ind m–2), ferred in some field studies (Reise 1987; Günther 1992), lugworm densities were below 5 ind m–2, which may in- coarser substrates in others (Brousseau and Baglivo dicate a negative interaction. However, lugworm densi- 1988; Beal 1989), or no particular substrate at all (Smidt 1951). Contradicting results were likewise reached on adult-larval interactions. Total bivalve spat was reduced M. Strasser (✉) · M. Walensky · K. Reise by high numbers of adult M. arenaria and Cerastoder- Wadden Sea Station Sylt, Alfred-Wegener-Institute for Polar and Marine Research, D-25992 List, Germany ma edule (L.) in an experimental arrangement on the e-mail: [email protected] Swedish west coast (André and Rosenberg 1991), but Tel.: 49-4651-956-0; Fax: +49-4651-956-200 numbers of M. arenaria recruits were not affected by 46 dense beds of M. arenaria in the Wadden Sea (Kühl In conclusion, several processes are known to have a 1955; Günther 1992). strong potential to shape the population structure of M. Among post-settlement processes, predation has been arenaria. Here we examined the relative importance of identified as an important regulator of community struc- some of these factors on a large scale (about 20 km). The ture of infaunal invertebrates including M. arenaria (e.g. aim of this study was (1) to determine abundances and Pihl 1982; Jensen and Jensen 1985; Reise 1985; Günther distribution of adults and spat, as well as death assem- 1992). Although young post-larval M. arenaria have the blages, as a prerequisite to investigating whether (2) adult potential for dispersal by byssus drifting (Armonies M. arenaria beds have an adverse effect on the abun- 1994), this was of minor importance for the distribution dance of spat and whether (3) the distribution patterns of pattern of M. arenaria recruits in the German Bight spat and adults of M. arenaria are influenced by tidal lev- (Günther 1990). However, on exposed sandy flats, juve- el, sediment type, A. marina density, the presence or ab- nile clams may be dislocated by bedload sediment trans- sence of seagrass, and projecting M. arenaria shells. port (Emerson and Grant 1991; Armonies 1998). Other physical and biological disturbances may occasionally affect M. arenaria populations. M. arenaria disappeared Materials and Methods from the Danish Wadden Sea after severe winters in the early 1940s (Smidt 1944). Ice scouring reportedly killed Sampling took place in the Sylt-Rømø Bight, a shallow tidal basin M. arenaria spat living close to the upper part of the sed- in the North Sea, 54°50’–55°10’ N ; 8°20’–8°40’ E. The Bight is bound on two sides by two barrier islands, and their respective iment surface, while deeper living adult specimens re- causeways to the mainland. The water exchange between the Bight mained largely unaffected (Kühl 1951). The lugworm, and the North Sea is confined to a narrow tidal inlet between the is- Arenicola marina (L.), a dominant bioturbator on the in- lands. Here, maximum tidal current velocities are about 1.3 m s–1. tertidal flats of the Wadden Sea, was found to have an The Sylt-Rømø Bight encloses 404 km2, of which one third be- longs to the intertidal zone. This zone consists largely of sand flats adverse effect on M. arenaria recruitment in a field ex- (72%) and to a lesser extent of muddy sand (25%) and mud flats periment (Flach 1992). (3%). The low water volume of the Bight is about 570×106 m3 and is doubled at high tide to a volume of about 1120×106 m3. The tides are semi-diurnal with a mean tidal range of about 2 m. Salini- ty normally remains close to 30 psu. Mean annual water tempera- Fig. 1A,B Mean abundance of Mya arenaria in the intertidal zone ture is 9 °C with a summer average of 14 °C and a winter average north of Rømø harbor, northern Wadden Sea, in 1995. For better of 5 °C. Localized water circulation patterns are generated by the visualization of the sampling sites the intertidal zone below the flood and ebb tides in combination with changing wind conditions. dotted line is enlarged by 110%. A Adults. B Juveniles. At perma- More about the area is given in Gätje and Reise (1998). nent sites (RI, RII, RIII) abundances are averaged over five sam- Spat, adults, and death assemblages of M. arenaria were sam- pling dates between July and September 1995 pled along the east coasts of the islands Rømø (northern part of 47 Fig. 2A,B Mean abundance of M. arenaria in the intertidal zone of Königshafen Bay, northern Wadden Sea, in 1995. A Adults; B juveniles the Bight) and Sylt (southern part of the Bight). The two areas dif- al level (MTL), (2) mid-intertidal zone: –0.5 to 0.0 MTL, (3) up- fered both in size and environmental conditions: per low intertidal zone: –0.7 to –0.5 MTL, (4) lower low intertidal zone: below –0.7 MTL. 1. The northern part (Fig. 1) is dominated by muddy sand. In ad- The sites were selected using a stratified random sampling pro- dition, an extensive mud flat is situated in the corner between cedure. The uneven number of sites at tidal levels and locations Rømø and the causeway. This mud was too soft to walk across (Table 1) roughly reflects the areal proportions in the Sylt-Rømø and was spared from sampling. The sampling area comprised Bight. For practical reasons, 11 sites in the western part of Köni- about 5.0 km2. gshafen Bay were sampled in a straight line (Fig. 2). Because 2. The southern part is dominated by sand and muddy sand. The these sites were situated at different tidal levels and on different intertidal zone of Königshafen Bay (Fig. 2) covers an area of sediments, they are treated as random samples. At each sampling about 4.0 km2. Sand flats make up 88% of this area. Muddy site the following parameters were recorded: (1) sediment type: sand and mud flats are only encountered in two small areas. sand (0 –10% of the particle fraction <0.063 mm) or muddy sand The area from List harbour to Munkmarsch (Fig.