Long‐term observations on the dynamics of three species of polychaetes living on tidal flats of the Wadden Sea: the role of weather and predator–prey interactions

Abstract

Summary 1. On the tidal flats of the Wadden Sea, the predatory polychaete Nephtys hombergii (Sav.) is sensitive to low winter temperatures, whereas its main prey, the smaller polychaetes Scoloplos armiger (O.F.M.) and Heteromastus filiformis (Clap.) are not. 2. Monitoring macrofauna dynamics during the last three decades (1970–97) in two distant Wadden Sea areas revealed that Nephtys abundance was severely reduced in all eight coldest winters (generally to an average of < 0·1 g ash‐free dry weight m–2), whereas its biomass reached mean values around 1 g AFDW m–2 during periods with mild winters (≤ 5 days with an air temperature remaining below 0°C all day). Over‐winter changes in Nephtys abundance were strongly related to temperature and 70% of the variability in late winter abundance of this species could be explained by temperature characteristics of the foregoing winter. 3. Biomass values of Nephtys observed in late winter predicted to a high degree (90%) the generally somewhat higher values in the subsequent summer. This seasonal change of predator abundance was generally not related to prey density. Only locally (in an exposed low‐biomass area) and rarely (one‐tenth of the years of observation) did low prey supply (if it was lower than simultaneous predator biomass) coincide with a decline, rather than the usual seasonal increase in Nephtys biomass. 4. A negative influence of predator abundance on prey biomass was observed in both prey species over the entire range of observed predator biomass values. The usual seasonal (winter to summer) increase of prey biomass was significantly smaller at high than at low Nephtys biomass and was even reduced to nil at the highest predator density observed. Scoloplos biomass was about four times higher after years with low (< 0·1 g m–2) than after years with high (> 1·0 g m–2) Nephtys biomass. Similar differences were observed in biomass of Heteromastus. 5. It is concluded that winter temperatures act as an important structuring factor in the Wadden Sea zoobenthic community by directly governing the densities of an important infaunal predator and indirectly affecting the abundance of at least two important prey species. Cold winters intervene by starting a new cycle of predator–prey interaction on average once per 3 or 4 years. 6. Predator abundance was more frequently determined by weather conditions than by food supply. Prey abundance was primarily governed by predator abundance and food supply and was only indirectly (via predator abundance) affected by the incidence of cold winters.