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-preyinteractions

1. On the tidal flats of the Wadden Sea, the predatory polychaete Nephtys h ombergii (Sav.) is sensitive to low winter temperatures, whereas its main p rey, the smaller polychaetes Scoloplos armiger (O.F.M.) and Heteromastus fi liformis (Clap.) are not. 2. Monitoring macrofauna dynamics during the last three decades (1970-97) i n 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 a sh-free dry weight m(-2)), whereas its biomass reached mean values around 1 g AFDW m(-2) during periods with mild winters (less than or equal to 5 day s with an air temperature remaining below 0 degrees 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 explai ned by temperature characteristics of the foregoing winter. 3. Biomass values of Nephtys observed in late winter predicted to a high de gree (90%) the generally somewhat higher values in the subsequent summer. T his seasonal change of predator abundance was generally not related to prey density. Only locally (in an exposed low-biomass area) and rarely (one-ten th of the years of observation) did low prey supply (if it was lower than s imultaneous predator biomass) coincide with a decline, rather than the usua l 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 val ues. The usual seasonal (winter to summer) increase of prey biomass was sig nificantly smaller at high than at low Nephtys biomass and was even reduced to nil at the highest predator density observed. Scoloplos biomass was abo ut 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 observe d 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 de nsities of an important infaunal predator and indirectly affecting the abun dance of at least two important prey species. Cold winters intervene by sta rting a new cycle of predator-prey interaction on average once per 3 or 4 y ears. 6. Predator abundance was more frequently determined by weather conditions than by food supply. Prey abundance was primarily governed by predator abun dance and food supply and was only indirectly (via predator abundance) affe cted by the incidence of cold winters.