CHIRONOMID LARVAL ASSEMBLAGES IN THE ENCLOSED RHINE-MEUSE DELTA - SPATIOTEMPORAL PATTERNS IN AN EXPOSURE GRADIENT ON A TIDAL SANDY FLAT

Chironomid species composition and distribution in relation to exposur e were studied on a tidal sandy flat at seven sites (average depth 0-5 0 cm), differing in the duration and frequency of the exposure periods . The chironomid assemblages can be divided into an 'upper eulittoral' one, consisting of Chironomus acutiventris, Ch. balatonicus, Einfeldi a carbonaria, Stictochironmus histrio, Cryptochironomus sp. and Cladot anytarsus sp., and a 'lower eulittoral' one consisting of Lipiniella a renicola, Ch. nudiventris and Procladius sp. Small S. histrio, L. aren icola and E. carbo naria larvae appeared first on frequently exposed s ediments and dispersed later over the entire gradient. Small Ch. nudiv entris larvae appeared first at submerged sedi ments. The high micro-p hytobenthos biomasses observed (chlorophyll-a >150 mg m(-2)) are proba bly responsible for the high chironomid densities and biomasses. Preda tion by waders may have had an important impact on chironomid densitie s and species composition at exposed sites. The influence of sediment texture, temperature stress, settlement and migration of early instar larvae, chironomid predator-prey interactions and competition for spac e between large and small chironomids on the establishment of a zonati on pattern are discussed.