Temporal and Spatial Dynamics of Planktonic Rotifers in the Elbe Estuary during Spring

Abstract

The spatial and temporal distribution of planktonic rotifers in the Elbe Estuary, Germany, was investigated at weekly intervals from March to July, 1995. Samples were taken at a fixed site in the Hahnöfer Nebenelbe, and the main channel was surveyed four times at eight stations from Hamburg to the upstream limits of the brackish-water zone. Abiotic and biotic parameters were determined and correlated with rotifer abundance to gain information about the forces that structure the rotifer community in this dynamic environment. A maximum density of 2048ind.l−1was observed at the beginning of June. More than 70 rotifer species were identified during the whole period, but only a few of them appeared at significant densities. The predominant species wereKeratella cochlearis, which accounted for over 32% of the total abundance,Keratella quadrata,Brachionus calyciflorusand species of the generaSynchaetaandPolyarthra. At the end of JulySynchaeta bicornisappeared at the low density of 23ind.l−1at the station farthest downstream. Except for this typical brackish-water species, all rotifers encountered belonged to freshwater taxa, which decreased in abundance rather rapidly toward the river mouth. From March to the middle of June, rotifers grazed predominantly on heterotrophic components of the microbial food-web, such as planktonic and aggregate-associated bacteria, detritus and heterotrophic flagellates. Toward the end of June, the chlorophyllaconcentration increased sharply, and a new rotifer community established itself, feeding mainly on autotrophic organisms in the Elbe Estuary.