Estuarine aggregates in the longitudinal profile of the River Elbe: seasonal changes in occurrence, characteristics and colonization.
Zimmermann-Timm, Heike; Hoberg, Marcus; Holst Henry, ; Müller, Stefan
Aggregates from the estuarine environment have not been intensively investigated previously. Differences in substrate and colonization were studied using single aggregates from the Elbe Estuary. The composition of the aggregates was determined by the planktonic community in the limnetic section of the estuary and, farther downstream, the terrestrial plant community as well, but exchanges between the sediment and water column in the brackish zone were also of great importance. In the estuary microaggregates, aggregates < 1200 μm, were the most abundant. The mean size of the Elbe aggregates was small, ranging between 50 and 2500 μm, but they reach high abundances in the range from 20 to 4000 aggregates 1-1 and so cause a high turbidity. In contrast to marine and lake snow, estuarine aggregates were densely colonized by bacteria, protozoans and sometimes metazoans. Bacterial numbers on aggregates were high, about 0.3 x 10 6 to 25 x 10 6 bacteria aggregate-1, while attached protozoans numbered from 98 to 22500 1-1. Many metazoa were facultatively attached to particles during the summer. These high numbers of bacteria, protozoans and metazoans associated with single microaggregates have not been reported before. The chemical composition of the aggregates indicates that a great deal of decomposition occurs as the aggregates move downstream. This change during horizontal transport is apparently comparable to that occuring during vertical transport in lakes and the sea. Our results indicate that estuarine aggregates are important microenvironments and must be included in the conceptual framework of estuarine studies because the density to which they are colonized indicates their importance in chemical and energy cycling.