Characteristics, Dynamics and Importance of Aggregates in Rivers - An Invited Review

Abstract

The current knowledge of micro- and macroaggregates in running waters as well as an assessment of their importance will be given in this overview. Micro- and macroscopic aggregates are an abundant component of running waters, considering both rivers and estuaries. They are composed of different kinds of organic and inorganic matter and elements, mainly from the aquatic but also from the terrestrial environment. Their production is determined by natural and anthropogenically influenced abiotic and biotic factors. Aggregate abundance, size and composition vary greatly along the longitudinal profile of running waters. Aggregates are actively or passively colonized by bacteria, protozoa (amoebae, ciliates and flagellates) and metazoa (copepods, nematodes, rotifers, veliger larvae) and they show higher loads of nutrients and organisms compared to the surrounding water. Detritus in running waters has been examined over a period of 100 years but investigations of single aggregates began only in 1996, after pioneering works in the marine field and in lakes. Since then studies have been focused on several aspects of aggregate formation, colonization, nutrient recycling and decomposition. Aggregates have been identified as foci of microbial activity. It has been shown that the significance of aggregate – associated organisms and processes varies both temporally and spatially and is also affected by the nutrient status of surrounding water. An important conclusion from recent studies is that the resuspension and sedimentation of aggregates connects pelagic and benthic food webs to an aquatic food web. It also underlines the importance of the microbial food web, with its fluxes among aggregate-associated pico- and nano-organisms, to organisms of higher trophic levels. Predators of higher trophic levels are only able to eat small-sized organisms if they are associated to aggregates.