A new sampling device for microaggregates in turbid aquatic systems

Abstract

Microaggregates (5 to 500 µm) are the main component of suspended particulate matter in shallow and turbid aquatic environments. Whereas their abundance and size structure have been studied by various optical systems, their composition, aggregation and disaggregation processes, and microbial colonization have been studied only little. This is largely due to inadequate sampling devices for microaggregates. Therefore, we established a new device, allowing the careful collection and documentation of aggregate size and abundance by laser illumination and a regular digital camera. The instrument simultaneously serves as a settling chamber to separate and subsample microaggregate fractions, differing in their settling velocities. Thus, it provides material for studying the biological and chemical composition and microbial colonization of microaggregates with a minimum of manipulation. First results from its application during a semitidal cycle in the German Wadden Sea demonstrate that it yields reliable results of the abundance and size structure of aggregates, reflecting the tidal changes in the hydrodynamic conditions. Total aggregate abundance covaried with dry weight of suspended particulate matter and turbidity. The biological and chemical analysis of aggregate fractions separated during 45 min of settling, showed that the various fractions exhibited distinct differences with respect to aggregate size, dry weight, particulate organic carbon, the carbon per nitrogen ratio (by weight), number of bacteria and bacterial production rates per aggregate. Hence, the application of this sampling device will lead to better insight into the dynamics and qualitative changes of microaggregates in turbid aquatic systems and deepen our understanding of their overall significance in these ecosystems.