Physical and biogeochemical controls of microaggregate dynamics in a tidally affected coastal ecosystem

Abstract

Tidal flat ecosystems exhibit pronounced tidal currents that cause high loads of suspended matter (SPM) and intense sedimentation. To identify systematic patterns of tidal SPM dynamics and the significance of physical forcing versus microbial processes in aggregation processes, we conducted a comprehensive study from January 2002 to October 2004 in a backbarrier tidal flat area of the German Wadden Sea. Further, various aggregate fractions were separated by their settling properties in June and October 2004, applying a new sampling device. Tidal dynamics of SPM, particulate organic carbon (POC), aggregate abundance and size, chlorophyll a (Chl a), the carbon to nitrogen ratio (C:N), numbers of bacteria, and DOC often exceeded seasonal dynamics of the tidal means of these properties. SPM, POC, Chl a, and aggregate abundance were positively correlated and aggregate size negatively correlated to the current. DOC concentrations and total bacterial numbers exhibited minima at high tide and maxima at low tide. Aggregate quality - i.e., POC:SPM, Chl a, size, amino acid content, and bacterial colonization - varied tidally among the fractions, relative to bulk SPM and was different in June and October. In June, tidal dynamics of these properties and bacterial biomass production were higher than in October. Aggregate abundance was substantially lower during the growing season and aggregate size larger than in fall and winter. Microbial processes were important during the growing season in affecting tidal dynamics of aggregation and sedimentation, whereas in fall and winter, physical forcing was the main factor controlling aggregate dynamics. © 2006, by the American Society of Limnology and Oceanography, Inc.