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

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. Tidal flat ecosystems at the transition zone between land and coastal seas are strongly affected by inputs of inorganic nutrients and organic matter both from terrestrial as well as marine origin and thus are one of the most productive marine ecosystems. They act as a filter and sink for a variety of land-born substances running off the coast and are, together with estuaries, of prime importance in land–sea interactions. They exhibit pronounced tidal currents whose dynamics cause intense sedimentation and resuspension of particulates, resulting in permanently turbid water masses with high loads of suspended matter (SPM). Phytoplankton primary production is strongly light-limited and rather low and benthic primary production contributes substantial amounts (Tillmann et al. 2000; Wolfstein et al. 2000). Because of the high input of organic matter, tidal flat ecosystems are usually net-heterotrophic and act as a sink for organic matter (Postma 1981). Despite this high input of organic matter, the SPM is dominated largely by inorganic constituents, and most of it is composed of microaggregates ,500 mm, undergoing pronounced changes and restructuring during current velocity changes (Eisma and Li 1993; Chen et al. 1994; Mikkelsen and Pejrup 1998). At slack water, rather low SPM concen1 Corresponding author: ([email protected]). Acknowledgments We thank O. Axe, C. Duerselen, J. Freund, L. Gansel, H. P. Grossart, O. Joerdel, C. Klotz, S. Kotzur, B. Kuerzel, J. Maerz, B. Rink, F. Roelfs, A. Schlingloff, A. Sommer, E. Stanev, R. Weinert, M. Zarubin, and the captain and crew of RV Senckenberg for technical assistance in the field and in the lab and for excellent cooperation. We also thank the marine physics group, University of Oldenburg, for providing the data from the measuring pole. This work was supported by the Deutsche Forschungsgemeinschaft within the Research Group BioGeoChemistry of the Wadden Sea (FG 432-TP5). trations occur because substantial amounts of the SPM settle out. Differential settling appears to be the most important mechanism to generate rather large aggregates at this time of low shear rates. At mean tide and towards the current velocity maximum (CVM), when high shear rates occur, resuspension results in high concentration of SPM, composed of rather small aggregates (,100 mm). Besides these dramatic changes of aggregate dynamics during tidal cycles, seasonal variations in SPM concentration and composition and in the aggregate size structure have been reported (Behrends and Liebezeit 1999; Mikkelsen 2002; Grossart et al. 2004). It is, however, not well understood how physical forcing—i.e., the flow field—and biological properties of the component particles, aggregates, and the dissolved phase interact and control aggregate dynamics seasonally, but also during tidal cycles. We do not know whether the various fractions of aggregates, differing in size, sticking and settling properties, vary in their biochemical and geochemical composition. Diatoms and bacteria produce mucus material, which affect aggregation (Passow 2002; Bhaskar et al. 2005). Further, diatom species vary in their stickiness and aggregation properties (Kiørboe and Hansen 1993; Passow and Alldredge 1995), and adsorption properties of dissolved organic carbon (DOC)—i.e., the hydrophobicity—in tidal flat systems undergo pronounced seasonal changes (Bakker et al. 2003). We hypothesize that these and other properties are important—but so far neglected—in controlling and thus understanding dynamics of aggregate formation and sedimentation in tidal flat ecosystems. The role of heterotrophic bacteria in the turnover of organic matter on aggregates has been studied quite extensively during the last decade (for review see Simon et al. 2002). Most of these studies have been carried out in pelagic systems, but rivers and estuaries have been studied as well.