Non-conservative behaviour of molybdenum in coastal waters: Coupling geochemical, biological, and sedimentological processes

Non-conservative behaviour of dissolved Mo was observed during specific time periods in the water column of the Wadden Sea of NW Germany. In July 2005 dissolved Mo declined within 36 h from a level only slightly below seawater (82 nM) to a minimum value of 30 nM, whereas in August 2002 dissolved Mo revealed a tidal cyclicity with maximum values up to 158 nM at low tide. In contrast, cruises in August 2003 and 2004 displayed an almost conservative behaviour of Mo. The decrease in dissolved Mo during July 2005 and elevated values in August 2002 were accompanied by Mo enrichments on aggregates in the water column of the Wadden Sea. Along with Mo, dissolved Mn showed unusual concentration patterns in July 2005, with values distinctly below the common summer level (by a factor of five). A direct relation between the loss of Mo and scavenging by freshly formed MnOx phases could not be inferred from our data because both metals revealed inverse patterns. Parallel to decreasing dissolved Mo concentrations dissolved Mn showed an increasing trend while particulate Mn decreased. Such finding is compatible with the formation of oxygen-depleted zones in aggregates, which provide suitable conditions for the rapid fixation of Mo and parallel release of Mn by chemically and/or microbially mediated processes. Our assumption is supported by biological (e.g. number of aggregate-associated bacteria) and sedimentological (e.g. aggregate abundance and size) parameters. The production of organic components (e.g. TEP) during breakdown of an algae bloom in July 2005 led to the formation of larger Mo-enriched aggregates, thus depleting the water column in dissolved Mo. After deposition on and incorporation into sandy tidal flats these aggregates are rapidly decomposed by microbial activity. Pore water profiles document that during microbial decomposition of these aggregates, substantial amounts of Mo are released and may replenish and even enrich Mo in the open water column. We postulate a conceptual model for the observed non-conservative behaviour of Mo in coastal waters, which is based on the tight coupling of geochemical, biological, and sedimentological processes. (c) 2007 Elsevier Ltd. All rights reserved.