Seasonal benthic nepheloid layer in the Gulf of Riga, Baltic Sea: Sources, structure and geochemical interactions

Abstract

The sources of particles, as well as the geochemical structure and interfacial exchange were studied for the summer benthic nepheloid layer of the shallow (50 m) Gulf of Riga. The material was sampled at nine stations during three cruises of August 2001–2003 with a main focus on the deep waters. Suspended particulate matter (SPM) and its major (N, Si, P, Al, Fe, Mn, Ca) elements were determined as the principal parameters. The results indicate the summer benthic nepheloid layer as an important component of the environmental regime of the Gulf of Riga. The nepheloid layer develops as a result of the near-bottom hydrodynamics and a corresponding saline structure below the thermocline. Benthic resuspension and retention of the settling surface particles in the salinity gradient strongly enhance water turbidity in the depths. The magnitude and sustainability of the nepheloid layer might be also promoted via flocculation of colloidal clay and Mn. Clay-associated Al, Fe and Si are the leading elements of SPM. Manganese is the next major contributor and is governed by its depositional input from the upper layer and cycling in the depths. A regression model indicates adsorption of dissolved silicate onto suspended solids in the silicate-rich nepheloid layer. Particulate organic matter and associated CaCO 3 are the dominating biogenic components, which are principally supplied by algae in the trophogenic zone. High sorbed phosphate proportions in the total suspended P agree with the P equivalents of apparent oxygen utilization in the nepheloid layer below the Redfield ratio, indicating phosphate scavenging and deposition with solid particles. These processes control the deep-water P concentration and thus the amounts of P entering the trophogenic layer. The processes operating in the nepheloid layer form a complex mechanism affecting the benthic exchange and hence biogeochemical cycling in the Gulf.