Effect of Resuspension on Solute Fluxes and Organic Carbon Degradation in Marine Sediments

Sammanfattning: Internal load has been discussed to be a reason for the still high concentrations of nutrients in the water column, in e.g. the Gulf of Finland (GoF), Baltic Sea, in spite of decreased nutrient load from land. Resuspension is a process that is believed to affect the transport of solutes across the sediment-water interface, and thus affect the internal load. The effects of resuspension have previously been investigated in laboratory and model studies, with contradictory results. In this study advanced in situ measurement techniques have been used to improve the knowledge of the effect of resuspension on benthic fluxes of dissolved oxygen, nutrients (NO3-, NH4+, PO43-, Si(OH)4), dissolved inorganic carbon (DIC), dissolved iron and dissolved manganese, and on sedimentary organic carbon degradation rates. One station, Måvholmsbådan (MB), situated in the Gothenburg archipelago was visited in January 2002. Three stations, PV1, Kasuuni, and XV1, in the GoF were visited during three cruises in June-July 2003, September 2004 and May 2005. MB is situated close to the river mouth of Göta Älv and has a water depth of 12 m. The organic material was refractory due to its terrigenous origin and due to winter conditions. PV1 is situated at an accumulation bottom at 70-75 m depth and the bottom water was anoxic or hypoxic periodically. Kasuuni, at a depth of 49-56 m, is situated on a transport bottom, and XV1, 36-38 m water depth, on an erosion bottom. The oxygen consumption clearly increased in the GoF due to resuspension, but no large effects were observed on the fluxes of nutrients, DIC, dissolved iron or dissolved manganese. The increase in oxygen consumption was probably due to stimulated re-oxidation of accumulated dissolved reduced substances in the sediment and not due to increased degradation rate of organic material. Thus, resuspension events do not contribute to increased internal load of nutrients to the water column. Instead, at low oxygen concentrations in the bottom water, the effluxes of ammonium decreased in 50 % of the cases due to resuspension, possibly as a result of adsorption on resuspended particles. The concentrations of nitrate, phosphate and iron were affected by low oxygen conditions and therefore also the fluxes, but there were no significant changes due to resuspension events. The increased oxygen consumption due to resuspension might lead to spreading of anoxic/hypoxic bottom areas. Thus, there might be indirect effects of resuspension on the benthic fluxes (i.e. on the internal load) of nutrients, dissolved iron and dissolved manganese.