Description: This study evaluates the effect of submarine ground water discharge (SGD) on biogeochemical processes of sandy sediments of Hel Bight (Poland) in the shallow southern Baltic Sea, using stirred benthic chambers combined to seepage meters, deep pore water profiles and a reactive transport model. The main impacts of fresh anoxic groundwater seepage are due to (1) the efflux of methane; (2) the efflux of phosphate and silicate; (3) the efflux of dissolved organic carbon (DOC) of aquifer origin. Methane from SGD is assumed to be only slightly oxidized within the sediments and potentially reach the atmosphere at a maximum rate of 30 mmol CH4 m−2 d−1. Silicate and phosphate supplied by SGD promote a seep-site net community production rate that is more than twice as compared to adjacent non seeping sites (70 and 30 mmol C m−2 d−1 respectively). However, oxygen uptake rates at the seep site during the night (30 mmol O2 m−2 d−1) are lower than those observed at the reference sites (50 mmol O2 m−2 d−1). We hypothesize that autogenic, relatively labile DOC is available at the reference site, leading to higher oxygen uptake rates as compared to the seep sites where it is being replaced by less reactive DOC originating from the ground water.

Description: Here, the pelagic carbonate system and the d13C signature of dissolved inorganic carbonate (DIC) were investigated in a tidal basin of the southern North Sea, the Jade Bay, with respect to tidal cycles and a transect towards the North Sea in winter time (January and November, 2010). Physical parameters, major and trace elements, and nutrient concentrations were considered, too. Primary production and pelagic organic matter respiration were negligible during winter time. Both, the compositional variations on the transects as well as during the tidal cycles indicate the mixing of North Sea with fresh water. The combined spatial co-variations of different parameters indicate an introduction of fresh water that was enriched in DI12C, metabolites (e.g., ammonia), protons, and dissolved redox-sensitive elements (e.g., Mn2+). During the January campaign, the discharge via the flood gates was limited due to ice cover of the hinterland drainage ditches, allowing for an observation of tidal variations without significant mixing contributions from surface water discharges. Considering a binary mixing model with North Sea and fresh water as end-members, the extrapolated fresh water end-member composition for this campaign is estimated to contain about 3.8 mmol/kg DIC , and enhanced concentrations of NH4+, Mn2+, and protons compared to North Sea water. The fast temporal response of dissolved geochemical tracers on tidal variations in the Jade Bay indicates a continuous supply of a fresh water component. The measured composition of fresh waters entering the Jade Bay via flood gates (end of October, 2010) did not match the values estimated by the binary mixing model. Therefore, the overall fresh water component likely is a mixture between sources originating from flood gates and (in January) dominating submarine groundwater discharge entering the Jade Bay. This model is consistent with the results obtained during the November campaign, when a more important contribution from flood gates is expected and a more variable fresh water end-member is estimated. The co-variations of the concentrations and the stable carbon isotope composition of DIC are applied to evaluate possible superimposed sink-source-transformation processes in the coastal waters and a general co-variation scheme is suggested.