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The discharge of groundwater into the sea affects surrounding environments by changing the salinity, temperature and nutrient regimes. This work reports the spatial effects of a submarine groundwater discharge (SGD) on the abundance and structure of the meiofaunal community in the shallow area of Puck Bay (Baltic Sea). Several field expeditions in the years 2009 and 2010 found that low-saline groundwater escapes into the bay from permeable, sandy, near-shore sediments. The SGD literature has grown rapidly during the current decade; however, the effects of this type of disturbance on the shallow sandy bottom fauna have thus far been little studied. We provide evidence that the discharge of groundwater has a clear effect on meiofaunal assemblages in the research area. This effect was reflected in a significant decline of certain meiofaunal taxa, mainly nematodes and harpacticoids, as well as in altered patterns of temporal distribution and small-scale (vertical) zonation of meiofaunal assemblages. Overlooking submarine groundwater discharge processes may lead to serious misinterpretations of ecological data. It is clear that groundwater discharge phenomena should be considered in future scientific studies.

Dissolved reactive Mn (dMn react ) has been determined at the redoxclines of two anoxic deeps from the Baltic Sea (Landsort Deep and Gotland Basin) and two seasonally anoxic freshwater lakes (Lake Dagow and Fuchskuhle, Germany). This dMn react fraction is rapidly oxidised under oxygen atmosphere and is assumed to consist predominantly of Mn(III). There is a distinct increase of dMn react from the outer regions towards the central part of the Landsort Deep. Although the presence of MnO x particles in the Gotland Basin is evidence of ongoing oxidation of reduced Mn species, almost no dMn react was detected. Since completely different processes of Mn oxidation appear rather unlikely, we suggest oceanographic properties are responsible. Lateral currents and intrusions in the Gotland Basin seem to prevent the formation of a stable suboxic zone, a prerequisite necessary for accumulation of dMn react . Such perturbations supply trace amounts of O 2 and H 2 S, causing either immediate oxidation/reduction of dMn react or deterioration of its stabilising ligands. dMn react has also been determined in Lake Dagow with values significantly exceeding the level of the Landsort Deep due to stable stratification of this lake. In contrast, H 2 S appearance throughout the entire water column and a pH react in Lake Fuchskuhle.

Experiments with water samples from the redoxclines of the Black Sea and the Baltic Sea identified a fraction of dissolved Mn which is completely oxidised to solid MnO x within less than 48 h at laboratory conditions. Disproportionation of this dissolved reactive Mn (dMn react ) into Mn (II) and Mn (IV) did not occur. Our data suggest that bacteria using oxygen are responsible for the fast oxidation of dMn react . The operational definition of dMn react is a Mn phase that passes a 0.45 μm filter, but can be separated from remaining dissolved Mn (II) by filtration 48 h after exposure to atmospheric oxygen. The application of this method to water samples from the redoxcline of the Black Sea reveals dMn react profiles comparable to published Mn (III) profiles analysed by polarography thus identifying Mn (III) as the dominating constituent of dMn react . As the degree of autocatalytic oxidation of dissolved Mn (II) by readily produced MnO x and microbial Mn (II) oxidation within the applied oxidation period is unknown, dMn react is at least a semi-quantitative measure of dissolved Mn (III). Furthermore, the present method helps to assess the full potential for oxidation of dissolved Mn within aquatic ecosystems. This method has the advantage that sample preparation can be easily done on site, followed by analysis of dissolved Mn by conventional methods.