Assessments of the environmental status of the Baltic Sea as called for by the Marine Strategy Framework Directive (MSFD) must be based on a set of indicators. A pre-core indicator is the diatom/dinoflagellate index (Dia/Dino index), which reflects the dominance of diatoms or dinoflagellates during the phytoplankton spring bloom. Here we explain the principles of the Dia/Dino index and the conditions for its calculation using examples from two very different water bodies, the Eastern Gotland Basin and Kiel Bay. The index is based on seasonal mean diatom and dinoflagellate biomass values. A precondition for its applicability is the coverage of the bloom. As a criterion, the maximum value of diatom or dinoflagellate biomass has to exceed a predefined threshold, e.g., 1000 µg/L in the investigated areas. If this condition is not fulfilled, an alternative Dia/Dino index can be calculated based on silicate consumption data. Changes in the dominance of these two phytoplankton classes impact the food web because both their quality as a food source for grazers and their periods of occurrence differ. If diatoms are dominant, their rapid sinking reduces the food stock for zooplankton but delivers plenty of food to the zoobenthos. Consequently, the Dia/Dino index can be used to follow the food pathway (Descriptor 4 of MSFD: "food web"). Moreover, a low Dia/Dino index may indicate silicate limitation caused by eutrophication (Descriptor 5 of MSFD: "eutrophication"). The Dia/Dino index was able to identify the regime shift that occurred at the end of the 1980s in the Baltic Proper. Diatom dominance, and thus a high Dia/Dino index, are typical in historical data and are therefore assumed to reflect good environmental status (GES). In assessments of the environmental status of the Eastern Gotland Basin and Kiel Bay, Dia/Dino index GES thresholds of 0.5 and 0.75, respectively, are suggested. The GES thresholds as calculated by the alternative Dia/Dino index are 0.84 and 0.94, respectively.

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Transition zones between marine and freshwater environments are characterized by a pronounced salinity gradient and concomitant variation in invertebrate species richness. Here we use the β-diversity concept to depict the species turnover of macrobenthic species along the salinity gradient of the Baltic Sea with salinities ranging from 34 in the transition zone to the North Sea to less than 5 in the Bothnian Sea. Based on 250 data sets from 72 locations that were grouped into 2 habitats defined according to their depths and sediment types (shallower: 15 to 19 m, fine to medium sand; deeper: 20 to 35 m, silt to silty sand), we calculated the Jaccard dissimilarity index (β 1-J ) as a measure of species turnover. To keep the focus on the salinity gradient, sediment characteristics and the time period covered by the data sets (spring and summer 1995 to 2005) were predefined. The mean hydrographic parameters, including temperatures, salinities and dissolved oxygen (DO) concentrations of the sample locations were derived from model calculations based on data gathered over a 3 yr period before sampling. At the deeper stations, the total number of macrofaunal species was 255, while at the shallower ones, 172 taxa were found. Statistical analyses revealed salinity to be the main structuring factor for macrobenthic species turnover. High correlations between the β 1-J index and mean salinities in both habitats (Spearman's rank correlation coefficient ρ = 0.88 in shallower and ρ = 0.86 in deeper areas) confirmed these findings. β-diversity values with median β 1-J varied between 51 and 65% within the salinity classes eu-, poly-, α-meso-and β-mesohaline, while values of 75 to 100% characterized between-group comparisons. Furthermore, these high β-diversity values depict a discontinuous change in the communities and are found at salinities of around 10, 18, and 30, which ties in fairly well with the existing salinity boundaries postulated by the Venice System. KEY WORDS: Species turnover · β-diversity · Salinity gradient · Macrofauna · Venice System · Baltic Sea Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 436: 101-118, 2011 102 agreed on the Venice System (Caspers 1959), in which the following generalized salinity class boundaries were defined: euhaline zone > 34 to 30, polyhaline zone 30 to 18, α-mesohaline zone 18 to 10, β-mesohaline zone 10 to 5, α-oligohaline zone 5 to 3, β-oligohaline zone 3 to 0.5, and limnetic zone 0.5 to 0. The System is widely used, e.g. in the implementation of the European Union's (EU) Water Framework Directive in the coastal waters of the Baltic Sea (von Weber 2004). The Venice System's boundaries are also consistent with distribution limits determined by coenosis (Hiltermann 1963). Nevertheless, other studies have claimed that no objective criteria exist for these boundaries (e.g. Bulger et al. 1993). Except for the minimum species richness at salinities from ~5 to 8, the boundaries of the Venice System cannot be derived from Rema...

Sediment cores of 20 cm diameter contammg the natural benthic fauna were subjected to low oxygen conditions in a laboratory microcosm system. After several days of oxic conditions ('oxic stage') the oxygen content of the water was reduced to 25% saturation for 15 d ('hypoxic stage'), followed by a 'reoxygenation stage'. Effective solute transport rates were calculated using measurements with the conservative tracer ion bromide. Profiles of oxygen and SC02 were measured and molecular diffusive as well as effective fluxes, accountmg for effective solute exchange, were calculated. The overall response of the benthic community was to compensate for low oxygen content of the overlying water by increased pumping activity. On average, effective diffusion coefficients (Den} were 3 times higher in hypoxia than under oxic conditions. Den reached 1.5 x cm2 ssl, a value 30 times that of molecular diffusion. During hypoxia we observed low molecular diffusive O2 flux, higher effective O2 flux, as well as an increase in SC02 within the sediment. We interpret this as a shift of transport away from diffusion within the bulk sediment interstices (oxic conditions) to the advective transport pathways along burrows during hypoxia. This facilitates fast transport of oxygen and bromide along burrows and contrasts with the slower transport of C 0 2 from the interstices governed by molecular diffusion. In this transient situation calulations based on gradients result in an unrealistic molar ratio of fluxes (C02 /02) as high as 11.