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Fluxes of dissolved nutrients in the organic-rich sediment of Makirina Bay (Croatia) were estimated in in-situ incubation experiments and compared to those calculated from a diagenetic model based on the concentration vs. depth profiles in the pore water extracted from a sediment core at the same sampling site. Benthic fluxes observed in the in-situ experiment were strongly influenced by the decomposition of sedimentary organic matter by bacterial sulfate reduction and precipitation of authigenic pyrite and calcite. Assimilation of dissolved nitrate and ammonia within the sediment resulted in peculiar nitrate and ammonia concentration profiles. Differences between the benthic and diffusive fluxes indicated that the effects of bioturbation and irrigation, which were not taken into account in the model calculation of diffusive fluxes, should not be considered as negligible. They are, however, extremely difficult to estimate because of the heterogeneity of the sediment. Sulfate reduction in the pore water could not be detected, although it was obvious from pyrite precipitation and the isotopic compositions of different sedimentary sulfur species. It is suggested that reoxidation of sulfide and upward diffusion of sulfate from the sediment layer accumulated during past salt production can account for the scattered sulfate vs. depth profile. The distribution and stable isotope compositions of sedimentary sulfur species were studied in more detail. The influx of sulfate was explained by rapid sulfate reduction in the sediment. The sulfate reduction rate was highest in the upper part of the sediment column, as concluded from delta(34)S values. In lower horizons, the concentration of organic sulfur increased, while concentrations of sulfide decreased. The mirror-imaged concentration vs. depth profiles of sulfide and organic sulfur, as well as their stable isotopic compositions, show that they were formed simultaneously, most probably in competitive processes.

Biometric features and physico-chemical conditions are responsible for many of the variables for metal concentrations in indigenous populations of mussels. In order to reduce variations and promote the utility of mussels as bioindicator organisms for environmental mercury concentration levels, individuals of approximately uniform biometric characteristics, selected from a culture of 1-year-old mussels (Mytilus galloprovincialis, Lmk., were transplanted to four locations in the Krka river estuary and nearby coastal area (Eastern Adriatic coast). Biometric parameters of mussels and their total and methylmercury content were analysed four times over a period of 270 days in 1988/89. It was found that total and methylmercury concentrations were significantly correlated with shell weight, wet weight and dry weight of mussels. The estimate of minimal methylmercury concentration in the water of the Krka River Estuary (approximately 20 pg/l), derived from the mussels data, is in a good agreement with recently determined concentration levels of methylmercury (between 50 and 100 pg/l) in waters of the same area. The accumulation efficiency for methylmercury is about 20-50 times higher than for total mercury. Only 1% of the total mercury content, and 20-50% of the methylmercury content in water filtered by the mussels, is accumulated in the shellfish tissues.