engleski

Effect of anoxic stress on density and distribution of sediment meiofauna

Background and purpose: Present study is designed to examine suitability of meiobenthic approach in an oxygen defficiency monitoring. Progressive oxygen depletion that occurred during the summer and autumn of 1989 in the central and eastern part of the northern Adriatic Sea culminated by the end of November in a short-term but extensive anoxic event. A final outcome was a strong disturbance in an ecological balance and mass mortality of macrobenthic organisms. There is a quantity of papers on the effects of acute oxygen defficiency in benthic communities, but most of them concerned macrobenthic component. Meiofauna response to acute oxygen deficiency is scarcely examined, and mainly accompanied by negative findings. The study reported here provides the comparative data from nine northern Adriatic stations distinguished by different level of oxygen deficiency. It aimed to establish if, in what manner, and how much anoxic conditions influence the local density and distribution of sediment-living meiofauna. The results obtained at the most affected stations were partially compare with an adequate data obtained before and after the anoxia occurrence. Material and methods: Sediment samples were collected by SCUBA divers, using plastic hand corers with an inner diameter of 3.5 cm. The material was treated by standard laboratory procedure including fixation, elutration, sieving (100 and 50 ľm mesh size), counting, and classification of the main meiofaunal taxa under a dissecting microscope. The raw data (means of three replicate per sample) were subjected to statistical analyses. Results and conclusion: The use of meiofaunal assemblages for the detection and quantification of perturbation induced by acute oxygen deficiency is examined. The effect of anoxic stress on density and distribution of meiofauna was clear and well-defined. Comparative investigations revealed an oxygen dependent zonation of total meiofauna, main taxa densities and Ne/Co ratio along a spatial gradient of disturbance. A meiobenthic respons was quantitative, taxon specific, and similar to respons tipically occured in chronically affected ecosystems (eg. Baltic Sea, Black Sea). It was found that the intensity of stress respons depends on the level of oxygen deficiency, and the time span of stress conditions. In parts of the hypoxic area distinguished by a lower intensity of oxygen deficiency (O2 saturations >20%) no signs of disturbance on meiofauna community level appeared. Parts of the hypoxic area distinguished by O2 saturation < 20% seem to be moderately affected by stress conditions, as shown by reduced density values. At stations hit by anoxic stress the meiofaunal response was evident by a significant decrease of Nematoda, Copepoda, and total meiofauna densities, in an increase of the Ne/Co ratio, and in an increase of percentage participation of nematodes in the communities composition. According to results of present study, a meiobenthic approach could be succesfully aplied in the detection and quantification of perturbation induced by an oxygen depletion. In distinction from monitoring of organic pollution, the high Ne/Co ratio seems to be useful indicator of anoxic stress conditions.

meiofauna; oxygen deficiency; anoxic event; spatial distribution; Nematoda; Copepoda

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