engleski

A comparative study on benthic macrofauna in two shallow Mediterranean lakes

The species richness, diversity and seasonal dynamics of benthic macroinvertebrates were studied and compared in two shallow Mediterranean lakes, one covered with dense submerged macrophytes, the other macrophyte poor. Investigation of Lake Jezero (0.37 km2) and Lake Ponikve (0.875 km2) was done during 2000 and 2001 as a part of study of their suitability for water supply on Krk Island (409.9 km2), Adriatic Sea, Croatia. Sampling was done monthly from November 2000 to October 2001. Quantitative samples of macrozoobenthos were collected at five stations in Lake Jezero (from 0.5-11 m depth) and at four stations in Lake Ponikve (from 0.5-7 m depth) using an Ekman grab (sampling area 0.01 m2). Physico-chemical parameters (dissolved oxygen, oxygen saturation, biological oxygen demand, temperature, pH, total dissolved solids, conductivity, nitrate, nitrite, ammonium, orthophosphate, chlorophyll a and transparency) were also measured. Comparing to previous investigations (from 1980s and 1990s) some changes, like gradual increase of dissolved nutrients and productivity, were noticed in both lakes. Considerable differences between Lake Jezero and Lake Ponikve were noticed in the community structure of macrozoobenthos and submerged vegetation. Once diverse and lush (six taxa in 1980s), submerged vegetation is recently completely absent in Lake Jezero. Species richness, average macrozoobenthos diversity and average abundance are recently much lower in Lake Jezero (25 taxa, H'=0.65 and 545 individuals m-2) than in Lake Ponikve (57 taxa, H'=1.56 and 9340 individuals m-2). Natural eutrophication processes in the shallow lake and its shift from vegetation-dominated clear water state to turbid non-vegetated state could be the reason for differences between investigated lakes (Lake Jezero is much older). An additional reason could be the long-lasting high water and sediment concentration of aluminium in Lake Jezero, as evidence of human influence from 1970s up to the present. In order to improve the water quality in Lake Jezero, low stock of grass carp and silver carp was introduced in 1974 and 1976. Emergent macrophytes were also cut periodically. Some benthivorous cyprinid fishes were also introduced occasionally for sport fishery purposes. All these activities could be responsible for present state in Lake Jezero, but they were all done without any careful management and monitoring program, so their influence on changes in benthos community and water quality is not so clear.

macrozoobenthos; diversity; Mediterranean karstic shallow lake; aluminium

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