IMPACT OF A NET CAGE FISH FARM ON THE DISTRIBUTION OF BENTHIC FORAMINIFERA IN THE NORTHERN GULF OF EILAT (AQABA, RED SEA)

Abstract

Two sediment transects, at 23 m depth, were sampled below a commercial fish farm at the northern end of the Gulf of Eilat in order to describe the benthic foraminiferal community along an organic enrichment gradient. Sediment organic matter, porewater ammonia and phosphate concentrations reflected the input from the overlying fish cages and there was generally a decrease in these variables with distance from the point source. Rose-bengal stained foraminifera were found at all of the stations sampled below the fish farm, i.e. there were no azoic sediments. Fifty-one species of foraminifera (>45 μm) were found in the sediments around the fish farm but most were rare species. The most abundant (generally > 83% of the foraminifer abundance) species (stained + unstained) found were Amphisorus hemprichii, Amphistegina lessonii, Assilina ammonoides, Borelis schlumbergeri, Challengerella bradyi, Hauerina diversa, and Peneroplis planatus. Five of these foraminifera are algal symbiont bearing forms. The diatom-bearing species, A. lessonii and A. ammonoides, constituted 40 to 50% of the total number of tests. The most abundant stained species were A. ammonoides, B. schlumbergeri, H. diversa and P. planatus. Abundances of foraminifera were highest in the “hypertrophic” zone, adjacent to the fish cages. The most abundant species showed negative correlations between total abundance of tests and organic matter in the upper centimeter of the sediment. Negative correlations were also found between abundances of both total and stained tests of the most abundant species and integrated ammonia concentrations. No clear indicator species of the organically-enriched benthos were identified. Deformed tests (mainly P. planatus ) were observed in the samples and it is not clear whether they formed as a result of adverse conditions related to the fish farm because such variants also occurred at unenriched sites.