Benthic foraminiferal response to the removal of aquaculture fish cages in the Gulf of Aqaba-Eilat, Red Sea

Abstract

Abstract For about 20 years, finfish were reared in floating cages at the northern end of the Gulf of Aqaba-Eilat, Red Sea. The benthic ecosystem at the fish cages area was severely impacted by organic enrichment, resulting in an environment with no living foraminifera. A government decision led to the fish cages' removal in June 2008, creating a unique opportunity to monitor and assess post-removal changes in the benthos. Three years of benthic foraminiferal assemblage monitoring, beginning in July 2008 and ending in July 2011, are summarized here. Monitoring was carried out monthly by collecting sediment samples from stations of varying distances from the fish farm location, and, after its appearance in the summer of 2009, sampling the native seagrass Halophila stipulacea. Living foraminifera first appeared in the sediment in January 2009, progressively increasing in abundance thereafter. A clear difference in the rate of the rehabilitation process was observed on a spatial scale, related to distance from the point source of the organic enrichment. Recovery began with the first appearances of a few living individuals of Ammonia spp., Amphistegina lessonii and Nonion spp. By July 2009, a significant increase in overall abundance was recorded in the stations furthest from the fish cages, with Operculina ammonoides strongly dominating the assemblages. Populations of O. ammonoides revealed polymorphism in the coiling mode of their shells. Inflated involute and semi-involute forms dominate the living assemblages, whereas flattened evolute tests are more common in the dead assemblages, representing the period that preceded the fish farms. Unlike previous interpretations in the literature, in which such morphological variation was attributed to hydrodynamic energy or depth habitats, here it is hypothesized that the inflated involute and semi-involute forms are a morphological trait characteristic of the pioneer assemblages colonizing the area after its recovery from fish cages eutrophication. The reestablishment of the native H. stipulacea seagrass community was an important factor enabling epiphytic foraminifera to colonize the previously impacted sediments. All living foraminiferal species found on the seagrasses were also found in the former assemblages, suggesting that seagrass meadows existed before eutrophication and were the main habitats of the dead assemblages.