UNSUSPECTED LOSS OF SEAGRASS COMMUNITIES BASED ON MOLLUSCAN DEATH ASSEMBLAGES: HISTORIC BASELINE SHIFT IN THE TROPICAL GULF OF EILAT (AQABA), RED SEA

Anthropogenic stresses over the last 60 years on the naturally oligotrophic Gulf of Eilat, a region of exceptional biodiversity, are suspected to have strongly affected coral reefs but the status of soft-sedimentary macrobenthos is unknown. Comparing living bivalve mollusks from 6 seasons at 15 and 30 m water depths with naturally accumulating death assemblages reveals significant live-dead discordance in community composition and structure. The largest contrast and thus strongest implied change is near historic sources of nutrients from sewage and aquaculture (Fish Farm FF sites), where the seabed is presently bare mud, but all sites including moderately to sparsely grassed distal sites near the DAN Hotel have apparently shifted. Living bivalve richness and evenness increase significantly from FF15 m to DAN15, with intermediate values at 30 m sites. Living assemblages everywhere are dominated by facultative deposit feeders (tellinids, semelids) and subsidiary suspension-feeders that prefer mud or muddy seagrass (Fulvia, Lioconcha). In contrast, death assemblages are dominated everywhere by lucinid bivalves specialized to sandy seagrass, especially Ctena and Anodontia; these species occur alive at only 2 sites (<1%). The implicit decline in lucinids indicates (1) a reduction in seagrass cover, especially in the Fish Farm area and (2) a change in the quality of seagrass meadows that still exist. Live-dead mismatch also suggests (3) slow ecological recovery given that point nutrient inputs at FF ceased in the 1990s. Siliciclastic smothering of seagrass is more likely at FF, near the mouth of Wadi Arava, arising from an increase in rainfall and/or the ‘Kinnet’ storm-water canal. However the shift to a detritus-based bivalve community everywhere suggests that primary production increased regardless of changes in siliciclastic influx. Dating of shells is underway to determine the timing of these changes, but live-dead mismatch indicates that the extensive soft-sedimentary benthos of the northern Gulf has in fact shifted from its pre-urban baseline. Death assemblages provide otherwise unattainable and critical information about the existence of change, the spatial extent of damage, and what a fully restored system would look like for the region, without the uncertainties of possible analogs in other systems.