Young fossil records can be a powerful means of detecting biotic response to anthropogenic stressors vs natural variations. The Gulf of Eilat is a region of exceptional biodiversity and well-studied marine ecosystem, but the extensive soft-sediment shelf is poorly known. Four very large cores (60 cm diameter) from 15 and 30m water depth were sampled 70cm down-core using a specially devised underwater sampling method. Cores were dated (sediment 210Pb and 14C of molluscan shells) and living bivalve communities documented over 1.5 years. The sites, located both proximal and distal to historic point sources of nutrients and watershed runoff, reveal significant change in composition and functional-groups of bivalve assemblages, with onset of urbanization (1950) dated to ~15cm in the deeper core sediments.
Total organic carbon shows continuous decline since the Little Ice Age (LIA), especially in shallow cores. Biological response seems to be related to grain size, which is linked to the runoff of fine sediments; sandy during Medieval Warm Period, muddier during LIA and then associated with post-settlement pollution. Stronger variations are detected in shallower sites, especially FF15 that is closest to the wadi mouth. Assemblages are dominated by chemosymbiont-bearing lucinid bivalves specialized to sandy seagrass. These results suggest the past widespread distribution of seagrass. Seagrass-dwelling lucinids become less abundant in favor of facultative deposit-feeding tellinids. Thus, widespread and sandy seagrass meadows, characterized by organic matter sequestration, were a persistent natural state of the shelf, which has changed significantly in the past decades and is especially associated with diverse stressors from urbanization of the watershed.
Combining the living and the down-core results for a complete ecological history of Eilat’s soft-sediment shelf provide otherwise unattainable and critical information about the existence of change, the spatial extent of disturbance, and what a fully restored system would look like for the region.