שינויים אקולוגיים משמעותיים במפרץ אילת על פי תיארוך בעופרת 210: היעלמות עשב הים כבית גידול ושינוי בחברת הצדפות כתגובה לשינויים אנתרופוגניים

יעל אדלמן-פורסטנברג , מכון גיאולוגי

Young fossil record can be a powerful means of detecting biotic response to human stressors in settings with little or no monitoring history. The Gulf of Eilat is a region of exceptional biodiversity and a well-studied water column and reef system, but the extensive soft-sediment shelf is poorly known. Two sites at 15 and 30 m, located both proximal (FF) and distal (DAN) to historic point sources of nutrients and watershed runoff, were cored to test for bivalve species and functional-group responses by bivalve assemblages (also living assemblages). Main stressors include fish farms (1996-2008), sewage (1980s-1995), and channelized, sediment-charged floodwaters (from 1960s). 60-cm diameter ‘suction cores’ acquired by divers were subsampled in 5cm increments to 70 cm core-depth and sieved through 2 mm. A separate 5.3 cm-diameter push core was analyzed for 210Pb using the fine-fraction only. We find that 1950 CE, which was the onset of urbanization, is ~15-20 cm in each core, which extend back to ~1750 CE. Sediment grain size and TOC change little over the centuries leading up to settlement, and spatial variation is modest. The 30 m sites are muddier than 15 m sites, with lowest mud content at FF15, closest to the natural mouth of the wadi. TOC levels are higher at 15 m sites (~0.8-1.2%) than at 30 m sites (0.4-0.9%). Assemblages are dominated by chemosymbiont-bearing lucinid bivalves typical of sandy seagrass, with subsidiary facultative deposit feeders and suspension feeders. These results suggest the past widespread distribution of seagrass. Sediments, bivalve assemblages, or both are quite different in post-settlement core increments at each site. Grain size (%mud) is unchanged at the distal DAN sites, but becomes variable at FF15 and drops sharply at FF30, consistent with more focused floodwater input. TOC declines notably at all sites except FF30, suggesting the loss of seagrass to trap and sequester it. At the same time, at all sites but DAN15, seagrass-dwelling lucinids become less abundant in favor of facultative deposit-feeding tellinids, consistent with the strong spatial gradient in seagrass evident today. Also, 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 pollution. The data provide compelling evidence that widespread and sandy seagrass meadows, characterized by organic matter sequestration, were a persistent natural state of the Eilat shelf, and that the system has changed significantly associated with diverse stressors from urbanization of the watershed.


תאריך 10/06/2018 10:30 12:00
הדפסשלח לחבר
עבור לתוכן העמוד