Intertidal lagoons in the western Arabian Gulf are summer dead zones

Summer temperatures in excess of 60°C, have been measured in surficial substrates in the exposed areas of intertidal mudflats along the coast of the Arabian Gulf in Bahrain and Saudi Arabia. The incoming flood tide in coastal lagoons may have a temperature as high as 42°C. As a result, large swaths of the intertidal zone in the western Arabian Gulf are barren of living meiofauna (e.g., foraminifera, ostracods, gastropods, and worms) during the hottest months of the year. Samples collected from the intertidal zone in summer yield only dead assemblages of foraminiferal, ostracod, and gastropod shells. Dead assemblages of the foraminifer Peneroplis collected from the upper reaches of the intertidal zone consist largely of juvenile specimens, implying that the living populations, when present, do not reach maturity. In summer, the full range of living meiofaunal organisms is only observed in tidal channels that remain submerged during low tide.

Our laboratory experiments have determined the upper temperature limit for the survival of gastropods and ostracods collected from coastal lagoons of the Western Arabian Gulf is in the range of 47 to 48.5 degrees C. At 50°C total mortality is observed. In Saudi Arabia, substrate temperatures exceeding the lethal temperature for these organisms are already observed in late April, and persist until late October.

The presence of a summer dead zone for intertidal calcifying organisms is consistent with predictions of climate models, which suggested that by 2050 portions of the Arabian Gulf may become too hot to support animal life. On the western coast of the Arabian Gulf, these predictions have already become a fact.