Chemical oceanography of the Arabian Gulf

Abstract

Most of the available information about basin-wide chemical processes in the Arabian Gulf is based on observations made during the expeditions of the Research Vessels Meteor (1965), Atlantis II (1977) and Umitaka-Maru (1993–1994) with the ongoing national programs in shallow territorial waters largely focusing on local environmental issues. Despite the acute paucity of data, it is clear that the Gulf's biogeochemistry and ecosystems are experiencing massive changes resulting from human activities. The two key components of these changes are a reduction in river runoff that has caused a large increase in salinity in the northern part of the Gulf, and an increase in nutrient loading largely through sewage discharge from the sprawling urban centers along the coasts. The change from a pristine, oligotrophic state to a human-perturbed, eutrophic system is not so clear from the long-term nutrient and chlorophyll trends, presumably because of rapid uptake of nutrients by the phytoplankton and consumption of the photosynthesized organic matter by the heterotrophs. However, eutrophication has led to large increases in the frequency and intensity of phytoplankton blooms since the 1990s. It may also account for the high total organic carbon (TOC) concentration in the water column and the recent development of summer-time hypoxia in near-bottom waters of the central Gulf, which have important implications for the future evolution of the oxygen minimum zone of the North-Western Indian Ocean. The high atmospheric dust deposition maintains high dissolved trace metal concentrations in seawater. There are several intriguing and hitherto unresolved aspects of nutrient budgets in the Arabian Gulf, mostly arising from the differences between nutrient concentrations in the inflowing and outflowing currents, highlighting the unknown roles and strengths of the local sources and sinks. However, even though the latter are being modified by human activities, the N:P concentrations in the deeper part of the Gulf vary in Redfieldian proportion. An interesting feature of nitrogen biogeochemistry in the Gulf is the accumulation of high nitrite in the euphotic zone. The relative contributions of nitrification and assimilatory nitrate reduction by phytoplankton to the nitrite build up are not known. Dissolved nitrous oxide concentrations are slightly above saturation. The extremely high thermohaline variability and biogenic carbonate production within the Gulf also make inorganic carbon cycling in the Gulf quite unusual. However, this remains one of the least studied topics of research in the Gulf. Another subject of disproportionately high significance is benthic biogeochemistry due to shallow depths and high temperatures within the Gulf. Since the small-sized Gulf is shared by eight countries, active regional cooperation is essential to understand its processes and come up with measures to address the serious threats faced by its biogeochemistry and ecosystems from human activities.