To appreciate the fragile, marginal and delicately balanced state of ecosystems in the Arabian Gulf, it is important to highlight some vital processes and hydrological features that contribute to the overall marine environment.
The Gulf is a sizeable but constrained and shallow basin with a maximum depth of about 120 metres and an average depth of thirty-six metres. The narrow link between the Gulf and the Indian Ocean through the Strait of Hormuz limits connectivity with adjacent species and has influenced the diversity and uniqueness of marine life in the Gulf. The temperature and salinity of the Gulf are both heavily influenced by the extreme climate, so that native species, although particularly well adapted to the extreme conditions, are, in many cases, living close to the limits of their environmental tolerance.
The waters off the coast of Abu Dhabi exhibit marked seasonal variability in physical, chemical and biological characteristics. Near the coast, the mean monthly sea surface temperature varies from 19ºC to 35ºC and the range declines with depth, with maximum temperatures recorded during the peak summer months of heat and humidity. Salinity varies with temperature and is another critically important parameter for marine life. Excessive evaporation leads to very high salinity, especially in the southern Gulf, with readings sometimes reaching seventy ppt (parts per thousand) in shallower bays. On average, seawater in the world's oceans has a salinity of about 3.5 per cent, or thirty-five ppt. These extremes limit the diversity and abundance of marine life and, in particular, the development of critical habitats such as coral reefs. In the winter and spring, the cold Shamal (northerly) winds often reach gale force, churning and mixing the water column, especially in the south of the Gulf, helping to dissipate these extreme water conditions. These seasonal changes in temperature and salinity help to drive the cyclic abundance of biomass within the Gulf.
There is growing evidence to suggest that climate change is also impacting temperature and salinity levels with extremes in the Arabian Gulf occurring with increased frequency. Consequently, ecologically destructive events such as toxic algal blooms or ‘red tides', mass fish kills and coral bleaching destroy species living at the limits of their tolerance. Unfortunately, these natural stressors on the marine environment are being exacerbated through the activities of Man, too often tipping the delicate hydrological balance of the Gulf in a precarious direction.
Under the Burning sun
Under the burning sun, at its peak during the summer months, the sea surface warms, causing evaporation levels to increase. The combined effects of wind and solar energy can lead to very high evaporation levels, especially in shallow lagoon areas. The evaporation rates are so high that the total annual input of fresh water from the Tigris and Euphrates rivers, in the north-west of the Gulf, is less than that which evaporates from the surface of the whole Gulf. This is compensated for by inflow from the Gulf of Oman through the Strait of Hormuz.
Seasonal Variations in Productivity
There is a well-defined seasonal cycle in the Gulf's biological characteristics. Zooplankton abundance is low during the summer months and increases as the waters cool in the autumn. Consequently, the abundance and distribution of fish populations, in particular smaller pelagic species, such as the Indian scad (Decapterus russelli), and their predators, such as the Kingfish (Scomberomorus commerson) and Yellow-spotted trevally (Carangoides fulvoguttatus), are closely associated with seasonal changes in temperature and secondary productivity.
Seabed (Benthic) Productivity
Benthic, or sea bed, productivity in the Gulf is one of the highest in the world. Due to the shallowness of the Gulf basin, sunlight penetrates and reaches much of the sea floor. Consequently, the lower waters support abundant marine flora, including extensive seagrasses, macro-algal beds and cyanobacterial mats, which support rich benthic communities. Extensive mudflats contain abundant infaunal organisms such as polychaete worms and molluscs that inhabit the soft substrate. These, in turn, support organisms higher in the food chain including crustaceans and fish species that sustain important fisheries.
As annual marine plants, seagrasses complete their life cycle by July and August, although this timing varies by location and species. By September and October in Abu Dhabi's coastal waters, new leaves (lamina) appear and the productivity of seagrass increases over the winter. During this period, dugongs (Dugong dugon) congregate in larger numbers to mate and to graze on the rich beds of vegetation. Calves are born during February in time to graze on the abundant vegetation. Green turtles (Chelonia mydas) also increase in numbers during this period, taking advantage of this seasonal bounty.
The Web of Marine Life-Building Blocks
Most people are familiar with the more visible and charismatic marine species, such as fish, corals or marine mammals. It is, however, the tiny, unseen life forms that are fundamental to life in the sea. The basis of life in the sea is the abundant tiny single cell algae known as phytoplankton, often referred to as primary producers.
Microscopic phytoplankton coexist with and sustain other larger organisms in the complex web of marine life. Each species consumes and/or is consumed by several other species at different (trophic) levels in the food chain. Since only about 10% of an organism's energy is passed on to its predator, animals at higher levels in the food chain tend to be larger in size but far fewer in number than at the lower levels. But the relationships between species at different trophic levels are delicate – if any link in the food chain is broken, the entire ecosystem faces potential collapse.
Phytoplankton are microscopic plants that convert sunlight energy into food energy through photosynthesis, a process that requires light, water, basic nutrients and the chemical chlorophyll. This leads to rapid growth. Even in ideal conditions, phytoplankton live for only a day or two. But after phytoplankton die, their productivity does not cease. Over millions of years, dead phytoplankton and the remains of other marine organisms sank to the ocean floor, accumulated and were gradually converted into valuable reserves of hydrocarbons (oil and gas). As primary producers, they are the foundation of the marine food chain. Phytoplankton are considered biotic indicators because they respond rapidly to changes in their environment and especially to the conditions and quality of seawater. For this reason, EAD has monitored phytoplankton in the coastal waters of Abu Dhabi since 2002.