The world’s oceans contain an abundance of life. However, there are areas of ocean that should be teeming with life that are virtual aquatic deserts. These areas are known as dead zones, and the cause, almost without exception, is man’s impact on nature.

The most recent dead zone has been created by the Deepwater Horizon explosion and the subsequent spillage of massive amounts of oil and gas into the water. Although the oil does pose serious environmental risks, recent research suggests that the levels of methane pumped out by the well runs the risk of greatly reducing oxygen levels in the water. A research team found that methane levels in the water around the ruptured well were far above normal levels, with levels of up to 100,000 times the norm reported.

Oxygen concentrations of 30 per cent lower than normal have also been reported, which poses a definite danger to marine life in the area.

Although the Deepwater Horizon disaster is drawing a lot of media attention at the moment, the dead zone being created there lies close to a much more insidious dead zone of around 6,000 to 7,000 square miles off the mouth of the Mississippi. This dead zone is caused by fertiliser and animal waste from intensive farming practices being washed down the river and into the Gulf.

Once there, the unnaturally high nutrient levels in the Gulf serve to feed massive blooms of algae. Although the increase in algae, which forms the base of the food chain in the oceans, might seem to be a good thing at first, it can have a catastrophic impact on the area over time.

According to John Bothwell, senior researcher at the Cayman Islands Department of Environment, the department has not conducted any local research on the effect of nutrient rich water on algae, as it is well established that as nutrient levels in waters increase so does the rate of algal growth. This is also referred to as eutrophication.

“Localised intense algal blooms can have a negative impact on other, non-coral marine life. This is especially common with phytoplankton, free-floating (often single-celled) algae,” says Bothwell.

Such blooms can lead to low oxygen levels, or anoxia, especially in enclosed or semi-enclosed bodies of water.

Although algae produce oxygen through photosynthesis, just like plants, the algae take up oxygen at night in order to stay alive. Once the algae dies, the problem becomes even worse as the micro-organisms that assist in the decomposition of the dead algae take up oxygen as well.

“The overall reduction in oxygen in the water can cause fish to die and stress other animals and plants,” says Bothwell.

Surface algae can also block sunlight from other plant on the ocean floor, thereby reducing the oxygen production of these plants.

In Cayman, the effect of anoxia can occasionally be observed in the inland ponds.

“[It] is the primary reason for the occasional ‘fish kills’ that we see in our various local ponds, usually over summer when algae blooms more naturally occur due to the extended periods of warm sunny weather with little wind to mix the water,” says Bothwell.

However, the algal blooms in these inland ponds are part of a natural process, and many local species have adapted to deal with the drop in oxygen levels that occurs during the summer months. According to Bothwell, Guppies and tarpon live near the surface where oxygen tends to be more plentiful and tarpon can even gulp air to supplement their oxygen intake. However, medium sized bottom dwelling fish like the introduced tilapia die off due to the reduced oxygen levels.

In spite of anoxia being more common in enclosed bodies of water, it can occur in the ocean as well as evidenced by the well-documented dead zone off the mouth of the Mississippi river. However, this generally requires the continuous introduction of large amounts of nutrients into the water in order to keep the eutrophication at such a level as to sustain a dead zone.

“In Cayman the Department of Environment has only observed such anoxic conditions in the end of poorly circulating canals, though such conditions could occur, especially at night, in other localised areas as well, such as the small eutrophied bay at the mouth of the dike road near to the GT landfill and sewage treatment plant, if conditions were right,” says Bothwell.

Yet dead zones are not bound to remain that way. The Black Sea was once the biggest dead zone in the world, due to large amounts of fertiliser that washed into it from Eastern Bloc countries. However, fertiliser became too expensive after the collapse of the Soviet Union, leading to a dramatic reduction in its use. This has lead to the Black Sea making a remarkable recovery over the last two decades, with fishing once again becoming a viable economic activity in the region.

Although the recovery of the Black Sea did not occur due to regulation, changes in the use of fertilisers or stricter control of industrial or sewage emissions can have similar effects on dead zones, allowing a slow but steady recovery.

However, algal blooms do not have to be continuous to have a negative impact on the marine environment, as even short-term blooms can have a very negative impact, depending on the type of algae involved, as is the case with red tides.

“In these cases the algae itself is often of a toxic variety and can kill birds, fish and other animals that eat enough of them or the toxin can result in poisoning to humans who eat shellfish that have been consuming the algae. Sometimes the concentration of toxin is so high that it can enter the water or the air causing allergic reactions in people,” says Bothwell.

However, according to Bothwell, he is unaware of any such cases having been reported in the vicinity of Cayman.

Apart from the dramatic impact algal blooms can have on marine life in general, the impact on coral can also be catastrophic.

“Increasing levels of algae on coral reefs normally result in the overgrowth of the corals by the algae causing the death of the corals,” says Bothwell.

Corals do not need high amounts of dissolved nutrients in water, and the algae that do grow under these conditions can be controlled by herbivores such as parrot fish and sea urchins, according to Bothwell. However, if there are abundant amounts of nutrients available in the water, the growth of the algae can outstrip what these grazers can consume.

Other events can also impact on the amount of algae present on the reefs.

“After the Caribbean-wide die-off of long-spined urchins in the 1980s many reefs around the Caribbean saw an increase in algal cover,” says Bothwell.

The die off, which occurred in 1983, is estimated to have claimed up to 97 per cent of the long-spined sea urchin population in the Caribbean. The resulting increase in algal cover had a negative impact on the coral, the biodiversity of the reefs and even on tourism in some of the smaller island locations. Fortunately, Cayman was not impacted as heavily as many other locations.

“Cayman’s marine parks and the protections they provided to parrot fish and other grazing species have probably contributed to restricting the spread of algae on our reefs after the loss of the diadema,” says Bothwell.

“Cayman’s lack of rivers and other natural sources of nutrient runoff has also probably contributed to our generally relatively good reef health,” according to Bothwell.

Apart from killing established corals, algae growth also prevents the settlement of new corals.

“In addition to overgrowing live corals algae also takes up space on bare rock on the reef that would otherwise be available for coral larvae to settle out on to. Again, the diadema were probably very important in helping to keep the reef rocks bare of algae and promoting coral recruitment,” says Bothwell.