Magazine article Oceanus

Red Tides and Dead Zones: The Coastal Ocean Is Suffering from an Overload of Nutrients

Magazine article Oceanus

Red Tides and Dead Zones: The Coastal Ocean Is Suffering from an Overload of Nutrients

Article excerpt

The most widespread, chronic environmental problem in the coastal ocean is caused by an excess of chemical nutrients. Over the past century, a wide range of human activities--the intensification of agriculture, waste disposal, coastal development, and fossil fuel use--has substantially increased the discharge of nitrogen, phosphorus, and other nutrients into the environment. These nutrients are moved by streams, rivers, groundwater, sewage outfalls, and the atmosphere toward the sea.

Once they reach the ocean, nutrients stimulate the growth of tiny marine plants called phytoplankton or algae. When the concentration of nutrients is too high, this growth becomes excessive, leading to a condition called eutrophication.

There is a clear connection between eutrophication and two significant environmental problems: harmful algal blooms (HABs) and the depletion of oxygen dissolved in bottom waters (hypoxia). The effects of both HABs and oxygen depletion are felt throughout the coastal ecosystem, with direct and indirect effects on human health, food supplies, and recreation.

For scientists seeking to understand it, eutrophication is a challenge because the physical and biological processes linking nutrients and their impacts are complex. For policymakers seeking to manage these impacts, the challenge is weighing the economic benefits of the activities that generate nutrients with the environmental costs of eutrophication.

Too much of a good thing

In the ocean, as on the land, photosynthesis combines energy from the Sun with carbon dioxide and nutrients such as nitrogen and phosphorus to produce carbon-rich plant material. This natural process is called primary production and forms the base of the marine food chain. It also provides most of the oxygen in the atmosphere. Without primary production, the world would be a much different (and a good deal less pleasant) place.

But every silver lining has a cloud. Of the thousands of species of algae, perhaps only a hundred are toxic. When these species occur in high concentrations, they can color the water and produce what are popularly referred to as "red tides" or "brown tides;' Scientists prefer to call these outbreaks harmful algal blooms or HABs. (See "The Growing Problem of Harmful Algae," page 34.)

Toxic algae enter the marine food chain when they are consumed by small marine animals called zooplankton and by fish or shellfish. The toxins that accumulate in these consumers are then passed up the food chain to marine mammals, seabirds, and even humans, where they can cause illness or even death.

Blooms of some non-toxic species of algae can also cause problems. For example, the North Atlantic right whale is in grave risk of extinction. This species feeds seasonally offCape Cod on concentrated patches of zooplankton called copepods. In some years, an algal species called Phaeocystis blooms in Cape Cod Bay. Although Phaeocystis is not toxic, large blooms essentially clog surface waters and right whales cannot find the copepod patches they need to eat.

Non-toxic HABs include large blooms of seaweed or macroalgae that can coat beaches, interfering with recreational activities. Other HABs clog seagrass beds and coral reefs, which provide nurseries for commercially important fish and support high levels of biological diversity necessary for a healthy environment

Harmful algal blooms occur in every part of the world. In the U.S. and other developed countries, monitoring efforts and fishery closures have reduced the incidence of human illness caused by toxic algae. However, both monitoring and closures have economic costs that can be substantial. Perhaps the most striking example of this is the complete loss of the wild shellfish resource in Alaska--which once produced 5 million pounds annually--to persistent paralytic shellfish poisoning.

It is difficult to assess the precise way in which human activities influence the occurrence and severity of HABs. …

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