Aquatic Species and Their Environments - Unwelcome Guests—aquatic Invasive Species

Numerous aquatic ecosystems have been degraded by invasive species. The primary source of aquatic invasive species has traditionally been ship ballast water, which is generally picked up in one location and released in another. In San Francisco Bay alone, it is estimated that a new invasive species becomes established every 14 weeks through ballast water. Invasive species are also established through transfer from recreational boating vessels; intentional release, usually in attempts to establish new populations for fishing; dumping of live bait; release of aquarium species; and accidental escapes from research facilities. The Nonindigenous Aquatic Nuisance Prevention and Control Act of 1990 and the National Invasive Species Act of 1996 are intended to help prevent unintentional introductions of aquatic nuisance species.

The zebra mussel is an invasive species that both degrades aquatic resources and threatens native species, particularly native freshwater mussels. Zebra mussels first appeared in the United States in 1988. Figure 5.9 illustrates the spread of zebra mussels throughout the Great Lakes and beyond during the following ten years. According to the U.S. Geological Survey, zebra mussels were found in Virginia in 2002, and in a lake in Kansas in 2003; signs of juvenile zebra mussels were detected in Nebraska, as well, in early 2004. This pest species reproduces rapidly and threatens aquatic habitats by clogging water passages and starving out native species. U.S. freshwater mussel species are in fact disappearing at an alarming rate. Figure 5.10 shows the decline in the number of pearly mussel species in the Mississippi River.

In the state of Georgia, invasive Asian eels have increased in number in many habitats. These species were brought over from Southeast Asia or Australia, where they are considered delicacies. The three-foot-long, flesh-eating eel preys on species such as largemouth bass and crawfish in and around the Chattahoochee River. The eels have gills but can also breathe air—this enables them to worm their way across dry ground to get from one body of water to another. Asian eels have few predators in their new habitat, and humans have found no effective way to control them. In March 2000 an Asian eel was reported near Florida's Everglades National Park, confirming fears that the eel would spread beyond Georgia.

FIGURE 5.9
Zebra mussel distribution, 1988 and 2003

Striking at the Base of the Food Chain

Phytoplankton (planktonic plant life) are microscopic photosynthesizing species that form the basis of nearly all marine food chains. (See Figure 5.11.) In many parts of the world, phytoplankton seems to be declining. The most severe damage appears to be in the waters off Antarctica, where phytoplankton are severely depleted. The depletion of phytoplankton has implications all the way up the food chain, affecting not only the zooplankton that consume them but larger species such as penguins, seals, and whales. Scientists believe that phytoplankton declines are a result of the thinning atmospheric ozone layer (caused by industrial pollutants such as chlorofluorocarbons, or CFCs), which allows increasing amounts of ultraviolet radiation to penetrate the Earth's surface. Ultraviolet radiation decreases the ability of phytoplankton to photosynthesize and also damages their genetic material.