may already have had an impact on the food web in Lake Erie. A filter feeder, it is thought to be responsible for an increase in water clarity in the lake during 1989 and 1990. Fishery scientists are concerned that the organism will divert enough primary and secondary production from pathways that support fish growth to affect the lake's economically important walleye fishery. No control techniques are currently available to address a problem of this magnitude. Although it is not yet found in U.S. waters outside the Great Lakes, the zebra mussel is expected to spread widely throughout the surface waters of the eastern United States over the next several years.
Acidification of poorly buffered lakes (and other surface waters) by acidic precipitation has been a major environmental issue in the United States and Canada (as well as parts of western Europe) for the past two decades. The ecological changes caused by acidification are fairly well understood (e.g., Schindler, 1988), but the severity of the problem is still controversial, despite more than a decade of extensive research. Acidification tends to simplify the biotic structure of lakes, as acid-sensitive species are lost and relatively fewer acid-tolerant species remain. However, ecological impacts generally are greater at the population level than at the community level, and effects on some integrative measures of ecosystem performance, such as total primary production and community respiration, have not been demonstrated conclusively, especially for mild levels of acidification.
In contrast, rates of decomposition of organic matter, especially leaves and other terrestrially produced materials, are slowed in acidic lakes (Perry et al., 1987; Brezonik et al., 1991 a), and certain pathways in the biogeochemical cycles of major elements such as nitrogen and sulfur may be altered or inhibited under acidic conditions (e.g., Rudd et al., 1988). Water column concentrations of several minor metals (manganese, iron, and especially aluminum) and trace metals (cadmium, lead, zinc, and mercury) are higher in acidic lakes because of increased solubility and decreased tendency to adsorb onto particles, and the free (uncomplexed) chemical forms of the metal ions trend to predominate in acidic waters. Other factors being equal this trend should increase metal bioaccumulation and toxicity to aquatic biota. Indeed, increased aluminum toxicity is thought to be a major factor in the loss of fish species in many acidic lakes, but the situation is less certain for other potentially toxic trace metals (Campbell and Stokes, 1985; Brezonik et al., 1991a). Increased competition for metal-binding sites on organisms by the higher H+ concentrations in acidic