The large increase in macrophyte coverage and biomass in waters infested with zebra mussels is the result of increased water clarity (Lyakhnovich et al. 1988, MacIsaac 1996). Improved water transparency allows sunlight to penetrate to deeper levels where macrophytes can now become established. This increased macrophyte abundance may act as a barrier hindering the influx of nutrients used by phytoplankton (Karatayev et al. 1997). The euphotic depth in Lake St. Clair, for example, has now expanded to include most of the benthic surface, and it has now become the host of large areas of macrophyte growth (Griffiths 1993). Increased water clarity in Lukomskoe Lake (Belarus) from 1.8 to 4 m resulted in an expansion of macrophyte cover (from 6 to 30-percent of total lake area) due to an increase in the depth at which macrophytes can grow (from 2.5 to 5 m) (Lyakhnovich et al. 1988).
Increased macrophyte growth can also have a recreational impact. In the summer months of 1994, large amounts of decaying macrophytes washed up along several United States and Canadian shorelines, fouling beaches and causing water quality problems. Due to high bacteria counts and the large amounts of decaying macrophyte stranding, some beaches were closed.