Re: What are the effects of high level pH on freshwater ecosystems(pH 7&8)

There are many freshwater ecosystems that are naturally acidic or alkaline, depending on the mineral content of the surrounding geology. These systems don’t usually stray more than a point or two away from pH 6-7. A lot is known about artificial acidification of these ecosystems from acid rain studies, but not much is known about artificially enhancing alkalinity. To offset the impacts of acid rain, some areas have tried "liming" their acidified lakes to "harden" the water back to its original pH. This has proven effective in helping some fish and invertebrate populations recover from pH stress, but these ecosystems often do not return completely to their pre-acid states.

In general, small increases or decreases in water pH do not change the diversity of species in aquatic ecosystems, although they may have a significant impact on the abundances of species that are pH sensitive. Larger changes in pH, however, can drastically decrease species diversity and change species composition in freshwater systems, as fewer species can tolerate such harsh environmental conditions. Fish, in particular, tend to be especially sensitive to changes in pH. One example of how variable levels of alkalinity can effect species diversity can be found at the Sand Hill lakes of Nebraska: http://csd.unl.edu/csd/illustrations/ra5a/lakes-wetlands.html

Another consequence of changing pH in an aquatic system is to change the concentrations of phosphates, nitrates, etc. and organic materials dissolved in the water. Primary producers (plants and algae) use these basic chemicals in their metabolisms. These primary producers are then eaten by animals in higher trophic levels. Thus, changing the concentrations of inorganic and organic molecules MAY have an "cascading" impact on all the species in that system by reducing plant production, but how pH influences these interactions is still not very well known.

References:

Anderson, N. John, Blomqvist, Peter, Renberg, Ingemar. 1997  An experimental 
and palaeoecological study of algal responses to lake acidification and 
liming in three central Sweden lakes.  European Journal of Phycology 32 (1): 
35-48.

Andersen, Ragnvald; Vollestad, Leif Asbjorn. 1996  Recovery of Piscivorous 
Brown Trout and its Prey, Arctic char, in the Acidified lake Selura after 
liming.  Nordic Journal of Freshwater Research 0 (73): 3-14.

Doka, S. E., Mallory, M. L., McNicol, D. K., Minns, C. K. 1997 Species 
richness and species occurrence of five taxonomic groups in relation to pH 
and other lake characteristics in southeastern Canada.  Canadian Technical 
Report of Fisheries and Aquatic Sciences 0 (2179): I-V, 1-57.

Simmons, Kenneth R.; Doyle, Katheryn.  1996  Limestone treatment of 
Whetstone Brook, Massachusetts: III. Changes in the invertebrate fauna.  
Restoration Ecology 4 (3): 284-292.

Traaven, T. S.; Frogner, T.; Hindar, A.; Kleiven, E.; Lande, A.; Wright, 
R.F.  1997  Whole-catchment liming at Tjonnstrond, Norway: An 11-year 
record.  Water Air and Soil Pollution 94 (1-2): 163-180.