Use of Phytoplankton in Large River Management1

Historically, rivers have served as sources of drinking water, fisheries resources, transportation routes, irrigation supplies, and waste removal systems. Human civilization has had many major effects on rivers, dating back more than 5000 years when Egyptians built dams on the Nile to supply water for crops and human consumption. Today, management of large rivers requires a balance between human needs and ecological integrity, although until quite recently, ecological principles have played a minor role in river management (Edwards 1995). Planktonic algae are an important part of these issues because they play a central role in the functioning of large rivers. Algal communities are major producers of organic carbon in larger rivers, are a food source for planktonic consumers, and may represent the primary oxygen source in many low-gradient rivers (Thorp and Delong 1994, Köhler 1995, Reynolds and Descy 1996). Phytoplankton are responsive to excessive supplies of inorganic nutrients and may pose problems in long stretches of rivers with cultural eutrophication, but may also enhance water quality for humans in rivers affected by agricultural or industrial uses. Algal communities of river systems consist not only of suspended algae, but also a diverse benthic assemblage of macrophytic forms, smaller epilithic species, epiphytes, and sediment-dwelling forms (Reynolds 1996). This review focuses on the use of planktonic algae in river management because of their central importance in larger rivers and because of the growing need for ecosystem-level studies on river plankton.