Green Tides: New Consequences of the Eutrophication of Natural Waters (Invited Review)

Abstract

In recent decades, alongside the comparatively well-studied bloom caused by phytoplankton, a bloom of marine and fresh waters caused by littoral benthic macroalgae of three genera—Ulva, Cladophora, and Spirogyra—have become a global phenomenon. In the present review, an attempt is made to gain an understanding of why it is these taxa of green filamentous algae that start to grow rapidly in the spring in many water bodies and streams, including oligotrophic waters, and then float up from the bottom, forming floating mats (metaphyton); then their decaying masses are washed ashore and cause substantial ecological and economical losses. Peculiar and common ecological and physiological features of Ulva, Cladophora, and Spirogyra favorable for the formation of green tides are considered. Although eutrophication (the supply of nitrogen and phosphorus from agricultural lands, industrial and domestic wastewaters, and aquaculture) is the evident cause of the increase in algal biomass, it is suggested that the location of external fluxes of inorganic nutrients (surface runoff or groundwater discharge), as well as the biogenic redirection of internal fluxes of nitrogen and phosphorus from pelagial to littoral (benthification), play a key role in the formation of green tides. Measures for controlling green tides are discussed. The necessity for detailed studies of the metaphytonic form of vegetation of benthic macroalgae is emphasized. Obviously, a revision of the present concept of oligotrophic/eutrophic waters which considers only the pelagic compartments of aquatic ecosystems is required.