Abstract

  The world's largest lakes, really inland seas, are characterised by long retention times, and are dominated by internal physical forcing, generally low nutrient loadings and predominantly internal recycling. Most are oligotrophic. Many are severely phosphorus deficient. Certainly few ever support large crops of phytoplankton. Horizontal heterogeneity sometimes permits enhanced production in shallow bays or behind thermal bars and occasional blooms develop under conditions of near-surface stratification.
   An assessment of published information on the composition and seasonality of phytoplankton in the open water habitats of these lakes confirms the oligotrophic nature of the world's great lakes. There is a predominance of diatoms at all latitudes; chrysophytes are seasonally prominent in some lakes at high latitude; other flagellates, including dinoflagellates, and species of cyanobacteria represent increasing proportions of the pelagic biomass towards the equator. There is an indication that species composition is influenced by underwater light availability and a positive correlation between Microcystis plankton and relatively higher concentrations of total phosphorus (TP>30 µg l-1) is suggested. Picoplankton is apparently abundant during periods of relatively high insolation of the water column. Although the carrying-capacity of the nutrients available is scarcely large, the production of biomass is strongly related to seasonal variability in the intensity and extent of water- column mixing and its relation to the periodicity and underwater penetration of photosynthetically- active radiation. Attainment of nutrient-limited crops generally coincides with shallow mixing whereas deep circulation suppresses production.   The differential effects of latitude, local climate and salinity upon this general deduction are also evaluated. The role of grazing, its contribution to nutrient recycling, and its contribution to sustaining pelagic food webs, is also considered. The paper makes some deductions about the threats placed on large-lake ecosystems by pollution, eutrophication and acidification and upon how their ecosystem health might be monitored and conserved in the future.

Keywords: Great Lakes; Phytoplankton; Population dynamics