Trophic Status Categories

Lakes with low levels of phosphorus usually support limited biological production (low algal biomass) and, thus, contain low concentrations of chlorophyll-a. These lakes, which are called oligotrophic, tend to have clear water and sufficient oxygen throughout the year to support fish and other aquatic organisms. Lakes with moderate levels of phosphorus support greater biological production, hence contain greater concentrations of chlorophyll-a, and are called mesotrophic. Water clarity in mesotrophic lakes is moderate, but there is an increased probability of oxygen depletion in the deepest areas. Eutrophic lakes contain even greater concentrations of phosphorus and chlorophyll-a. These lakes tend to experience high densities of large aquatic plants (macrophytes) and algae, but also support productive fisheries. Surface accumulations or 'blooms' of algae may occur during the warmest months. Water transparency can be significantly reduced to the point of discouraging contact recreational activities such as swimming or waterskiing. Also, oxygen depletion in deep waters may occur throughout the year as a result of excessive microbial decomposition of macrophytes and algae. Lakes with very high concentrations of phosphorus and chlorophyll-a are called hypereutrophic. Algal blooms are common in these lakes, often lasting throughout the summer and well into autumn. Oxygen depletion can occur throughout the year and may extend to the surface, leading to fish kills.