Oligotrophic

Oligotrophic describes bodies of water, such as lakes, reservoirs, and some rivers, that have low nutrient concentrations, particularly phosphorus and nitrogen. This limited nutrient supply restricts primary production, leading to clear water and low algal biomass.

Characteristics include high water transparency, often with Secchi depths of several meters, cold and well-oxygenated conditions, and deep light penetration. The bottom sediments are typically low in decomposable organic matter, and communities tend to be composed of species adapted to low productivity and clear water, such as certain cold-water fish. Chlorophyll-a concentrations are usually low, reflecting limited phytoplankton in the water column.

Ecology and formation: Oligotrophy commonly arises in landscapes with low nutrient inputs, such as glacially formed basins or areas with efficient nutrient retention in soils and sediments. Nutrients tend to stay bound in sediments or be rapidly utilized, maintaining low productivity. Seasonal stratification can occur, but oxygen typically remains high in both surface and deep waters.

Measurement and indicators: Scientists assess oligotrophy using Secchi disk transparency, chlorophyll-a concentration, and total phosphorus. In many oligotrophic lakes, Secchi depths exceed 6 meters and chlorophyll-a is low, often below 2 μg/L, with high dissolved oxygen levels throughout the water column.

Human impact and management: Nutrient enrichment from agriculture, wastewater, and urban runoff can shift systems toward mesotrophic or eutrophic states, reducing clarity and oxygen, promoting algal blooms. Management aims to reduce nutrient inputs, especially phosphorus, through land-use practices, wastewater treatment improvements, and restoration measures.