Government of New Brunswick
Aging and eutrophication

Lakes like other biological systems age over the passage of time. Lakes age naturally over a geologic time scale and they should not exhibit age-related changes over a human’s lifetime. Lake succession is a term used to describe the process of continuous aging in a lake in terms of their physical and biological properties. Lake succession is expressed in terms of age, from young, to middle-age and finally to old-age. As the biological properties of a lake age, they reach different biological growth states referred to as trophic states. The term trophic status refers to a level of primary productivity or growth in the algae and plant community of a lake. Young lakes have a low primary productivity whereas old-age lakes have high levels of primary productivity. The term eutrophication refers to the change in a lake from having lower level of primary productivity to a higher level of primary productivity.
Lakes also undergo physical aging over time. A lake can become filled with sediment and particulate matter causing it to become shallower. The two main factors that control natural lake succession are the mean depth of the lake and the addition of nutrients from the surrounding drainage basin. Over time a lake will become filled in with sediments and particulates deposited from in-lake processes and those carried by inflowing waters and deposited. Eventually over time a lake can transition into a wetland or even to a dry land environment.

Lakes can age prematurely due to human influences on the lake’s natural aging process. Eutrophication caused by human influences typically occurs over decades and results in the excess growth of algae and plants, diminished lake clarity, light penetration and oxygen concentrations which can have impacts on aquatic life and the use and enjoyment of a lake. (Figure 1)

Trophic states of lakes

Trophic status is a measure of the degree of eutrophication of a lake and the extent of its nutrient enrichment or primary productivity. It is indicated on a scale from low to high primary productivity; the lower productive oligotrophic state, the moderately productive mesotrophic state and the highly productive eutrophic state.

Trophic status can be determined using measurements of nutrient concentrations (nitrogen or phosphorus), water transparency, or some measure of algal growth.  Other measurements that can help to determine trophic status include dissolved oxygen profiles, frequency and intensity of algal blooms, characteristics of plant and animal communities, and the physical dimensions of the lake (e.g. shape, depth, size, etc.)

Oligotrophic state: Clear waters with little organic matter or sediment and minimum biological activity.  These lakes are usually deep and the shoreline is sparsely populated with aquatic plants.

Mesotrophic state: Waters with more nutrients and, therefore, more biological productivity.  These lakes are intermediate with respect to depth, chlorophyll concentration, water clarity, and aquatic plants.

Eutrophic state: Waters extremely rich in nutrients, with high biological productivity.  These lakes have higher concentrations of phosphorus and chlorophyll and poorer clarity.  Typically, they are shallow, often muddy and contain an abundance of aquatic plants.