by Alex Moot

Most of the lakes in New Hampshire formed as the glaciers receded, carving out basins in the landscape. In these newly formed lakes, the edges and bottom were exposed rock, which doesn’t erode very quickly, meaning there were not many nutrients available. As a lake ages, sediment from the watershed is washed in, filling in the bottom of the lake. This sediment is rich in nutrients, and therefore increases the fertility of the lake. More fertile lakes have more nutrients and therefore more plants and algae.

A lake is usually classified by scientists as being in one of three possible classes: oligotrophic, mesotrophic or eutrophic:

Oligotrophic lakes are pristine and ideal for swimming and trout fishing. Such lakes are characterized by high water clarity, low nutrient concentrations (phosphorus and nitrogen), minimal levels of aquatic plant growth (weeds), and low algae concentrations. In oligotrophic lakes, oxygen is found at high levels throughout the water column. In addition, low algal concentration allows deeper light penetration and less decomposition. When algae, zooplankton and fish die, they sink to the bottom and are decomposed by microbes and invertebrates. This decomposition process uses up oxygen. Since oligotrophic lakes are less fertile and have less algae and other organisms, there is less decomposition and the oxygen doesn’t get used up. The fish found in oligotrophic lakes like cold, high oxygenated water, such as rainbow trout. Chocorua Lake is an oligotrophic lake.
Mesotrophic lakes are commonly clear water lakes and ponds with beds of submerged aquatic plants, medium levels of nutrients, and increasing levels of algae and/or weeds. Mesotrophic lakes may have mostly clear water but can experience algal blooms in late summer. Mesotrophic lakes behave differently than oligotrophic lakes in that they stratify, meaning they separate into layers in the summer. The top layer of water becomes warm from the sun and contains algae. Since the by-product of photosynthesis is oxygen, oxygen concentration remains high at the surface of the lake. The bottom layer remains cooler and can become anoxic (oxygen depleted) in mid-summer. This change occurs because as all the algae and other organisms die and are decomposed at the bottom of the lake, oxygen gets used up. Since this bottom layer of water does not mix with the top layer of water in the summer, oxygen cannot be replenished. The implications of anoxia are that no fish or other organisms can live where there is no oxygen; therefore, in late summer, fish move shallower where there is still oxygen available.
Eutrophic lakes are shallow, green and have murky water and mucky, soft bottoms. They also have a lot of plants and/or algae. Such lakes are characterized by low water clarity, high levels of aquatic plant growth, high algae concentrations, high nutrient concentrations, and very low dissolved oxygen concentrations near the lake bottom. Eutrophic lakes are found where the soils are more fertile and where there is a lot of farmland.

It is important to remember that lake trophic state is not interchangeable with water quality. Water quality is subjective and depends on how you intend to use the water body. A lake that is good for duck hunting is not necessarily good for water skiing. In turn, a lake that is great for swimming may not be great for bass fishing.