Which type of lake is known for its high nutrient levels and high productivity, often leading to low dissolved oxygen at the bottom?

Eutrophic lakes are characterized by high nutrient levels, particularly phosphorus and nitrogen, which stimulate significant plant and algae growth. This high productivity can lead to algal blooms, and as the organic matter from dead algae and plants decomposes, it consumes dissolved oxygen in the water. As a result, the bottom layers of a eutrophic lake often experience low levels of dissolved oxygen, particularly in warm months or during stratification when mixed layers do not freely exchange oxygen. This hypoxic condition can harm aquatic life, leading to fish kills and reduced biodiversity.

Oligotrophic lakes, in contrast, are nutrient-poor and often support less biological productivity, while mesotrophic lakes lie somewhere in between, with moderate nutrient levels and productivity. Senescent lakes refer to older lakes that are transitioning towards eutrophication, but they may not necessarily exhibit the high productivity or nutrient levels typical of fully eutrophic lakes. Thus, the defining characteristics of eutrophic lakes make that choice clearly the most appropriate answer.