What term describes very old shallow lakes that may eventually become marshes?

The term that describes very old shallow lakes that may eventually become marshes is "senescent lake." This term specifically refers to the later stages of a lake's life cycle, where it has aged significantly, leading to shallower depths and changes in biodiversity and chemistry. Over time, these lakes often undergo sedimentation, accumulation of organic matter, and nutrient enrichment, which can result in changes in vegetation and a transition towards marshland or other wetland ecosystems.

Senescent lakes typically experience a decline in water clarity and can become more biologically productive, eventually supporting different types of aquatic plants and wildlife. The transformation into marshes is part of the natural succession process as the lake continues to fill in and nutrient-rich sediments increase. This ecological progression is crucial as it affects the types of species present in the area and the overall health of the aquatic ecosystem.