Final answer:
Decomposition in open-water systems such as lakes and oceans primarily occurs in deeper layers, especially near the bottom, where decomposers break down organic material in the nutrient-rich, oxygen-poor environment.
Step-by-step explanation:
In open-water systems such as lakes and oceans, decomposition occurs mostly in deeper layers rather than the surface layer. This is because the oxygen-rich water at the surface, which is ideal for decomposition, eventually moves to the bottom of the lake during the seasonal mixing process known as fall turnover.
During this process, colder water at the surface sinks and displaces water at the bottom, mixing the layers and redistributing oxygen throughout the water column. However, as temperatures drop, such as in winter, the surface can freeze, preventing further oxygenation of the water below. Consequently, the water at the bottom, being oxygen-poor due to ongoing decomposition, becomes even less favorable for decomposition until the next turnover event.
Furthermore, nutrient-rich water tends to be found near the bottom due to the decomposition of sunken organic material by decomposers residing there. Thus, most decomposition is likely happening closer to the bottom, where these organisms break down dead material, releasing nutrients back into the lake or ocean ecosystem.