Of extreme importance is the fact that water absorbs light. This leans to a selection for different photosynthetic groups as different depths. Figure 9.13 will enlighten you as to various types of algae and photosynthetic bacteria found at various depths as determined by the absorption spectra of their photosynthetic apparatus. Sediments of freshwater and saltwater represent an environment which has characteristically been considered anaerobic and within a short distance of the top of the sediment devoid of life. However, this view is changing with our new appreciation of the active microflora of the subsurface.
Freshwaters often show a characteristic stratification.
Hypolimnion low temperatures, low oxygen and elevated nutrients. Less dense warm water rests on denser cold water. This acts as barrier to movement between hypo and epilimnion. Turnover is a seasonal mixing of these layers. What makes it occur.