Our current best conceptual understanding of the stressor ‘aquatic sediments’ is shown in Figure 1.
Figure 1. Potential causes of a change to aquatic sediments and the condition responses observed as a result of this change.
The introduction of both fine and coarse sediments into aquatic systems is an important natural process and is integral to the natural functioning of these systems. However, human induced increases in the natural loads of sediments, and most particularly fine sediments (<63 µm) entering systems can have a number of undesirable impacts.
Within an estuary, fine sediments can remain in suspension, they may be deposited in low energy areas such as mangroves, forming muddy deposits, or are transported out of the estuary via tidal currents and are deposited in nearshore coastal areas. Increases in suspended sediments reduce light penetration making water appear darker and murkier leading to reduced primary production and the loss of benthic plants (e.g. seagrasses) and algae. When sediments settle out of the water column they can smother sessile benthic animals and plants. Human disturbance of catchments (mainly the clearing of vegetation) has resulted in large increases in the loads of fine sediments entering estuaries. Such increases lead to a general increase in the ‘muddiness’ of estuarine and nearshore coastal areas. Other human disturbances, such as dredging, mining, trawling, alteration of hydrodynamics and boat wash, cause increases in suspended sediments and also affect the distribution of fine sediments within estuaries. The presence of impoundments in catchments may trap some fine sediments and thus reduce loads entering the estuarine reaches. However, the associated reduction in freshwater inflows reduces flushing of the estuary which often results in increased siltation, particularly in the upper estuary.
Factors that may work to modify the impacts of aquatic sediments include the water exchange rate as estuaries with higher flushing rates are less likely to retain additional sediment than those with smaller velocities. The natural turbidity of estuaries also varies widely depending on the estuary type. Macrotidal estuaries (tide-dominated estuaries) will naturally have higher levels of suspended sediments due to high tidal velocities which help resuspend sediments. At the other end of the scale coastal lagoons, which have negligible tidal flows tend to have much clearer waters as the slow water velocities allow finer sediments to settle out of the water column. Higher salinities also result in better water clarity as they result in suspended sediment flocculation and settlement. Changes to the hydrodynamics or freshwater flow regime of a system, as well as habitat removal can alter the amount of aquatic sediments present.