Probably many other less intensively studied organisms are affected in similar ways. By lowering the pH of sea water, silt can kill oyster eggs and cause larvae to develop abnormally. Turbidity also reduces the oyster food supply, by blocking sunlight and underwater photosynthesis by algae. Regulations now prohibit dredging between June 1st and September 30th, so that sediment will not be stirred up when oysters are spawning and larvae are settling. A wide variety of human activities disturb the soil and can cause turbidity and siltation, including road and bridge construction, grading of building sites, and plowing of farm fields.
Flooding causes turbidity and siltation as it scours river banks and bottoms.
Flood waters can also roil up and scatter the beds of newly settled larvae. This has happened occasionally during great storms like the hurricane of 1938. However, serious flooding is occurring more often as the watershed is developed, due to runoff from non-absorbent surfaces like roads and roofs and loss of flood storage capacity when flood plains are tilled. Many studies have been done on the effects of heavy metals, pesticides, and petroleum hydrocarbons on oysters. Juvenile stages were found to be more sensitive than adults.