Humans+Impact

Human and their Footprints in the Sand

Humans have definitely left and are continuing to leave our footprints all over the San Francisco Bay Estuary. Most of the vegetative wetlands were destroyed either to be used as fuel to power housing and industry or cleared to make way for levee construction. Ninety-seven percent of the original 550 square miles of fresh water marshes have been diked off and plowed for farmland. The loss of habitat has dramatically effect on many native organisms especially the native bird populations that nest in the marshes. Many species have gone extinct such as the Bank Swallow, Willow Flycatcher, and Yellow-billed Cuckoo and the species that remain are confined to the ever-shrinking wetlands. The destruction does not stop there; sides of rivers have been stripped of vegetation and replaced with cement to control flooding. This causes a decrease in the amount of dead plant material, which is a main part of the zooplankton diet. The biggest impact humans have is pollution, ever since the gold rush when miners would let mercury run in the bay. Then in the early 20th century, raw sewage was the problem, which resulted in a dramatic reduction of the blue oyster and soft shell crab populations.

Since the 1950’s run off was the major problem. Large amounts of pollutants enter the estuary from storm-water runoff. Urban runoff carries metals, PAHs, PCBs and pesticides washed from streets, lawns and industrial properties. Agricultural runoff contains pesticides, herbicides, nitrates and phosphates applied as fertilizer, and selenium leached from the soil. Oil and petroleum products enter the estuary from oil spills and leaks in the engines of the many ships, which pass through the bay. The increased pollution has actually risen the contaminate levels in the organisms so high consuming too much fish or waterfowl from the Bay could be a health risk.

The influx of phosphorous known as eutrophication can cause rapid algae growth called blooms. Eventually the algae will die and sink to the bay floor where decomposers break down the algae and in the process; they use much of the dissolved oxygen in the water. The drop in dissolved oxygen can cause aerobic organisms such as fish to die if the oxygen levels are not replenished quick enough. (Algae Bloom)

Lastly, the damming of rivers in that flow in to the estuary has caused a lower net flow of water in the estuary and this causes less zooplankton and some species of fish and an increase in salinity of about 1-2 ppt.



(Phosphorus Cycle)