seasonal variations. Maximum deposition takes place in the winter and early spring when river runoff is highest due to increased rain, when the longshore drift is coming from the south, and when winds are from the southwest. In the summer, deposition is low because of lower runoff, southward longshore drift and northwest winds. Turbulent mixing within the estuary increases the amount of suspended sediment and therefore increases the turbidity within Yaquina Bay. Turbidity has been found to be most prominent when river runoff is low, typically during the summer. Chlorophyll concentrations and other measurements of water chemistry (see Chemistry section below) verify these observations. Geology Yaquina Bay was formed roughly 2.2 million years ago in the Oligocene Epoch of the Cenozoic Era. Increased sedimentation and subsidence contributed to formation of Yaquina Bay. Sedimentary rocks in this area are mainly "sandstone, siltstone and clay-containing mudstone", as indicated in geological samples from the sediments. The oldest rocks found in the Coast Range and in Yaquina Bay date back to Paleocene and Eocene Epochs, about 40-60 million years ago. The seafloor to the west of Oregon is continually spreading and there is evidence of subduction taking place along the coast of Oregon and Washington. Biology of Yaquina Bay Yaquina Bay comprises several different habitats that provide resources for many species. These habitats include intertidal eelgrass beds, mudflats, and sandflats, and subtidal areas. Mudflats and sandflats are typically covered by water during high tide and exposed to air during low tide, while subtidal areas are covered by water almost all of the time. Life in Yaquina Bay Beds of native eelgrass (Zostera marina) and beds of invasive eelgrass (Zostera japonica) grow separately and provide a distinct habitat for certain organisms. Birds, including gulls, ducks, shorebirds, crows, geese, egrets, rails, pelicans and cormorants are present in Yaquina Bay using the eelgrass and mudflats as habitats. Mud shrimp also live in mudflats, and they play an important role in nutrient cycling within the estuary. Burrows in the mud made by mud shrimp pump oxygen deeper into the sediment, which makes it available for microbes to use. Mud shrimp activity has also been shown to increase the movement of carbon and dissolved inorganic nitrogen in and out of the mud. Some species of diatoms, a type of phytoplankton, can be found on the sediments of Yaquina Bay wetlands. These diatoms are also important in nutrient cycling within