<P> The San Francisco Estuary together with the Sacramento--San Joaquin River Delta represents a highly altered ecosystem . The region has been heavily re-engineered to accommodate the needs of water delivery, shipping, agriculture, and most recently, suburban development . These needs have wrought direct changes in the movement of water and the nature of the landscape, and indirect changes from the introduction of non-native species . New species have altered the architecture of the food web as surely as levees have altered the landscape of islands and channels that form the complex system known as the Delta . </P> <P> This article deals particularly with the ecology of the low salinity zone (LSZ) of the estuary . Reconstructing a historic food web for the LSZ is difficult for a number of reasons . First, there is no clear record of the species that historically have occupied the estuary . Second, the San Francisco Estuary and Delta have been in geologic and hydrologic transition for most of their 10,000 year history, and so describing the "natural" condition of the estuary is much like "hitting a moving target". Climate change, hydrologic engineering, shifting water needs, and newly introduced species will continue to alter the food web configuration of the estuary . This model provides a snapshot of the current state, with notes about recent changes or species introductions that have altered the configuration of the food web . Understanding the dynamics of the current food web may prove useful for restoration efforts to improve the functioning and species diversity of the estuary . </P> <P> The San Francisco Bay is both a bay and an estuary . The former term refers to any inlet or cove providing a physical refuge from the open ocean . An estuary is any physiographic feature where freshwater meets an ocean or sea . The northern portion of the bay is a brackish estuary, consisting of a number of physical embayments which are dominated by both marine and fresh water fluxes . These geographic entities are, moving from saline to fresh (or west to east): San Pablo Bay, immediately north of the Central Bay; the Carquinez Strait, a narrow, deep channel leading to Suisun Bay; and the Delta of the Sacramento and San Joaquin rivers . </P>

Three top predators in aquatic ecosystems in the bay area living today