ISOHALINE POSITION AS A HABITAT INDICATOR FOR ESTUARINE POPULATIONS

Populations of native and introduced aquatic organisms in the San Fran cisco Bay/Sacramento-San Joaquin Delta Estuary (''Bay/Delta'') have un dergone significant declines over the past two decades. Decreased rive r inflow due to drought and increased freshwater diversion have contri buted to the decline of at least some populations. Effective managemen t of the estuary's biological resources requires a sensitive indicator of the response to freshwater inflow that has ecological significance , can be measured accurately and easily, and could be used as a ''poli cy'' variable to set standards for managing freshwater inflow. Positio ning of the 2 parts per thousand (grams of salt per kilogram of seawat er) bottom salinity value along the axis of the estuary was examined f or this purpose. The 2 parts per thousand bottom salinity position (de noted by X(2)) has simple and significant statistical relationships wi th annual measures of many estuarine resources, including the supply o f phytoplankton and phytoplankton-derived detritus from local producti on and river loading; benthic macroinvertebrates (molluscs); mysids an d shrimp; larval fish survival; and the abundance of planktivorous, pi scivorous, and bottom-foraging fish. The actual mechanisms are underst ood for only a few of these populations. X(2) also satisfies other rec ognized requirements for a habitat indicator and probably can be measu red with greater accuracy and precision than alternative habitat indic ators such as net freshwater inflow into the estuary. The 2 parts per thousand value may not have special ecological significance for other estuaries (in the Bay/Delta, it marks the locations of an estuarine tu rbidity maximum and peaks in the abundance of several estuarine organi sms), but the concept of using near-bottom isohaline position as a hab itat indicator should be widely applicable. Although X(2) is a sensiti ve index of the estuarine community's response to net freshwater inflo w, other hydraulic features of the estuary also determine population a bundances and resource levels. In particular, diversion of water for e xport from or consumption within the estuary can have a direct effect on population abundance independent of its effect on X(2). The need to consider diversion, in addition to X(2), for managing certain estuari ne resources is illustrated using striped bass survival as an example. The striped bass survival data were also used to illustrate a related important point: incorporating additional explanatory variables may d ecrease the prediction error for a population or process, but it can i ncrease the uncertainty in parameter estimates and management strategi es based on these estimates. Even in cases where the uncertainty is cu rrently too large to guide management decisions, an uncertainty analys is can identify the most practical direction for future data acquisiti on.