One of the most conspicuous anthropogenic disturbances to estuaries worldwide has been the alteration of freshwater and tidal influence through the construction of water control structures (dikes, tide gates, culverts). Few studies have rigorously compared the responses of differing groups of organisms that serve as contrasting conservation targets to such anthropogenic disturbances in estuarine ecosystems. Elkhorn Slough in central California includes a spectrum of tidally restricted habitats behind water control structures and habitats experiencing full tidal exchange. To assess community composition for several different taxa in habitats with varying tidal exchange, we employed a variety of field approaches and synthesized results from several different studies. Overall, we found that communities at sites with moderately restricted tidal exchange were fairly similar to those with full tidal exchange, but those with extremely restricted tidal exchange were markedly different from other categories. These differences in community composition are likely the result of several factors, including restricted movement due to physical barriers, differences in water quality characteristics, and differences in habitat structure. Indeed, in this study, we found that water quality characteristics strongly vary with tidal restriction and may strongly influence patterns of species presence or absence. We also found that different conservation targets showed contrasting responses to variation in tidal exchange. Full exchange appears to favor native oysters, commercially valuable flatfish, migratory shorebirds, and site-level biodiversity. Minimal tidal exchange due to water control structures supports a suite of estuarine endemics (including the tidewater goby and California brackish snail) not represented elsewhere and minimizes invasions by nonnative marine species. Altogether, our results suggest that total estuary-wide biodiversity may be enhanced with a mosaic of tidal exchange regimes.
Estuarine restoration is a major focus of coastal management. To set estuarine restoration targets, coastal managers need to understand natural baselines and human modifications. The goal of this study was to characterize baseline environmental conditions for the purposes of restoration planning at Elkhorn Slough, a regionally significant California estuary. We reconstructed baseline salinity, sediment sources, sediment accumulation rates, and wetland plant distribution by collecting and analyzing sediment cores from the upper and lower reaches of the estuary, and marsh extent by synthesizing previously published stratigraphic descriptions of sediment cores. The results of this study show strong contrasts between current and baseline conditions. Sediment accumulation rates have recently increased, whereas flood deposits have disappeared from marsh sediments. Representation by freshwater and brackish plants has also declined. Extent of marshes increased in the recent past, likely as a result of anthropogenic sediment loading from early EuroAmerican land use changes. Many of these marshes have degraded in past decades, but marsh extent today is still higher than in previous periods. Reconstruction of natural baselines and processes suggests that restoration strategies for the estuary should focus on increasing sediment supply and freshwater inputs to the marshes in order to restore the processes that naturally sustained marsh accretion and diversity. This study highlights the importance of revealing human modifications when designing restoration strategies for dynamic and highly altered systems such as estuaries.
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