Comparing zooplankton and snail densities among reference and restored marshes

Significant loss of wetland habitat with the subsequent loss of suitable habitat for juvenile fish in the San Francisco Estuary has led state and federal agencies along with university researchers to unite in the CALFED Bay Delta Program, which has restored several previously non-tidal marshes in the Estuary to various degrees of tidal amplitude. Hypothetically, reference/historical marshes that have not been diked, dredged or filled should yield greater zooplankton densities (a primary source of food for juvenile fish) compared to marshes recently restored to tidal action. This study examines the hypothesis by comparing zooplankton population densities among reference and restored marshes and testing for a correlation between increased zooplankton densities and increased fish densities within the marshes. Zooplankton sampling occurred approximately monthly (not all sites were accessible each month) at two major reference sites (Edith West Reference Marsh and Statelands Reference Marsh) and at six restored sites (Inner McNabney/Shell Marsh, Outer McNabney/Shell Marsh, Waterfront Marsh, Pt. Edith Marsh, Navy Marsh, all in southern Suisun Bay, and Tubbs Island Marsh in San Pablo Bay). Limited zooplankton sampling was also conducted at two additional reference marshes 10 km up the Delta from Suisun Bay (Northern Brown's Island Marsh and Sherman Island Marsh). At each marsh, zooplankton samples were obtained near the entrance of the tidal channel. The hypothesis of higher reference marsh zooplankton densities was rejected for several marshes with tidal pools connected to the tidal channels; these marshes yielded significantly higher zooplankton densities than the reference marshes. Magnitude of the mean differences ranged from 300-1000%. Furthermore, increased zooplankton densities in these restored marshes with connected tidal pools correlated with increased densities of small fishes. However, two restored marshes lacking tidal pools and one marsh with a small tidal pool yielded significantly lower overall zooplankton densities when compared to the reference marshes though the magnitude of differences in the mean were small (approximately 1-3%). Both additional reference marshes further up the Delta (Northern Brown's Island, and Sherman Island) yielded low zooplankton densities similar to that of the original reference marshes. Additionally, other researchers involved in our CALFED study noted reference marshes and restored marshes lacking tidal pools yielded fewer fish and epibenthic animals (amphipods, cumaceans, isopods, etc.) than did the marshes with tidal pools connected to the tidal channels. Generally, calanoid copepods were the predominant taxa at all sites and other taxonomic lists were fairly similar among the reference and restored marsh sites. Since restoration is a relatively new area of study, restoration efforts should proceed cautiously with restoration plans tailored to meet the unique environmental conditions present at each site. Conservation of existing sites (sites that have not been dredged, diked or filled) should be the focus with restoration as an experimental alternative solution. However, because there are few marsh habitats left to conserve in the San Francisco Bay Estuary, and these results are encouraging even for restoration to partial tidal action, it is likely that restoration efforts will continue and improve at future sites.