Marshes as sources or sinks of an estuarine mysid: demographic patterns and tidal flux of Neomysis kadiakensis at China Camp marsh, San Francisco estuary

The population characteristics, seasonal abundance and tidal flux of Neomysis kadiakensis were studied to determine if tidal marshes serve as sources or sinks for mysids in China Camp, located in the San Francisco estuary. Monthly surveys of the zooplankton community were conducted during spring tide periods between May 2002 and May 2003, using a fyke net (500 μm mesh) fixed at the mouth of a fourth order tidal channel system in a tidal marsh. Mysids were collected and preserved hourly throughout full tidal cycles. Neomysis kadiakensis was the dominant mysid species, comprising 94-100% of mysids throughout the year. Monthly mean abundance of N. kadiakensis ranged from 14 mysids m−3 in January 2003 to 244 mysids m−3 in March 2003. Length frequency distributions indicated that reproduction and recruitment were nearly continuous, with abundance peaks occurring throughout the year (e.g., spring, summer and early fall). The resulting flux estimates during this period suggested that China Camp marsh was a sink for N. kadiakensis. The mean daily (ca. 24.8 h) fluxes of N. kadiakensis corresponded to an instantaneous daily population mortality rate within the marsh channel of 0.29 d−1. Although all sizes and stages experienced a net import to the marsh, a significant positive relationship was observed between mysid length and mean annual flux, indicating that larger, mature mysids experienced a greater degree of tidal import to the marsh than smaller, juvenile mysids. These analyses suggest that size-specific predation, perhaps from fishes, shrimp, and/or birds may have had a disproportionate impact on large, mature mysids. Mortality rates in the marsh greatly exceeded the overall population growth rates, indicating that predation pressure on mysids was greater in the marsh than in the subtidal estuary. Mysids, therefore, supply a net flux of energy from the subtidal estuary to the marsh during spring tide series; however, the ultimate fate of this energy is unknown (remaining within the marsh system vs. export back to the estuary via trophic relay or other processes).