PLIGHT OF FORAMINIFERA AND OSTRACODA IN AN URBAN LAGOON

California’s Lake Merritt (the nation’s first wildlife refuge) is a heavily polluted tidal lagoon supporting an impoverished fauna of foraminifera and ostracodes. It was created in 1869 by damming San Antonio Slough off from Oakland Inner Harbor in San Francisco Bay. Soon thereafter, urban development obliterated its perimeter of marshes and mudflats and the aquatic environment rapidly degraded. A narrow channel permits tidal cycling, but tidegates are closed at low tide when rain is predicted, making the lagoon a catchment basin for runoff from four culverted creeks and 60 storm drains. Although its substrate is mostly black mud reeking of H₂S and its bottom water is periodically disaerobic, this open-water estuarine habitat currently supports a thriving community of predominantly exotic invertebrates, many of which have exploited introduced hard substrates.

In anticipation of Lake Merritt’s long overdue restoration, we are seeking data that could enhance the utility of certain microfauna in monitoring water pollution. Substrate samples collected in January and June 2003 yielded 14 species of Foraminifera and 10 species of Ostracoda. Ammonia tepida s.l. dominates foraminiferal assemblages throughout the lagoon, whereas Cyprideis beaconensis is the most common ostracode. This fauna is typical of marginal marine environments, but few live foraminifers were recovered and live ostracodes were rarely encountered beyond the tidally winnowed sector. Common teramorphs of Ammonia may reflect high concentrations of heavy metals attributed to urban runoff. Trochammina hadai, a foraminifer introduced into San Francisco Bay via ballast from Japan in the 1980’s, has infiltrated the lake in low numbers. We suspect that glassy microspherules, unusually common in Lake Merritt, formed during the firestorm that swept nearby hills in 1991.