THE CONTINUING PLIGHT OF MICROFAUNA IN AN URBAN LAGOON

California’s Lake Merritt is a heavily polluted tidal lagoon supporting an impoverished fauna of foraminifera and ostracodes. Soon after it was created in 1869 by damming San Antonio Slough off from Oakland Inner Harbor in San Francisco Bay, urban development obliterated its wetland perimeter and rapid aquatic degradation ensued. When rain is predicted at low tide, tidegate closure transforms the lagoon into a catchment basin for four culverted creeks and 60 storm drains. High nutrient levels lead to algal blooms that periodically create patches of low-oxygen bottom water above black mud reeking of H2S. Cement and wood substrates along the margins support a community dominated by exotic invertebrates.

As Lake Merritt begins restoration, we have been collecting microfaunal data to monitor the health of this ecosystem and to enhance our understanding of how these organisms respond to environmental parameters. Forty-one substrate samples collected between January 2003 and April 2004 yielded 18 species of Foraminifera, 1 species of Thecamoebia, and 16 species of Ostracoda. The highly dominate species Ammonia tepida s.l. and Cyprideis beaconensis characterize estuaries along the northeast Pacific margin. Live foraminifers were detected only in the most recent samples from the shallow lake margin where salinity and dissolved oxygen levels are highest. Common foraminiferal teramorphs, most apparent in the ubiquitous Ammonia populations, may reflect high concentrations of heavy metals derived from urban runoff. Trochammina hadai, a foraminifer introduced into San Francisco Bay in the 1980’s via ballast from Japan, has infiltrated the lake in low numbers that appear to be increasing.

The ostracode fauna consists of 6 freshwater, 2 brackish, and 8 nearshore marine species, distributed into three biotopes reflecting bottom facies that differ in salinity (ranging 11 to 38 ‰) and dissolved oxygen (ranging 0.17 to 10.32 mg/L). The ratio of nodose vs. non-nodose ecophenotypes of Cyprideis beaconensis increases proportionally with elevated levels of salinity. As with the foraminifers, live ostracodes thrive primarily along the lake margin of maximum tidal influence.