LAKE MERRITT: A TIDAL SLOUGH OF SAN FRANCISCO BAY FOREVER LINKED HYDROLOGICALLY AND BIOLOGICALLY

San Francisco Bay (SFB) consists of three subembayments—North Bay (including Suisun and San Pablo Bays), Central Bay, and South Bay, whereas Lake Merritt is a tidal slough in NE South Bay that connects to the Bay through the former San Antonio Creek as it was inundated by rising sea level. This long-standing connection between the two has allowed organisms in SFB to migrate into the lake as it formed, especially mobile ones (fish, seals, sea lions, and otters) as well as non-indigenous species. The SFB estuary is the largest in surface area on the US west coast (~306,411 acres). In contrast, Lake Merritt is only 140 acres. SFB is generally quite shallow, averaging only 20 feet water depth, with North and South Bays averaging 10-13 feet, Central Bay averaging 36 feet, and the deep tidal channels reaching 30 to 66 feet. The average depth of Lake Merritt is 8-10 feet and 10 feet at its deepest location. The Sacramento and San Joaquin Rivers enter the SFB estuary in the northeast from the Sacramento-San Joaquin Delta and drain 40% of the State of California. Water from the Delta generally moves through North and Central Bays and out to the Pacific Ocean through the Golden Gate, but also into South Bay. In contrast, the Lake only receives rainfall and runoff from the local 4,670-acre watershed through 60 storm drains. The average water temperature in SFB is ~10°C and in the Lake is ~15-16°C. In SFB, salinity values fall somewhere between the two end members of oceanic water (~33–35 psu) and the freshwater Delta (~0 psu). During periods of abundant freshwater input into this system, the boundary between these two (i.e., the 2 psu line), which normally lies somewhere from the Delta to Suisun Bay, may shift substantially oceanward into San Pablo Bay. The salinity in Lake Merritt also varies by location, with seasonal freshwater flowing in by means of the creeks and marine water by tidal flushing. Yet, because Lake Merritt is connected to the Bay only by a channel which construction projects have filled over the years as well as the addition of tidal gates, the flow has been greatly restricted. The tidal range is only 1-2 feet compared to ~6.4 feet for SFB, with a residence time of 4 days when the gates are open. If sea level continues to rise as predicted, the area encompassed by the Lake will become flooded and indistinguishable from SFB, no longer the microcosm that it is now.