California Sea Grant College Program

Over the past 150 years, Mission Bay, San Diego, has undergone a variety of changes that have affected benthic ecosystems. River diversion, dredging, filling, over-harvesting, pollution, and the introduction of exotic species have greatly altered bivalve assemblages in the bay. A gradient of flushing and organic enrichment also has been established by patterns of circulation and input of urban runoff. This results in relatively depauperate macrofaunal communities in the poorly-flushed back bay, peaks of abundance mid-bay, and high species richnesses in the mid-bay and nearer the mouth.

One of the most striking changes in Mission Bay has been the invasion of a small, Asian mussel, Musculista senhousia. This rnytilid, which often forms byssal mats on intertidal and soft sediments, was first found at least two decades before it carne to dominate portions of the bay, highlighting that a lag may exist between the invasion and subsequent population explosion of invasive species. The invasion of M. senhousia can be viewed both as a consequence and a cause of ecological change. As is often observed in urbanizing systems, anthropogenic changes, coupled with increases in the global movement of species, probably have made Mission Bay more vulnerable to invasion and may account for an increased rate of appearance of exotics over the last two decades.

Musculista senhousia can cause ecological changes by its physical alteration of the benthos. A variety of small macrofauna are found within natural mussel mats at higher densities than adjacent, mat-free sediments. Manipulative experiments using artificial mussel mats, living mussels, and mussel mimics suggest that the presence of the physical structure alone can explain much of this facilitation, although the biological activities of living mussels do affect some taxa. The effects of the mats are scale-dependent, however. Competition experiments demonstrate that the growth and survivorship of large, surface-dwelling, suspension-feeding clams are inhibited by M. senhousia. These experiments suggest that M. senhousia may have contributed to observed changes within a remnant salt marsh system (Northern Wildlife Preserve). Rapid colonization of a newly-created mitigation site could affect ongoing restoration efforts in this marsh.