Benthic invertebrate response to habitat complexity in South Bay salt ponds

Background/Question/Methods

In the south San Francisco Bay, California, the South Bay Salt Pond Restoration Project is restoring over 15,000 acres of former salt ponds to tidal marshes and managed pond habitats. Benthic invertebrates are a vital piece of the South Bay food web as they provide the sustenance for other invertebrates, fish, shorebirds and waterfowl. Improving the health of benthic communities in existing ponds is expected to attract more birds to those ponds and improve the overall health of the South Bay. One approach to increasing invertebrate populations is to provide structural heterogenity.  This study assessed the relationship between invertebrate diversity, species richness, and density at two types of structures--bird islands and wood posts--in two managed ponds in the South Bay.  The managed ponds included a deep, long-term ponded habitat and a nearby pond newly-restored to muted tidal action.  Soil cores and water samples were taken in June and September 2011 at ten wood structure and ten non-structure sites in the deep pond, as well as seven bird island structures and seven non-structure sites in the restored pond.  I tested the effects of pond type, structures, and seasonal water quality on different invertebrate parameters. 

Results/Conclusions

Results showed that neither type of structure significantly affected benthic parameters, although there was a trend toward more organisms and diversity at structure versus non-structure sites.  However, invertebrate populations showed significant changes based on season. Invertebrate populations in the deep pond showed a decline from summer to fall, while populations at the restored pond showed increased abunance in fall compared to summer.  Invertebrate decline in the deep pond may be attributed to seasonal water quality changes, especially increased temperature and salinity, and decreased dissolved oxygen. Invertebrate species indicative of poor water quality were present during the fall months.  The increased invertebrate numbers in fall at the restored site seem less due to water quality than the departure of migratory shorebirds between the summer and fall sampling, as invertebrates that are key shorebird food sources showed increases.  These results suggest that while structures may support larger more diverse populations, restoring tidal action is expected to have a greater beneficial effect on invertebrate populations and the bird species that feed on them.