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| California Science Center |
U.S. Geological Survey Open-File Report 2005-1279
By Edward Moon, Michelle K. Shouse, Francis Parchaso, Janet K. Thompson, Samuel N. Luoma, Daniel J. Cain and Michelle I. Hornberger
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Trace elements in sediments and clams (Macoma petalum) (formerly reported as Macoma balthica (Cohen and Carlton 1995)), clam reproductive activity and benthic community structure are reported for a mudflat one kilometer south of the discharge of the Palo Alto Regional Water Quality Control Plant in South San Francisco Bay. This report serves as a continuation of the Near-Field Receiving Water Monitoring Study, which began in 1974. Prior to this report, trace metal and reproductive activity/community structure data were reported separately (see Moon et al. 2004 and Shouse et. al. 2004). The data for 2004, herein, are interpreted within that context and are consistent with those previously reported.
Metal concentrations in both sediments and clam tissue samples are within the range of values previously observed due to seasonal variability. Copper and zinc concentrations in sediment and bivalve tissue display continued decreases over the last decade. In 2004, copper in sediment was observed to drop below the ERL (Effects Range-Low) concentration for the fourth consecutive year. The concentration of zinc in sediment never exceeded the ERL. Yearly average concentrations of copper, zinc and silver in Macoma petalum for 2004 are some of the lowest recorded since monitoring for metals began in 1975. The concentrations of mercury and selenium, during April and January 2004, respectively, were the highest values observed for these elements during this study. However, the concentrations of these elements in sediments and clams at Palo Alto were similar to concentrations observed elsewhere in the San Francisco Bay.
Analyses of the benthic community structure of a mudflat in South San Francisco Bay over a 31-year period show that changes in the community have occurred concurrent with reduced concentrations of metals in the sediment and in the tissues of the biosentinal clam Macoma petalum from the same area. Analysis of the reproductive activity of Macoma petalum shows increases in reproductive activity concurrent with the decline in metal concentrations in the tissues of this organism. Reproductive activity is presently stable with almost all animals reproducing during the two reproductive seasons (spring and fall) of most years. The community has shifted from being dominated by several opportunistic species to a community where the species are more similar in abundance, a pattern that could be indicative of a more stable community that is subjected to less stress. In addition, two of the opportunistic species (Ampelisca abdita and Streblospio benedicti) that brood their young and live on the surface of the sediment in tubes have shown a continual decline in dominance coincident with the decline in metals. Heteromastus filiformis, a subsurface polychaete worm that lives in the sediment, consumes sediment and organic particles residing in the sediment, and reproduces by laying their eggs on or in the sediment has shown a concurrent increase in dominance. These changes in species dominance reflect a change in the community from one dominated by surface dwelling, brooding species to one with species with varying life history characteristics.
Table of contents
List of figuresList of tablesAbstrac
Introduction
Environmental Monitoring
Trace Metals
Benthic Community Response to Trace Metals
RWQCB and NPDES
Objectives
Approach
Study Site
Methods
Sampling Frequency
Sediment
Clams
Metals Analysis
Reproductive Activity
Community Analysis
Results and Discussion
Salinity
Sediments
Metals in Clams
Reproduction of Macoma petalum
Benthic Community
Summary
Long-term Observations
2004
Value of Long-Term Monitoring
References
Figures
Tables
Appendix A
Appendix B
Appendix C
Appendix D
Appendix E
Appendix F
Appendix G
Appendix H
For Additional Information Write to:
Samuel N. Luoma,
MS 465 U.S. Geological Survey
345 Middlefield Road
Menlo Park, CA 94025
Copies of this report may be obtained from the authors or
U.S. Geological Survey
Information Center Box 25286, MS 517
Denver Federal Center
Denver, CO 80225
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