HR: 0800h
AN: B31D-0614    [Abstracts]
TI: Bivalve Forensics: Sclerochronological Constraints on the Timing of a Biological Invasion in San Francisco Bay 
AU: * Goodwin, D H
EM: goodwind@denison.edu
AF: Denison University, Department of Geosciences 100 Sunset Hill Drive, Granville, OH 43023, United States
AU: Cohen, A N
EM: acohen@sfei.org
AF: San Francisco Estuary Institute, 7770 Pardee Lane, Oakland, CA 94621, United States
AU: Roopnarine, P D
EM: proopnarine@calacademy.org
AF: California Academy of Sciences California Academy of Sciences, 875 Howard Street, San Francisco, CA 94103, United States
AB: Clams are biological chart recorders: their shells contain a record of environmental conditions in the form of periodic growth increments and geochemical variation. In most species, this archive begins with the deposition of the dissoconch following metamorphosis and ultimately ends with the death of the individual. In fossil specimens, the exact date of the beginning or end of the archive cannot be determined. Therefore, sclerochronologic and geochemical records from fossils represent short intervals of time that cannot be directly tied to absolute time—so called "floating chronologies." In contrast, the date of the end of the archive from live- collected specimens can be precisely resolved—in many cases to the day collection. Using this date as a pinning point, one can establish the timing of events throughout the remainder of the chronology relatively precisely. Here, we use this approach to constrain the timing of a biological invasion event in San Francisco Bay. The basic idea is simple. The timing of an invasion event (i.e., successful recruitment), can be resolved if the date of the initiation of shell deposition is established from the first specimens to appear in a new area. To absolutely establish the timing of an invasion, one must assume that specimens represent the initial cohort of invaders. If this assumption cannot be met, then this method provides minimum estimates for the timing of invasion events. This technique is best suited for regions where species composition is closely monitored and the likelihood of collecting the initial cohort is high. To demonstrate this technique, we examined specimens thought to represent the initial cohort of the oyster Crassostrea gigas to establish a population in South San Francisco Bay. All individuals were collected live in July or August of 2006. Sclerochronological examination suggested that the specimens were at least four years old. These age estimates were confirmed using stable oxygen isotope (δ¹⁸O) and stable carbon isotope (δ¹³C) variation. For each specimen, δ¹⁸O and δ¹³C profiles representing the complete ontogenetic history were obtained by sampling the resilifer in the left valve. In all specimens, δ¹⁸O and δ¹³C values were positively correlated and show strong seasonal variation. Comparison of predicted and observed δ¹⁸O variation suggests that both temperature and the δ¹⁸O of the water in which they grew—a function of Delta outflow—strongly influence the pattern of δ¹⁸O observed in the shell. Together, sclerochronological and geochemical analysis indicate that this invasion of C. gigas in South San Francisco Bay occurred in the Spring of 2002. The ability to establish the timing of a biological invasion is likely to help scientists better understand the dynamics of invasion events. Furthermore, these results may help environmental managers and policy makers manage invasive species, as well as develop strategies to prevent future invasions of C. gigas and other non- native species.
DE: 0424 Biosignatures and proxies
SC: Biogeosciences [B]
MN: 2007 Fall Meeting