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Abstract: Do clams like dams? Effects of river discharge on growth rates in marine bivalve mollusks as revealed by sclerochronological and stable isotope techniques
Before the upstream dams and diversion projects, the Colorado River delivered approximately 18 million cubic meters per year of fresh water to its estuary. Today, only a trickle enters the upper Gulf of California. How did this environmental change affect growth rates in two common marine bivalve mollusks in the area today?
We examined the growth rates of 183 specimens of Chione spp. (111 C. cortezi and 72 C. fluctifraga). 86 specimens were alive during times of river flow and 97 specimens alive during times when the river did not reach the upper Gulf. The presence of fresh water influx was recognized by del 18-O values below those caused by annual temperature variation. Growth rates during the second and third years were estimated by the amount of shell added between sclerochronologically and isotopically defined winter growth bands. Intra-annual variation in growth was estimated by measuring circadian and fortnight growth increments between winter bands.
Annual growth during times of river flow differs significantly from annual growth during times of no flow. The sharply reduced freshwater flow increased growth rates from 10.8 to 27.5% in both species. Examination of intra-annual growth clearly indicates that fresh water discharge can depress growth rates in Chione spp. In the absence of river flow, growth increases to a maximum in May/June and reaches a second, but smaller maximum in fall. Growth in specimens alive during river influx showed similar growth rates in spring and autumn. Autumn rates during times of river flow were significantly higher than today’s growth rates, but significantly lower than today’s growth rates during spring. Historic and isotopic evidence indicates that maximum river flow (spring snowmelt in the Rocky Mountains) occurred at the Colorado delta from May to July. This pulse of fresh water may have reduced the overall growth in these two species. The snowmelt-derived water masses presumably carried large amounts of nutrients to the estuary, which fed the clams during fall. Today this additional meal is not provided.
Sclerochronological and isotopic techniques applied to Recent and subfossil shells can decipher the results of an unintentional experiment on the biota of the upper Gulf of California.
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