THE SOURCE OF SUSTENANCE FOR SIPHONING SHELLFISH IN THE COLORADO RIVER ESTUARY: NITROGEN ISOTOPES IN BIVALVE SHELLS REVEAL CHANGES IN NUTRIENT DELIVERY

Stable isotopes of N can be used to detect the source of nutrients in modern and past estuarine ecosystems. We examined δ¹⁵N from the organic matrix of shells in live-collected and sub fossil bivalve mollusks from the Colorado River and its estuary. In most years since 1960, the river no longer reaches the sea, so it also fails to deliver sediments and nutrients to the Gulf of California. The absence of terrestrially-derived nutrients may be impairing the estuarine ecosystem, but the utilization of these nutrients in the “predambrian” estuary has not been documented. We compared δ¹⁵N measurements from three species of bivalve mollusks: Corbicula fluminea, an introduced freshwater species, Mulinia coloradoensis, a brackish species once abundant in the estuary, and Chione fluctifraga, a fully marine species. All three are found alive today, and the latter two also occur in predambrian shell deposits. δ¹⁵N values obtained from modern Co. fluminea are low (δ¹⁵N = 3.3 ‰) with respect to live-collected M. coloradoensis and Ch. fluctifraga (average δ¹⁵N = 9.8 ‰), documenting that fluvial N signals are distinct from marine N signatures. Some component of these differences could be attributable to interspecies variation in physiology and behavior. Preliminary data also show that predambrian M. coloradoensis and Ch. fluctifraga are isotopically much lighter than modern specimens of the same species (δ¹⁵N = 3.5 ‰ and 6.0 ‰, respectively). This indicates that nutrients from the Colorado River Basin reached the river's estuary during predambrian times and were utilized by the local biota. Because productivity in the estuary may have been fueled by its river, conserving and restoring the estuarine ecosystem may require the resumption of some Colorado River flow. N isotopes from mollusk shells may reveal changing sources and levels of nutrients in other modern and ancient coastal ecosystems.