STABLE ISOTOPES OF CARBON AND NITROGEN IN CORBICULA CLAMS RECORD SPATIAL VARIATION IN AQUATIC DIC AND NUTRIENT SOURCES IN A JAPANESE LAKE

Bivalve mollusks such as the Japanese clam Corbicula japonica can be used to monitor spatial variation in aquatic chemistry, as they integrate pollutants over a period of time and may store a record of environmental conditions in the accretionary growth of shell material. We measured δ¹³C and δ¹⁵N in the adductor muscle and foot tissue of C. japonica obtained from sites across Lake Shinji, Japan, as well as δ¹⁵N in the organic matrix of shells in live-collected individuals. Lake Shinji receives freshwater at its western end from the Hii River and several agricultural drainage canals; it then supplies water to brackish Lake Nakaumi via the Ohashi River.

δ¹³C values measured in the tissues of C. japonica collected from Lake Shinji range between –25‰ and –20‰, rising in magnitude toward the eastern end of the lake. Surface sediments (upper 1cm, δ¹³C –26‰ to –23‰) exhibit a similar, albeit slight, trend (Dettman and Seto 2004). The low C/N ratio of organic matter in the sediments indicates that it derives primarily from aquatic algae. The δ¹³C gradient may reflect the occasional counterflow of Nakaumi water, which is dominated by marine dissolved inorganic carbon (DIC), through the Ohashi River. Tissue samples obtained from Corbicula living in canals and in a western site occupied by dense Phragmites show δ¹³C values <–25‰, perhaps indicating greater local influence from terrestrial carbon sources. C. japonica tissue δ¹⁵N values range between 8‰ and 13‰, also increasing from west to east. This may record low-δ¹⁵N agricultural drainage water entering the western end of Lake Shinji and high-δ¹⁵N runoff and wastewater from the city of Matsue at the eastern end. Isotopic measurements on N trapped within the shell organic matrix show a comparable spatial pattern, however high variability in δ¹⁵N values between individual shells from the same collection sites likely precludes the use of buried shells for precise temporal reconstruction of aquatic nutrient conditions.

Dettman, D.L. and K. Seto, 2004, Organic matter in Lake Shinji and Nakaumi sediments: Sources inferred from nitrogen and carbon isotope geochemistry. International seminar on restoration of damaged lagoon environments, Shimane, Japan, January 2004.