The role of the alien clam Corbicula fluminea in the regulation of organic carbon in the San Joaquin River watershed

Science Center Objects

Sources and fate of various forms of organic carbon in the Sacramento-San Joaquin Delta (Delta) and San Joaquin River watershed are of concern because of the importance of identifying the sources of carbon contributing to the oxygen depletion zone on the San Joaquin River near the city of Stockton, the need to understand the causes of the low primary and secondary production in the Delta, and the potential for limited restoration success of the system given the aforementioned low productivity.

USGS scientist sampling Corbicula fluminea clams.

A USGS scientist sampling Corbicula fluminea clams.

Recent studies in the San Joaquin River drainage have focused on characterizing sources, loads and fate of organic carbon delivered from the San Joaquin River to the Delta. Those studies have not been concerned with ecosystem processes such as grazing that may underlie observed loads of organic carbon even though such processes have been shown to be extremely important in the Delta. The purpose of this study is to examine the external (tributary) source of phytoplankton to the Delta and specifically, to ask if phytoplankton may become depleted by grazing of the alien clam Corbicula fluminea within source waters before reaching the Delta, effectively reducing the external riverine loading of high quality, labile organic carbon to the Delta.

The objective is addressed using a simple numerical model based on field-derived estimates of clam density and size distribution and laboratory-derived grazing rates.

A limited number of chlorophyll-a and suspended particulate matter samples are collected in conjunction with the clam samples to support the modeling effort. Model predictions are compared with existing measures of organic carbon flux in the system to determine if clam consumption of phytoplankton is a significant component of the organic carbon budget of the system and assess whether it is possible that clam consumption of phytoplankton has contributed substantially to the long-term decline in Delta primary productivity. The results of the study are reported in a professional journal article.