Regional processes affecting dissolved organic material in the Sacramento-San Joaquin River system

The Central Valley of California, USA, is drained primarily by the Sacramento and San Joaquin River System into a delta that interacts tidally with the San Francisco Bay estuary. We use historical data along with molecular and isotopic tracers to determine the impact of land use, water impoundments, and diversions on dissolved organic material (DOM) concentration and quality. River-borne DOM supports two thirds of the heterotrophic demand of the estuary, lowers the quality of drinking water diversions from the delta, and affects the transport and methylation of mercury. DOM concentration in the rivers and delta varies by over a factor of 6 throughout the year, with a peak in early spring. Our previous results indicated that the delta DOM contribution to the estuary varies seasonally, supplying from 10 percent to 50 percent of the DOM exported by the river system into the estuary, with the greatest contribution occurring during winter and spring. Recent results using molecular source indicators suggest the DOM is largely added by local aquatic production rather than by terrestrial inputs, and is substantially altered by the heterotrophic microbial community. The molecular and isotopic results suggest that water management and land use significantly impact the timing and composition of DOM.