Long Island Sound (LIS) is a large estuary with seawater input from the Atlantic Ocean and freshwater input from the Connecticut River (~72%), Housatonic River (~12%) and Thames River (~9%). As a result, the modern salinities in LIS are below open ocean salinities and range from 22‰ to 32‰. The composition of carbonate tests of benthic foraminifera that form from LIS waters reflects the water salinity and water temperature in their d18O, Sr/Ca and Mg/Ca values. The d13C of carbonates reflects the isotopic values of dissolved carbonate in LIS waters, which depends on the mixing proportions of river and sea water, the d13C of carbonate in river water, and the degree of oxidation of organic carbon in the Sound. The latter process is responsible for the seasonal hypoxia in LIS. We hypothesize that the d13C in foraminiferal calcite is influenced by the amount of organic carbon oxidized, and thus can be used as a tracer for the degree of hypoxia that was prevalent when shells were being formed. We established a mixing model for modern LIS through analyses of river and LIS water samples for salinity, d18O, dissolved carbonate, d13C in dissolved carbonate and Ca, Mg and Sr. Bottom water oxygen contents and temperatures were measured when living foraminifera for analyses were being collected. The elemental and oxygen isotopic composition of the carbonates was calibrated for salinity and temperature using the field measurements. We determined the difference between measured d13C and predicted d13C, which is a measure of the amount of oxidized organic matter in the system. Our field results in the western part of LIS show that when oxygen levels are at or below 4mg/l, significantly lighter carbon isotopic compositions are observed than those predicted. Results from the mixing model calibrations will be applied to carbonates from 4 short cores to establish changes in temperature, salinity and dissolved oxygen in LIS over the last 400 years.