Elucidating the influence of oceanic biological activity on the organic composition of sea spray aerosol (SSA) is crucial to understanding marine cloud properties relevant to climate. Numerous marine field studies designed to address this topic have yielded conflicting results mainly as a result of the inability to distinguish primary SSA composition from terrestrial and marine secondary organic aerosols. In this study, two laboratory-induced phytoplankton blooms were conducted in an isolated system without background aerosol contributions. Values for δ 13 C were measured for SSA (δ 13 C SSA ) along with seawater particulate and dissolved organic carbon (δ 13 C POC and δ 13 C DOC ) to track changes in carbon transfer and composition between seawater and SSA. Contrary to common assumptions, δ 13 C SSA values were not equivalent to δ 13 C DOC . The consistently less negative δ 13 C SSA values indicate that nascent δ 13 C SSA reflects specific changes in relative contributions to SSA from the available seawater carbon pools, as a function of biological activity. A dual-source isotopic mixing model revealed that the difference between δ 13 C SSA and δ 13 C DOC was explained by increased relative contributions of "freshly produced" organic carbon (OC) to SSA, with the largest contribution of "freshly produced" OC occurring 2−3 days after the maximum chlorophyll-a concentrations. This finding is consistent with previous mesocosm studies, showing that organic enrichment in SSA requires processing by heterotrophic bacteria after periods of high primary productivity. This work examining the biological influences on SSA organic composition and nascent δ 13 C SSA values provides new insights into ocean-to-SSA carbon transfer dynamics, which can be used in future field studies to improve estimates of anthropogenic influences on the carbon composition of the marine environment.