We describe the use of the 14C-AMS technique to study the influence of upwelling waters to the local biological production, at Arraial do Cabo, Brazil. Results on Δ14C from seaweed tissues incubated into three different types of water mass show the association of primary production with the upwelling waters. We also discuss the importance of high- precision 14C - AMS measurements for this kind of study, and the properly application of isotopic fractionation corrections on radiocarbon results.