Copper titrations of known concentrations of chelators in artificial seawater illustrated that the growth depression of Thalassiosira pseudonana in response to copper could be used to quantitate the chelator levels to within ± 5% at concentrations down to 10−7 M EDTA. These results suggest that measured variations in T. pseudonana—copper bioassays to samples from Narragansett Bay and vicinity could be due to organic chelators in seawater.