Samples for phosphate, nitrate, and nitrite were immediately filtered through 0.22 µm filters and frozen. After storage, aqueous samples were thawed and re-filtered prior to analysis via ion chromatography (Dionex ICS-2100 ion chromatograph, Dionex IonPac AS18 column; SI Section S2 Figures for calibration curves). The MRL for phosphate was determined to be 0.027 mg-P•L -1 and the MRL range was 0.022 to 0.037 mg-P•L -1 (SI Section S2, Table ).