Measuring carbon isotope ratios of microphytobenthos using compound-specific stable isotope analysis of phytol

Abstract

Carbon stable isotope analysis is one of the most common methods for validating theories about the role microphytobenthos (MPB) plays in estuarine food webs. However, difficulties extracting MPB from sediments to determine a pure isotope signature have hampered such studies. We have developed compound-specific isotope analysis (CSIA) of phytol δ13C (δ13Cphytol) as an accurate proxy for δ13C of bulk MPB (δ13Cbulk). In most circumstances MPB is the dominant source of phytol in estuarine sediments, particularly in shallow water environments, so our method circumvents the need to extract MPB. We have demonstrated in the laboratory the relationship between δ13Cphytol and δ13Cbulk, and could predict δ13Cbulk within 1.8‰ (95% CI) of observed values at moderate replication (n = 5). If water temperature is included, δ13Cphytol predicts δ13Cbulk within 1.3‰. With greater replication (n = 10), predictions are extremely good (within 1.0‰). A two-source mixing model assessed the usefulness of each method for estimating MPB contribution to consumer nutrition. At moderate replication (n = 5), for a gap between MPB and another producer of 6.0‰, estimates were more precise (i.e., 95% CIs were smaller) when both δ13Cphytol and temperature were used (95% CI 0.40) rather than only δ13Cphytol (0.46). The greatest difference in precision was for a gap of 6.0‰, close to the average gap in the literature (5.5‰). Given the difficulty of extracting MPB from sediment, carbon isotope studies of food webs could benefit by using CSIA of phytol, especially in conjunction with simple measurements of water temperature. The advance in methodology will allow a reevaluation of the trophic importance of MPB.