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

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 (δ¹³C_phytol) as an accurate proxy for δ¹³C of bulk MPB (δ¹³C_bulk). 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 δ¹³C_phytol and δ¹³C_bulk, and could predict δ¹³C_bulk within 1.8‰ (95% CI) of observed values at moderate replication (n = 5). If water temperature is included, δ¹³C_phytol predicts δ¹³C_bulk 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 δ¹³C_phytol and temperature were used (95% CI 0.40) rather than only δ¹³C_phytol (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.