Environmental correlates of large-scale spatial variation in the δ15N of marine animals

Nitrogen stable isotopes can be used to estimate the trophic position of consumers in food webs. However, the nitrogen stable isotope ratios (delta(15)N) of primary producers at the base of food webs are highly variable and must be accounted for in these estimates. To assess spatial variation in the delta(15)N of primary producers, we measured the delta(15)N of phytoplankton-feeding bivalve molluscs (queen scallops Aequipecten opercularis) at sites in the north-east Atlantic (Irish Sea, English Channel, North Sea). Queen scallops are good monitors of spatial patterns in the delta(15)N of phytoplankton because their slow rate of tissue turnover integrates variability in the delta(15)N of their diet. A significant proportion of spatial variation in delta(15)N was statistically explained by widely recorded environmental variables such as salinity, depth and temperature. Accordingly, we developed a linear model to predict and map large-scale spatial patterns in scallop delta(15)N from the environmental variables. We used the model, in conjunction with new data on the spatial variation In delta(15)N of two predatory fishes, to show that 51% and 77% of spatial variance in dab Limanda limanda an whiting Merlangius merlangus delta(15)N, and hence apparent trophic level, could be attributed to differences in delta(15)N at the base of the food chain. Since temperature and salinity are correlated with base delta(15)N, and since gradients in these physical variables are particularly pronounced in coastal areas and close to estuaries, spatial comparisons of trophic position are easily biased If fine-scale information on base delta(15)N is not available. Conversely, in offshore regions, where temperature and salinity show little variation over large areas, variations in base delta(15)N and the associated bias will be less.