Nitrogen stable isotopes can be used to estimate the trophic position of consumers in food webs. However, the nitrogen stable isotope ratios (δ~(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 δ~(15)N of primary producers, we measured the δ~(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 δ~(15)N of phytoplankton because their slow rate of tissue turnover integrates variability in the δ~(15)N of their diet. A significant proportion of spatial variation in δ~(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 δ~(15)N from the environmental variables. We used the model, in conjunction with new data on the spatial variation in δ~(15)N of two predatory fishes, to show that 51 and 77 of spatial variance in dab Limanda limanda and whiting Merlangius merlangus δ~(15)N, and hence apparent trophic level, could be attributed to differences in δ~(15)N at the base of the food chain. Since temperature and salinity are correlated with base δ~(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 δ~(15)N is not available. Conversely, in offshore regions, where temperature and salinity show little variation over large areas, variations in base δ~(15)N and the associated bias will be less.
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