Nitrogen stable isotopes can be used to estimate the trophic position of consumers in food webs. However, the nitrogen stable isotope ratios (d15N) 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 d15N of primary producers, we measured the d15N 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 d15N of phytoplankton because their slow rate of tissue turnover integrates variability in the d15N of their diet. A significant proportion of spatial variation in d15N 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 d15N from the environmental variables. We used the model, in conjunction with new data on the spatial variation in d15N of two predatory fishes, to show that 51% and 77% of spatial variance in dab Limanda limanda and whiting Merlangius merlangus d15N, and hence apparent trophic level, could be attributed to differences in d15N at the base of the food chain. Since temperature and salinity are correlated with base d15N, 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 d15N is not available. Conversely, in offshore regions, where temperature and salinity show little variation over large areas, variations in base d15N and the associated bias will be less.
|Number of pages||10|
|Publication status||Published - 2003|