Publication Abstract

Environmental correlates oflarge-scale spatial variation in the delta N-15 of marine animals

S. Jennings and K. Warr

Nitrogen stableisotopes 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 offood webs are highly variable and must be accounted for in these estimates. To assessspatial variation in the d¹⁵N of primary producers, we measured the d¹⁵Nof phytoplankton-feeding bivalve molluscs (queen scallops Aequipecten opercularis) atsites in the north-east Atlantic (Irish Sea, English Channel, North Sea). Queen scallopsare good monitors of spatial patterns in the d¹⁵N of phytoplankton becausetheir slow rate of tissue turnover integrates variability in the d¹⁵N of theirdiet. A significant proportion of spatial variation in d¹⁵N was statisticallyexplained by widely recorded environmental variables such as salinity, depth andtemperature. Accordingly, we developed a linear model to predict and map large-scalespatial patterns in scallop d¹⁵N from the environmental variables. We used themodel, in conjunction with new data on the spatial variation in d¹⁵N of twopredatory fishes, to show that 51% and 77% of spatial variance in dab Limanda limanda andwhiting Merlangius merlangus d¹⁵N, and hence apparent trophic level, could beattributed to differences in d¹⁵N at the base of the food chain. Sincetemperature and salinity are correlated with base d¹⁵N, and since gradients inthese physical variables are particularly pronounced in coastal areas and close toestuaries, spatial comparisons of trophic position are easily biased if fine-scaleinformation on base d¹⁵N is not available. Conversely, in offshore regions,where temperature and salinity show little variation over large areas, variations in based¹⁵N and the associated bias will be less.

Reference:

S. Jennings and K. Warr, 2003. Environmental correlates of large-scale spatial variationin the delta N-15 of marine animals. Marine Biology, 142(6): 1131-1140.