Environmental correlates of large-scale spatial variation in the d 15 N of marine animals Received: 16 September 2002 / Accepted: 18 December 2002 / Published online: 20 February 2003 Ó Springer-Verlag 2003 Abstract Nitrogen stable isotopes can be used to estimate the trophic position of consumers in food webs. However, the nitrogen stable isotope ratios (d 15 N) of primary pro- ducers at the base of food webs are highly variable and must be accounted for in these estimates. To assess spatial variation in the d 15 N of primary producers, we measured the d 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 pat- terns in the d 15 N of phytoplankton because their slow rate of tissue turnover integrates variability in the d 15 N of their diet. A significant proportion of spatial variation in d 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 d 15 N from the environmental variables. We used the model, in conjunction with new data on the spatial vari- ation in d 15 N of two predatory fishes, to show that 51% and 77% of spatial variance in dab Limanda limanda and whiting Merlangius merlangus d 15 N, and hence apparent trophic level, could be attributed to differences in d 15 N at the base of the food chain. Since temperature and salinity are correlated with base d 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 d 15 N is not available. Conversely, in offshore regions, where temperature and salinity show little vari- ation over large areas, variations in base d 15 N and the associated bias will be less. Introduction Nitrogen stable isotope ratios (d 15 N) are often used to define trophic position because the d 15 N of consumers is typically enriched by 3.4& relative to their prey (Min- agawa and Wada 1984; Post 2002). However, it is im- possible to define trophic position without an appropriate isotopic baseline because the pools of ni- trogen that support marine food webs have different d 15 N (Owens 1987), and because primary producers use different sources of nitrogen in different areas and sea- sons (Dugdale and Goering 1967; Mullin et al. 1984; Dugdale and Wilkerson 1991). Measuring the production-weighted mean d 15 N of phytoplankton on large scales, as would be required to estimate the mean d 15 N at the base of food webs, is logistically and analytically challenging. This is because phytoplankton have very short life histories, fast tissue turnover times and are difficult to separate from other organic material. An alternative approach is to use phytoplankton feeders to integrate the short-term and spatial variability in the d 15 N of their diet. Zooplank- ton are not ideal for this purpose, as they also have fast tissue turnover times (Rolff 2000), but suspension feeding bivalve molluscs have longer life histories and slower tissue turnover times, and the d 15 N of their tissues reflects the assimilation-weighted mean d 15 N of their diet in preceding months (Goering et al. 1990; Lorrain et al. 2002). Moreover, suspension feeding bivalve molluscs feed on a combination of phyto- plankton and associated material that supports benthic production (Graf et al. 1982, 1984; Billet et al. 1983). Bivalve molluscs have been used to establish isotopic baselines in marine and freshwater ecosystems (Hobson and Welch 1992; Cabana and Rasmussen 1996; Mc- Kinney et al. 1999). The trophic positions of consumers are expressed relative to this baseline, by assigning a base trophic position (often 2 or 2.5) to the phyto- plankton feeder and assuming a constant fractionation (often 3.4&) of d 15 N with each ‘step’ in the food chain Marine Biology (2003) 142: 1131–1140 DOI 10.1007/s00227-003-1020-0 S. Jennings Æ K. J. Warr Communicated by J.P. Thorpe, Port Erin S. Jennings (&) Æ K. J. Warr Centre for Environment, Fisheries and Aquaculture Science, Lowestoft Laboratory, Lowestoft, NR33 OHT, UK E-mail: S.Jennings@cefas.co.uk Tel.: +44-1502-524363 Fax: +44-1502-513865