Tracking large-scale latitudinal patterns of d13C and d15 N along the E Pacific using epi-mesopelagic squid as indicators

Abstract

Because consumers integrate components of their habitat through diet over time and space, stable isotope ratios from animal tissues can track spatial variation in baseline values across marine systems. To understand large-scale geographic patterns in the eastern Pacific ocean, muscle d13 C and d15 N from epi-mesopelagic squid (n = 404) were collected from 398 S to 538 N and analyzed in relation to hemisphere, latitude, geographic area and current systems. Geographic patterns were controlled for effects of secondary factors such as squid size, species (Dosidicus gigas and Sthenoteuthis oualaniensis), tissue and year of collection. Joint latitudinal variation of d13 C and d15 N was also described for the first time. Both d13 C and d15 N, as well as the standardized difference between them, had distinct patterns by latitude. d13 C was the highest at 228S and decreased north and south of that latitude, with lower values at mid-northern latitudes than near the equator. d15 N had the lowest values near the equator and gradually increased towards mid-latitudes. The standardized difference between d13 C and d15 N was highest (C was higher relative to N) near the equator and declined to mid-latitudes. Overall, the d13 C and d15 N geographic patterns agreed with previous studies for d15 N from surface NO3, but not for d13 C in plankton, POM and squid. We suggest that the biochemical processes for carbon and nitrogen are spatially more variable than what has previously been reported because squid isotope ratios varied also among current systems and geographic areas. These geographic patterns in d13 C and d15 N, indicated by consumers such as cephalopods, could improve our understanding about the interaction between the carbon and nitrogen cycle and the heterogeneity in biochemical cycling processes in marine systems.