Tracking large‐scale latitudinal patterns of δ¹³C and δ¹⁵N 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 δ 13 C and δ 15 N from epi‐mesopelagic squid ( n = 404) were collected from 39° S to 53° 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 δ 13 C and δ 15 N was also described for the first time. Both δ 13 C and δ 15 N, as well as the standardized difference between them, had distinct patterns by latitude. δ 13 C was the highest at 22°S and decreased north and south of that latitude, with lower values at mid‐northern latitudes than near the equator. δ 15 N had the lowest values near the equator and gradually increased towards mid‐latitudes. The standardized difference between δ 13 C and δ 15 N was highest (C was higher relative to N) near the equator and declined to mid‐latitudes. Overall, the δ 13 C and δ 15 N geographic patterns agreed with previous studies for δ 15 N from surface NO 3 − , but not for δ 13 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 δ 13 C and δ 15 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.