Tracking large-scale latitudinal patterns of δ13C and δ15N along the E Pacific using epi-mesopelagic squid as indicators

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 delta C-13 and delta N-15 from epi-mesopelagic squid (n = 404) were collected from 39 degrees S to 53 degrees 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 delta C-13 and delta N-15 was also described for the first time. Both delta C-13 and delta N-15, as well as the standardized difference between them, had distinct patterns by latitude. delta C-13 was the highest at 22 degrees S and decreased north and south of that latitude, with lower values at mid-northern latitudes than near the equator. delta N-15 had the lowest values near the equator and gradually increased towards mid-latitudes. The standardized difference between delta C-13 and delta N-15 was highest (C was higher relative to N) near the equator and declined to mid-latitudes. Overall, the delta C-13 and delta N-15 geographic patterns agreed with previous studies for delta N-15 from surface NO3-, but not for delta C-13 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 delta C-13 and delta N-15, 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.