Controls on the carbon-isotope compositions of compounds in Peru surface waters

We sampled suspended organic matter from two east-west transects across the Peru upwelling region and determined the taxonomic and regional variability of the relationships between photosynthetic fractionation (ε(P)) and environmental conditions for diatoms, dinoflagellates, and total photosynthate. We also examined controls on the δ¹³C values of zooplankton-derived sterols and suspended particulate organic carbon (SPM). Ratios of chlorophylla (chl-α) to SPM suggest that light limited diatom growth rates along the northern transect, while nutrients apparently limited growth along the southern transect. In the southern transect, three different diatom-derived sterols exhibit similar ε(P)-growth rate relationships, consistent with previous results indicating that diatoms in Peru waters employ active transport of bicarbonate and were growth-limited by silicate. Phytol, largely synthesized by primary producers, and zooplanktonic sterols exhibited relationships similar to the diatom sterols, but correlation coefficients between their respective ε(P) values and environmental conditions were significantly lower. Values of ε(P) calculated from particulate organic carbon (POC) did not correlate with specific environmental variables, possibly due to the overprint of spatial variations in phytoplankton assemblages on δ¹³C(POC). These results suggest that the quality of environmental interpretation based upon carbon-isotopic composition decreases as particulate organic matter and target biomarkers become less taxon-specific due to contributions from multiple sources.