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 variabilit y of the relationships between photosynthetic fractionation (epsilon(p)) an d environmental conditions for diatoms, dinoflagellates, and total photosyn thate. We also examined controls on the delta(13)C values of zooplankton-de rived sterols and suspended particulate organic carbon (SPM). Ratios of chl orophyll-a (chl-a) to SPM suggest that light limited diatom growth rates al ong the northern transect, while nutrients apparently limited growth along the southern transect. In the southern transect, three different diatom-der ived sterols exhibit similar epsilon(p)-growth rate relationships. consiste nt with previous results indicating that diatoms in Peru waters employ acti ve transport of bicarbonate and were growth-limited by silicate. Phytol, la rgely synthesized by primary producers, and zooplanktonic sterols exhibited relationships similar to the diatom sterols, bur correlation coefficients between their respective epsilon(p) values and environmental conditions wer e significantly lower. Values of epsilon(p) calculated from particulate org anic carbon (POC) did not correlate with specific environmental variables, possibly due to the overprint of spatial variations in phytoplankton assemb lages on delta(13)C(POC). These results suggest that the quality of environ mental interpretation based upon carbon-isotopic composition decreases as p articulate organic matter and target biomarkers become less taxon-specific due to contributions from multiple sources. (C) 1999 Elsevier Science Ltd, All rights reserved.