Relationships between the Chemical Composition of Particulate Organic Matter and Phytoplankton Distributions in Recently Upwelled Waters off Peru

Abstract

The offshore advection of the nutrient-rich, recently upwelled surface layer off Peru was accompanied by changing phytoplankton assemblages and declining nutrient concentrations. The succession of large, chain-forming diatoms in nutrient-rich inshore waters to an assemblage of microflagellates, monads, small chain-forming diatoms, ciliates, and dinoflagellates in offshore moving waters resulted in a gradient in the chemical composition of the particulate and dissolved organic matter (POM and DOM) in the euphotic zone over the Peruvian slope and shelf. The “advanced” assemblages had higher proportions of lipid and lower protein and water soluble carbohydrate within the POM, high C/chlorophyll a and C/N ratios and low C/ATP ratios. Primary productivity was low, ranging from 0.2–1.2 $gc\cdot {{m}^{{}^{-}2}}\cdot {{d}^{{}^{-}1}}$. The more recently upwelled waters were typified by large chain-forming diatoms, e.g., Chaetoceros spp. and Schroderella delicatula, and higher proportions of protein and water soluble carbohydrate in the POM. Lipid fractions, C/N and C/chlorophyll a ratios declined while C/ATP ratios increased relative to offshore assemblages. Primary productivity exceeded 6 $gc\cdot {{m}^{{}^{-}2}}\cdot {{d}^{{}^{-}1}}$. Extracellular dissolved saccharide concentrations also increased substantially inshore. These data indicate diatom-dominated upwelling waters have a unique chemical signa-ture in total organic matter and composition, facilitating maintenance of an upwelling facies of organic-rich sediments, a high benthic oxygen demand and anaerobic conditions through sedimentation of the POM.