Abstract
Latitudinal variations in the carbon:nitrogen (C:N) ratio of particulate organic matter are well-established, but vertical (depth) variations within the euphotic zone remain largely unexplored. Here, we found that C:N ratios (mol:mol) of three size fractions of particulate organic matter exceeded the canonical Redfield ratio of 6.6 within the low-nutrient upper euphotic zone in the stratified northwest Sargasso Sea. In larger particles ( >20 μm), these ratios reached as high as 10–16. Elevated C:N ratios in all size fractions of particles (0.3 to >20 μm) in the upper euphotic zone coincided with the accumulation of glucose-rich carbohydrates. In contrast, particles within or below the nitracline, within the lower euphotic zone (~90–140 m), exhibited C:N ratios of <6.6 and contained comparatively less carbohydrate and greater amounts of amino acids. Additionally, patterns in the composition of individual carbohydrate monomers suggest that the vertical variations originate from both primary production and degradative processes. These variations in particle composition over depth will affect the stoichiometry in models of primary production, microbial remineralization, and carbon export.
Primary production and degradative processes impact the carbon to nitrogen ratio of particulate organic matter with depth, according to analysis of seawater samples from the northwest Sargasso Sea.