In low-latitude, open-ocean water columns, the euphotic zone often extends far below the surface mixed layer, creating depth-stratified habitats characterized, in part, by variations in light intensity and nutrient availability. In examining the organic and isotopic properties of particles at different depths across the euphotic zone, we have identified large differences in the composition of particulate organic matter that may be indicative of both variations in the biomass of primary producers and specific pathways of heterotrophic metabolisms. Using recent results from analysis of carbohydrates and compound-specific isotopic analysis of amino acids from thermally stratified euphotic zones, I will present hypotheses about the overall organic matter cycling in two distinct habitats of the euphotic zone: the high-light, low-nutrient upper euphotic zone, in which C:N ratios of particulate organic matter are high and nitrogen is efficiently cycled between autotrophs and heterotrophs, and the low-light, high-nutrient lower euphotic zone, in which C:N ratios of particulate organic matter are low and excess nitrogenous waste is generated by heterotrophs. Combined with particle dynamics processes active at these depths, these organic compositional distinctions may influence the elemental stoichiometry and biomarkers exported into the deep ocean.