Abstract
Variations in the isotopic and chemical composition of particulate organic matter have been observed globally in the surface ocean. Still, few studies have investigated variations over depth, especially within the euphotic zone. Using detailed sampling and analytical approaches, this dissertation identifies underappreciated vertical variations in the chemical composition of particles within the euphotic zone at the Bermuda Atlantic Time-series Study site. Further, these compositional variations within the euphotic zone impact the composition of organic particles within the mesopelagic and are modulated by seasonally variable water column conditions.
I present data from size-fractionated particles collected across three seasons at this well-studied site, with particular focus on thermally stratified conditions that lead to vertical gradients in particle composition. I newly identify significant variations in the isotopic, elemental, and biochemical composition of particulate organic matter over depth and across particle size class within the euphotic zone. First, I identify unique carbon isotopic signatures of photosynthetic communities in the lower versus upper euphotic zone and describe how this discovery is important for the interpretation of carbon isotopes more broadly in oceanographic studies. I also find significant and systematically higher carbon:nitrogen (C:N) stoichiometry in particles within the upper euphotic zone compared to the lower euphotic zone, driven by the relative carbohydrate and amino acid content of the particles. Finally, I discuss how seasonal variation in the depth of water column mixing correlates with significant differences in the vertical distribution of C:N ratios and carbohydrate content of particles in the upper 500 m. These new data highlight how environmental conditions and water column structure influence the growth requirements of primary producers and the vertical distribution of biochemicals that are essential for heterotrophic metabolism in both the euphotic and mesopelagic ocean.