Characterizing seasonal contribution of particles from the surface ocean to the mesopelagic food web through amino acid compound specific isotopic analysis and 234Thorium measurements

The mesopelagic food web is a major contributor to the biological carbon pump but is largely unconstrained mainly due to inadequate sampling of the deep ocean. Recent results of amino acid compound specific nitrogen isotope analyses (AA-CSIA) indicate the possibility of refractory suspended particles contributing to the mesopelagic food community in addition to more labile rapidly sinking particles from the surface oceans. Here we combine amino acid concentrations and AA-CSIA with ²³⁴Thorium (²³⁴Th) measurements to constrain seasonal differences in the downward flux of particles from the surface ocean at station ALOHA in the subtropical North Pacific Ocean. We determined the concentrations and isotopic composition of specific amino acids as well as the ²³⁴Th-normalized flux of C, N and amino acids throughout the upper 400m of the water column during winter and summer of 2014. Results exhibit distinct seasonal profiles. The amino acid flux was ~10 times greater in the summer, indicating increased primary production and export from the surface oceans to the deep. In contrast, the fraction of each specific amino acid compared to the total amount of amino acids remained reasonably constant between seasons and depths. Throughout the water column δ¹⁵N values of small particles (1-53µm) increased with depth, consistent with previous results from deeper samples (up to 750 meters) and indicating microbial degradation. The large particles (>53µm) have differing isotopic composition, allowing the contribution of small and large particles to be distinguishable within the mesopelagic food web. This study highlights the dependence of the deep ocean on the strength of the biological pump and our results potentially can be used to interpret the seasonal dietary composition of higher trophic level mesopelagic organisms.