Sources of N to the pelagic food web of the Tropical Atlantic: a stable isotope study

• Main Author: Sandel, Vera Marita
• Format: Thesis
• Language: English
• Published: 2013
• Subjects: Course of study: MSc Biological Oceanography; North Atlantic
• Online Access: https://oceanrep.geomar.de/id/eprint/21947/; https://oceanrep.geomar.de/id/eprint/21947/1/M.Sc.%202013%20Sandel,%20V.M.pdf

Primary production in the oligotrophic oceans has traditionally been thought to be supported mainly by upwelled deep-water nitrate and nitrogen compounds that are remineralized within the photic zone. Nonetheless, deficits in marine nitrogen budgets, especially in the tropical North Atlantic, initiated critical reevaluation of diazotroph contribution to the N cycle. In this study we assessed the spatial variability of bulk nitrogen and carbon isotopes of different functional groups of epipelagic zooplankton in a N-S (17°N-4°S at 23°W) and E-W transect (20-24°W at 18°N) in the tropical East Atlantic to identify potential shifts in the source supporting biological production. Incubation experiments were made on board to measure carbon-specific excretion rates. Results were compared to water column observations of nutrient and POM distribution as weil as colony abundance of the filamentous cyanobacterium Trichodesmium. Upwelling of deep-waters predominated in the region of the equatorial upwelling and were reflected in high zooplankton δ15 (>8‰), while the oligotrophic notih Atlantic around 5°N was characterized by strong thermal stratification, low δ15N (~2‰) and the presence of presumable diazotroph consumers. Along the N-S transect, δ15N was negatively correlated to Trichodesmium abundance. This was not the case along the E-W transect, where an eddy was sampled and high abundances of Trichodesmium coincided with relatively high δ15N values. At these stations the nutricline was shallow and high integrated chlorophyll-a values indicated that nondiazotroph production was high below the surface Trichodesmium bloom. Still, nitracline and phosphocline thickness and depth proved to be significant predictors of zooplankton stable nitrogen isotope values. Furthermore, a tight negative correlation was found between δ15N and δ13 C. Excretion rates of ammonia and phosphate differed between species, but did not correlate with biochemical properties of the watercolumn.