Internships

Samuel Dresner ’13

My summer project attempted to elucidate the role of zooplankton in the Sargasso Sea nitrogen cycle. My personal contribution to this larger question was to develop and carry out a method to analyze the δ¹⁵N (in permil versus atmospheric N₂ = {[¹⁵N/¹⁴N)sample/(¹⁵N/¹⁴N)atm] – 1} x 1000) of zooplankton fecal pellets. As fecal pellets constitute much of the export flux from the sunlit surface waters to the deep ocean (Urrere and Knauer, 1981; Angel, 1983), we might expect the δ¹⁵N of fecal pellets at depth to be similar to those found near the sea surface. Thus, by measuring the δ¹⁵N of fecal pellets, we are essentially creating a profile of the sinking flux in the Sargasso Sea. There is conflicting evidence regarding the δ¹⁵N of fecal pellets (particularly in reference to the food source and biomass of the zooplankton). At the Bermuda Atlantic Time-series Study site (BATS; 31° 40’ N; 64° 10’ W), there is an observed discrepancy between the δ¹⁵N of the export flux of organic matter out of the surface ocean (δ¹⁵N = ~3 ‰; Altabet 1988) and the suspended particulate N (δ¹⁵N = -3 to 1‰; Altabet 1988, 1989; Fawcett et al. 2010 submitted). My project consequently seeked to help resolve the δ¹⁵N discrepancy and to “balance” the nitrogen isotope budget in the Sargasso Sea. Studying the marine nitrogen cycle has direct implications on the carbon cycle, particularly how much CO₂ is sequestered in the depths of the ocean and consequently removed from the atmosphere.