The biochemical composition of phytoplankton in the Laptev and East Siberian seas during the summer of 2013

The Laptev and East Siberian seas which are generally viewed as terrestrial organic matter (TerrOM)-dominated seas, are among the least biologically understood regions in the Arctic Ocean. During the summer of 2013, however, the TerrOM signature was negligible in our samples. We investigated the biochemical composition (carbohydrates [CHO], proteins [PRT], and lipids [LIP]) of phytoplankton-dominated particulate organic matter in order to improve our understanding of the physiological status of resident phytoplankton. Our chlorophyll-a values and the presence of SCMs and resting spores were associated with a cessation of the phytoplankton bloom. Despite the low inorganic nitrogen nutrients in the water column, the cellular PRT (39%) were comparable to CHO (42%) contents and the inorganic (dissolved nitrogen:dissolved phosphate) and organic (PRT:CHO) indices did not indicate a nitrogen stress of phytoplankton metabolism. Altogether, the phytoplankton were likely in a growth transition from the exponential to the stationary phase, resulting in CHO-dominated cells with moderate PRT. By comparing our biochemical analyses with the LIP-dominated (> 50%) ones in the Chukchi Sea (the summers of 2011 and 2012), we conclude that more severe nitrogen-limited conditions occurred in the Chukchi Sea. In a quality aspect, we suggest that consumers which feed on LIP-rich phytoplankton could have an advantage to overwinter while those feeding on CHO-rich phytoplankton will gain energy efficiently in a short term. Therefore, the biochemical composition of phytoplankton could be a valid integrator of surrounding environments in which phytoplankton grow and can be a good indicator of their nutritional value.