EARLY SPRING PHYTOPLANKTON DYNAMICS IN THE SUBPOLAR NORTH ATLANTIC: THE INFLUENCE OF HETEROTROPHIC-PROTIST HERBIVORY

Abstract

To assess the importance of herbivory by heterotrophic protists in relation to mixed-layer depth prior to the spring phytoplankton bloom, we measured phytoplankton growth and heterotrophic-protist grazing rates during the March/April 2012 EuroBasin Deep Convection cruise in the subpolar North Atlantic. We performed 15 dilution experiments during 2-4 visits at one shelf (160 m) and two deep (~1300 m) stations. Of the two deep stations, one had a mean mixed-layer depth of 476 m, whereas the other was stratified (46 m). Euphotic depth averaged ~70 m at both stations. Initial chlorophyll-a varied from 0.2 to 1.9 μg L-1 at the deep mixed layer station and from 0.5 to 1.0 μg L-1 at the stratified station. In 80 % of the experiments, growth rates exceeded grazing mortality rates, regardless of mixed layer depth. Large mixed layer depth coincided with phytoplankton growth and grazing mortality rates that varied over a similar range from ≤0 to 0.6 d-1, and to an average grazing-impact representing 50% of primary production (PP). At the stratified station, phytoplankton growth rates varied from 0.18 to 0.41 d-1, grazing mortality rates varied from 0.11 to 0.34 d-1, and a temporal shift from a positive to a negative balance between growth and grazing rates caused the proportion of PP consumed to increase from 60% to 180%. Variations in in situ chlorophyll-a could not be explained where the mixed layer was deep, whereas at the stratified station the balance between rate estimates of phytoplankton growth and grazing mortality rates explained 98% of measured changes in chlorophyll-a. These results suggest a difference in the dominant surface loss process at the two stations: grazing at the stratified station vs. potential sinking aided by vertical mixing where mixed layer was deep.