Early spring phytoplankton dynamics in the subpolar North Atlantic: The influence of protistan herbivory

Abstract

AbstractWe measured phytoplankton‐growth (μ) and herbivorous‐protist grazing (g) rates in relation to mixed‐layer‐depth (MLD) during the March/April 2012 EuroBasin cruise in the subpolar North Atlantic. We performed 15 dilution experiments at two open‐ocean (∼ 1300 m) and one shelf (160 m) station. Of the two open‐ocean stations one was deeply mixed (476 m), the other stratified (46 m). At the shelf station, MLD reached the bottom. Initial chlorophyll a (Chl a) varied from 0.2–1.9 μg L−1 and increased up to 2.7 μg L−1 at the shelf station. In 80% of experiments, regardless of MLD, growth‐rates exceeded grazing‐mortality rates. At the open‐ocean stations, the deep ML coincided with μ and g that varied over the same range (≤ 0–0.6 d−1), whereas stratification corresponded to μ and g that ranged from 0.14–0.41 d−1 to 0.11–0.34 d−1, respectively. At the stratified station, the balance between μ and g explained 98% of in situ variations in Chl a, whereas at the deep‐ML station, rate estimates had no explanatory power. The consistent relationship between μ and g, which corresponded to a grazing‐removal of 64% of primary production, suggests that g might be predictable if μ is known, and that a coefficient of 0.64 may be a useful parameter for subarctic carbon models. Composition and persistence of the plankton assemblages differed at the stations and may have been a significant driver of grazing‐pressure. Overall, these results showed no association of MLD with grazing‐pressure and highlight the need to assess to what extent MLD represents the depth of active‐mixing to understand the effects of protistan‐grazing on the development of the North Atlantic spring bloom.