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

We 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.