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

Abstract

We measured phytoplankton-growth (l) and herbivorous-protist grazing (g) rates in relation to mixedlayer-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 lg L 21 and increased up to 2.7 lg L 21 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 l and g that varied over the same range ( 0-0.6 d 21 ), whereas stratification corresponded to l and g that ranged from 0.14-0.41 d 21 to 0.11-0.34 d 21 , respectively. At the stratified station, the balance between l 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 l and g, which corresponded to a grazing-removal of 64% of primary production, suggests that g might be predictable if l 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.

Seasonal similarity in rates of protistan herbivory in fjords along the Western Antarctic Peninsula

“…Nonetheless this suggestion conflicts with findings that polar athecate dinoflagellates and ciliates can achieve high growth rates at low in situ temperatures (Franzé and Lavrentyev 2014). While we did not measure grazers' growth rate in the present study, their estimated biomass was similar during both seasons and at some sites greater than what has been reported for more forgiving locales, such as the North Atlantic in spring for example (Gifford et al 1995;Morison and Menden-Deuer 2015). Thus low grazing pressure may not only be due to lack of grazers.…”

Dynamics of transparent exopolymer particle production and aggregation during viral infection of the coccolithophore, Emiliania huxleyi

“…Conversely, an abundance of smaller sized aggregates would be more coupled to grazing by smaller microzooplankton species. High E. huxleyi ‐specific microzooplankton grazing rates were measured for EhV‐infected cell populations in the North Atlantic (Laber et al ., ), and were comparable to rates in previous studies in the North Atlantic (Gifford et al ., ; Morison and Menden‐Deuer, ). The interaction of infection with grazing and fecal pellet production likely contributed to the enhanced export fluxes of PIC and POC observed in these infected blooms.…”

High export via small particles before the onset of the North Atlantic spring bloom

“…Details of the microbial community and microzooplankton grazing during our cruise are discussed, respectively, by Paulsen et al . [] and Morison and Menden‐Deuer [].…”