Grazing by microzooplankton and mesozooplankton in the high‐latitude North Atlantic Ocean: Spring versus summer dynamics

Abstract

Grazing on chlorophyll by microzooplankton (<200 •um) and copepods was measured in the mixed layer of the high-latitude North Atlantic Ocean during May and August 1991. No significant grazing by microzøoplankton occurred in May during a spring bloom dominated by colonial Phaeocystis pouchettii and Nitzschia spp. As the bloom declined, the size distribution of chlorophyll shifted from dominance by the > 20 •um chlorophyll fraction to dominance by the <20 chlorophyll fraction. The impact of grazing by microzooplankton increased as the bloom declined, with microzooplankton consuming 100% of potential daily chlorophyll production following the bloom. In August,.when the phytoplankton was dominated by the <20 #m chlorophyll fraction, m•cr0zooplankton consumed 37-53% (mean = 41%_+11% s.d.)'of potential daily chlorophyll production. Averaged over all experiments, microzooplankton grazing accounted for 81% of daily chlorophyll production. The grazing impact of Calanus finmarchicus stages C4 and C5, which dominated mesozooplankton biomass in the upper euphotic zone in both spring and late summer, was concentrated on chlorophyll > 20/tin in both seasons; C. f in mar c h ic u s did not consume significant amounts of chlorophyll < 20 •um in either season. Compared to the microzooplankton, copepods did not consume a significant fraction of total chlorophyll in either season, accounting for only -1% of daily chlorophyll production. Introduction The objective of this study was to evaluate losses of chlorophyll due to grazing by microzooplankton and mesozooplankton in the mixed layer of the high-latitude North Atlantic Ocean under contrasting biological and hydrographic conditions during spring and summer. The research was conducted on RV Endeavor cruises EN224-3 and EN227 in spring and late summer 1991 under the aegis of the Marine Light Mixed Layers (MLML) Accelerated Research Initiative, a multidisciplinary effort focused on the biological, physical and optical dynamics of the mixed lay_cr. The MLML study site was occupied t'or 8 days •n May 1991 and 9 days in August/Scptcmber 1991. Paper number 94JC00983 0148-0277!95!94JC-00983505.00 tic Front, an area characterized by extreme seasonal forcing of biology and hydrography. Convective cooling of the water column during winter produces adccp mixed layer of at least several hundred meters, which provides new nutrients to the system [Robinson et al., 1979; Plueddemann et al., this issue]. Irradiance and phytoplankton standing stocks are low during winter when phytoplankton growth is believed to be very low. With the onset of stratification, typically in April, standing stocks of chlorophyll increase IWil!iams and Hopkins, 1974; Plueddemann et al., this issue; Stramska et al., this issue], and a strong diatom bloom reaching 2-5/•g L '•. Calanus finmarchicus and other copepod species appear in the euphotic zone at this time, but their grazing appears to have little impact on the bloom [Dam et al., 1993; Morales et al., 1991; Weeks et al., 1993]. Transition to the summer phytoplankto...

Microzooplankton grazing activity in the temperate and sub-tropical NE Atlantic: summer 1996

“…However, in contrast to the findings of our study, aloricate ciliates dominated at 60°N during the summer of 1989 (Burkill et al 1993) and 59°N during the spring and summer periods in 1991 (Gifford et al 1995). Nevertheless, variations in the fixation methods used could account for the discrepancy between these data through the potential losses of ciliates (Stoecker et al 1994a).…”

“…The MLML study site was occupied during both the initial phase of the spring bloom (when Phaeocystis spp. and small diatoms dominated the phytoplankton) during spring 1991 and the post-bloom phase during summer 1991 (Gifford et al 1995). Therefore, our northerly study falls within the range of conditions previously described in the temperate NE Atlantic.…”

“…These studies demonstrated that microzooplankton were important controllers of the fate of phytoplankton production in surface waters between 47 and 60°N, and between 20 and 21°W. Results from experiments using the dilution technique of Landry & Hassett (1982) showed that microzooplankton herbivory accounted for up to 115% of daily phytoplankton production (Burkill et al 1993, Gifford et al 1995. However, these studies focused on the spring and summer phytoplankton communities in northern, eutrophic waters and did not investigate the role of microzooplankton grazing in the southerly, oligotrophic region of the NE Atlantic.…”

Distinct flavobacterial communities in contrasting water masses of the North Atlantic Ocean

“…The concentration of chlorophyll a remains high during summer and it gradually returns to winter concentrations (Henson et al, 2006). During summer, there is a shift from dominance by the 420 mm chlorophyll fraction to dominance by the o20 mm chlorophyll fraction (Gifford et al, 1995). In contrast, the NAST is an oligotrophic area with low levels of primary production (Longhurst, 1995).…”

Seasonal abundance and feeding patterns of copepods Temora longicornis, Centropages hamatus and Acartia spp. in the White Sea (66°N)

“…On the other hand, the pre-screening procedure may reduce the number of large naked protists and thus affect the food spectra. However, excluding the prescreening procedure increases the number of small zooplankters, which may have more grazing effect and thus lead to overestimation of the copepod grazing rates (Gifford et al 1995). The potentially high grazing impact of microzooplankton smaller than 200 lm was minimized by using fine mesh of 100 lm for pre-screening, and the small remaining effect was corrected for by using bottles without copepods as controls.…”