Dian J. Gifford

Protozoa in the diets of Neocalanus spp. in the oceanic subarctic Pacific Ocean

Copepod species of the genus Neocalanus dominate the zooplankton biomass of the oceanic subarctic Pacific Ocean. Neocalanus spp. populations in the subarctic Pacific environment are successful: they feed, accumulate lipid, and persist from year to year. Prior experimental observations derived from a variety of methods indicated that, although their functional morphology is such that they clear the small phytoplankton cells characteristic of the oceanic subarctic Pacific environment efficiently, Neocalanus spp. do not consume sufficient phytoplankton to meet even basic metabolic requirements in that environment. Hence, their success in the subarctic Pacific must depend on their ability to obtain nutrition from other sources. As part of the SUPER (SUbarctic Pacific Ecosystem Research) program, experiments were performed to test the hypothesis that N. plumchrus and N. cristatus obtain a significant portion of their nutrition from planktonic Protozoa. The experiments demonstrate that Protozoa alone do not provide sufficient nutrition for N. cristatus to meet its basic metabolic needs. Protozoa constitute the major dietary component of N. plumchrus however, in agreement with the predictions of Frosts (1987) model of the subarctic Pacific ecosystem. At a minimum this diet permits N. plumchrus to meet basic metabolic requirements. Copepod grazing activities appear to be sufficient to control protozoan stocks in the oceanic subarctic Pacific during late spring and early summer when Neocalanus spp. inhabit the upper water column.

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

Grazing on chlorophyll by microzooplankton ( 20 μm chlorophyll fraction to dominance by the 20 μm in both seasons; C. finmarchicus did not consume significant amounts of chlorophyll <20 μm 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.

Direct and indirect effects of grazing by Neocalanus plumchrus on plankton community dynamics in the subarctic Pacific

The effects of grazing of Neocalanus plumchrus C5 copepodids on plankton trophic coupling in the subarctic Pacific were examined in shipboard microcosm experiments during June 1987. Mixed-layer seawater was incubated for 5d in 601 containers under simulated in situ conditions and copepod densities ranging from 0 (control) to 0.75 copepods 1−1. Direct grazing effects were determined from temporal changes in abundances of chlorophyll, diatoms, and ciliates. Indirect effects were evaluated from measured rates of primary production (14C-bicarbonate uptake), bacterial secondary production (3H-thymidine incorporation), and 15N-ammonium uptake and regeneration. Phytoplankton grew to higher than natural levels in all microcosms over the course of the incubations, but copepods reduced the rates of increase by factors suggesting time-averaged clearance rates of 120, 420, 450 and 170ml copepod−1d−1 for chlorophyll, Nitzschia spp., centric diatoms and ciliates, respectively. Of the rates measured, those largely attributable to phytoplankton growth (i.e. primary production and ammonium uptake) declined with increasing macrozooplankton grazing, in proportion to phytoplankton standing stock measured as chlorophyll a. In contrast, rates associated with microbial loop activity (thymidine incorporation and ammonium regeneration) were enhanced by macrozooplankton grazing. Consequently, increased copepod grazing resulted in a larger fraction of phytoplankton production being processed through the microbial loop.

Planktonic Microbes in the Gulf of Maine Area

In the Gulf of Maine area (GoMA), as elsewhere in the ocean, the organisms of greatest numerical abundance are microbes. Viruses in GoMA are largely cyanophages and bacteriophages, including podoviruses which lack tails. There is also evidence of Mimivirus and Chlorovirus in the metagenome. Bacteria in GoMA comprise the dominant SAR11 phylotype cluster, and other abundant phylotypes such as SAR86-like cluster, SAR116-like cluster, Roseobacter, Rhodospirillaceae, Acidomicrobidae, Flavobacteriales, Cytophaga, and unclassified Alphaproteobacteria and Gammaproteobacteria clusters. Bacterial epibionts of the dinoflagellate Alexandrium fundyense include Rhodobacteraceae, Flavobacteriaceae, Cytophaga spp., Sulfitobacter spp., Sphingomonas spp., and unclassified Bacteroidetes. Phototrophic prokaryotes in GoMA include cyanobacteria that contain chlorophyll (mainly Synechococcus), aerobic anoxygenic phototrophs that contain bacteriochlorophyll, and bacteria that contain proteorhodopsin. Eukaryotic microalgae in GoMA include Bacillariophyceae, Dinophyceae, Prymnesiophyceae, Prasinophyceae, Trebouxiophyceae, Cryptophyceae, Dictyochophyceae, Chrysophyceae, Eustigmatophyceae, Pelagophyceae, Synurophyceae, and Xanthophyceae. There are no records of Bolidophyceae, Aurearenophyceae, Raphidophyceae, and Synchromophyceae in GoMA. In total, there are records for 665 names and 229 genera of microalgae. Heterotrophic eukaryotic protists in GoMA include Dinophyceae, Alveolata, Apicomplexa, amoeboid organisms, Labrynthulida, and heterotrophic marine stramenopiles (MAST). Ciliates include Strombidium, Lohmaniella, Tontonia, Strobilidium, Strombidinopsis and the mixotrophs Laboea strobila and Myrionecta rubrum (ex Mesodinium rubra). An inventory of selected microbial groups in each of 14 physiographic regions in GoMA is made by combining information on the depth-dependent variation of cell density and the depth-dependent variation of water volume. Across the entire GoMA, an estimate for the minimum abundance of cell-based microbes is 1.7×1025 organisms. By one account, this number of microbes implies a richness of 105 to 106 taxa in the entire water volume of GoMA. Morphological diversity in microplankton is well-described but the true extent of taxonomic diversity, especially in the femtoplankton, picoplankton and nanoplankton – whether autotrophic, heterotrophic, or mixotrophic, is unknown.