James R. Postel

Temporal variation in the structure of autotrophic and heterotrophic communities in the subarctic Pacific

Autotrophic and microheterotrophic plankton populations were monitored in the euphotic zone of the eastern subarctic Pacific during 6 one-month cruises in spring and summer, 1984, 1987 and 1988. Transmitted light, epifluorescence, and electron microscopy were used to identify, enumerate and estimate the biomass of size-populations of species. The 2–10μm size class dominated the biomass of both autotrophs and heterotrophs. The autotrophic flagellate, Phaeocystis pouchetii, was frequently observed in its non-colonial phase. Temporal variation in all the stocks was evident and could be explained only partially by the physical, chemical or biological factors investigated here. The general structure of the autotrophic community was similar to that in the North Atlantic, but major, unexplained variations between cruises occurred. Variation in mixed-layer depth and day length (but not variation in daily insolation) explained 25% of the variation in autotrophic doubling rate. Heterotrophic biomass comprised, in decreasing order of importance, non-pigmented flagellates, dinoflagellates, and ciliates. Ciliates rarely contributed more than 40% to the total. Microheterotrophic biomass rarely exceeded 30μg C 1−1 (avg 15μg C 1−1, 0–60m) whereas autotrophic biomass averaged 20μg C 1−1, 0–60m, and reached 74μg C 1−1 on one occasion, yet the grazing capacity of these microheterotrophs averaged 100% of primary production.

Abundance, variability, and potential grazing impact of planktonic ciliates in the open subaratic Pacific Ocean

Abstract The abundance and variability of planktonic ciliates in the open subarctic Pacific were determined during four month-long cruises in 1987 and 1988. The ciliate community, numerically dominated by relatively small aloricate choreotrichs, was comparable in abundance to communities in a range of oceanic and neritic environments, including waters with much higher average chlorophyll concentrations. Integrated (0–80m) ciliate biomass was typically 100–200mgC m −2 , although 3- to 4-fold higher levels were observed on two occasions in spring. Ciliate community biomass, in general, was dominated by large (>20 μ m width) individuals, although in August 1988 the biomass of smaller cells was as great or greater. The estimated grazing impact of the ciliate community averaged 20% of the primary production. On one instance in May 1988, however, a large biomass of ciliates led to an estimated grazing impact equivalent to 55% of phytoplankton production. While ciliates may be major phytoplankton grazers during sporadic ciliate “blooms”, dino- and other heterotrophic flagellates, which make up the bulk of microheterotroph biomass, must normally be of equal or greater importance as herbivores in this ocean region.

A comparison of membrane and glass-fibre filters for productivity experiments

Three types of glass-fibre and one membrane filter were compared for the amount of radioactivity retained under conditions typical of primary productivity experiments. All filters showed comparable results in terms of total radioactivity retained. It did not matter whether filters were removed from the filtration apparatus before or after the vacuum had been taken off. Filtration times of membrane filters were 3–6 times longer than those of glass-fibre filters. All filters tended to significantly increase the background levels of the scintillation fluor as a result of passage of water through them followed by fuming with concentrated HCl.