U. Christaki et F. Vanwambeke, SIMULATED PHYTOPLANKTON BLOOM INPUT IN TOP-DOWN MANIPULATED MICROCOSMS - COMPARATIVE EFFECT OF ZOOFLAGELLATES, CILIATES AND COPEPODS, Aquatic microbial ecology, 9(2), 1995, pp. 137-147
Succession of microbial populations and carbon flow were studied exper
imentally in a 2-stage linked system where phytoplankton growth was se
parated from decay or consumption. Three phytoplankton loss processes
were considered: bacterial lysis, protozoan grazing and mesozooplankto
n grazing. An axenic culture of the diatom Phaeodactylum tricornutum w
as transferred to 3 second-stage microcosms (150 1) kept in the dark.
The first vessel (L: lysis batch) contained no herbivores and was top-
limited by bacterivorous flagellates, the second (G: grazing batch) by
ciliates and the third (Z: zooplankton batch) by copepods. In the L c
ontainer there was no evidence of grazing on phytoplankton; the partic
le-associated protozoa were abundant (3.7 x 10(3) cell l(-1)). In the
G vessel, ciliates (up to 96 x 10(3) cell l(-1)) controlled both the p
hytoplankton and nanoflagellate populations and ciliates ingested phyt
oplankton at rates from 1.7 to 16 phytoplankton cell ciliate(-1) h(-1)
(average, 7.5 cell ciliate(-1) h(-1)). The average growth yields for
bacterivorous flagellates and ciliates were 35% and 45%, respectively.
In the Z treatment, the concentration of >10 mu m protozoa (able to i
ngest P. tricornutum) was apparently kept low due to copepod grazing p
ressure. Consequently, a considerable fraction of the phytoplankton wa
s channelled through the detrital pathway instead of the classical her
bivorous pathway. The general trend of the parameters describing micro
bial activity was Z > G > L. An influence of the complexity of the sys
tem on the turnover rate of organic matter was evident with the ratio
'cumulative biomass/POC' in the different vessels showing a lower leve
l of detritus when predators of phytoplankton were present.