The Ecology of Phytoplankton

264 MORTALITY AND LOSS PROCESSES IN PHYTOPLANKTON
monimolimnetic deepening and the enhanced
production of picoplanktic cyanobacteria. Bacterial
consumption of primary DOC yields between
25 and 130 g C m −2 a −1 to the pelagic food web,
which, in turn sustains the producton of up to
23 g copepod C m −2 a −1 . Much of this (16 g C m −2
a −1 )iseatenbyplanktivores in generating the net
annual production of 1.1 g C m −2 a −1 Stolothrissa
plus Limnothrissa and 0.3 g C m −2 a −1 Lates.
Adults of many families of fish will eat zooplankton
when it is sufficiently abundant to be
an attractive and satisfying resource. The majority
of adult pelagic fish (and of many that inhabit
shallower margins) are carnivorous on other fish
and/or on other large prey. However, many of
these species produce large numbers of small
eggs that give rise to juvenile stages, which are
initially mesoplanktic. They feed on microplankton
(Sarvala et al., 2003), often in direct competition
with and exposed to predation by adult
planktivores (e.g. O’Gorman et al. 1987). According
to the systematic simulation modelling of
Letcher et al. (1996), metabolic growth capacity,
rather than foraging ability or resistance to
starvation, is the leading bottom--up component
in larval survival. Predator size is a powerful
influence on survival but has only a weak effect
on the variability in the composition of the
available prey. Letcher et al. (1996) also deduced
that whether young fish died through starvation
or predation usually depended most on the
availability of their smallest prey organisms.
Reference should also be made to pelagic
squid (Loligo spp.) whose juvenile hatchlings (or
paralarvae) are released into the open water.
They are free-living and self-propelling, using
rhythmic contractions of the mantle to force
a series of alternating bursts of water flow
and recovery. Being barely 2 mm in length,
paralarvae are, unmistakeably, initially (albeit
briefly) mesoplanktic (Barón, 2003).
On the basis of this brief survey, it is clear
that the precise structure of the pelagic web
of consumers is highly variable and subject to
dynamic forces. So far as the impacts upon the
phytoplankton is concerned, outcomes hinge on
the numbers and sizes of the herbivores present
and the sustainability of the feeding modes
available.
6.4.2 Impacts of filter-feeding on
phytoplankton
Moving on from qualitative description of the
structural components of the phagotrophic
plankton, consideration is now given to the quantitative
impacts of their feeding on the producer
mass. It is conceptually easier to deal first
with the impacts of filter-feeders. Although this
method of food gathering is far from universal,
it can be the most striking and complete in its
impact. Moreover, its effects are relatively easy to
model. These are good enough reasons to explain
the additional fact that a large literature on filterfeeding
has accumulated.
Supposing that, to the potential planktic
consumer, the relatively most abundant food
resource is the nanoseston -- algae, large bacteria,
detrital particles measuring 2--20 µm across -- and
that particles are generally well dispersed within
the medium, then the development of some
means to sieve and to concentrate such particles
is likely to be favoured by evolution (Gliwicz,
2003a). The coupling of a filter and the means
of generating a water current across is a characteristic
of the feeding apparatus of many zooplankters,
including ciliates and rotifers. However,
it is at the mesoplanktic scale, of crustaceans
and tunicates, that filter-feeding has a
significant impact on the availabilty of food in
the entire medium, or at least beyond the immediate
environment of the individual animal. This
difference is most due to viscosity. Where the
smallest turbulent eddy is in the order of 1
mm or so, the typical microplankter (