Life in the shelf seas - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 169

of a diatom bloom) and the generation of increased numbers of grazers, as the
microzooplankton need to mate and produce eggs, and the eggs need to hatch.
Depending on the species, the entire process from mating through to new adult
copepods could take between a week and several months. This decoupling
between the microzooplankton population and their large-celled phytoplankton
food resource is nicely illustrated in Fig. 5.18 . Notice that the first stage of the
copepod life cycle, the production of eggs by the females, is correlated with
bloom timing ( Fig. 5.18a , b ), but by the time the copepods become adults
there is no correlation with the bloom ( Fig. 5.18c ). This explains why diatoms
exist; the larger celled phytoplankton have larger but slower-growing preda-
tors, and these predators cannot keep up with a sudden spurt of growth in
their prey.

Summary

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The autotrophic fixation of carbon, driven by the energy provided by sunlight, lies at
the heart of the marine food chain and determines the biologically mediated fluxes of
carbon between the atmosphere and the long-term storage pools of carbon in slope
sediments and the deep ocean. Physical processes, particularly vertical turbulent
mixing, control the light and nutrients available to the autotrophs. In the shelf seas
the high levels of turbulent mixing, combined with the proximity of the seabed
sediments to the photic zone, help to drive the high rates of primary production
compared to the open ocean.
The autotrophic phytoplankton range in size from cyanobacteria of O(1 m) size

m

up to diatoms of O(100 m) size. There is a fundamental physical constraint on the

potential nutrient uptake by phytoplankton, governed by cell size. Nutrient input to
larger cells is severely limited by the rate at which molecular diffusion is able to
replenish the zone around the cell wall. Taking advantage of the autotrophic supply
of organic food is a grazing community with a size range from the heterotrophic
bacteria O(1

m

m) in size, through to the microzooplankton (O(1-10 mm)) and
extending up to fish, marine mammals and humans. In plankton food chains, we
can say roughly that predators, or grazers, tend to be a factor of 10 larger in size
compared to their prey (a notable exception to this rule-of-thumb is the grazing by
bacteria on sinking faecal pellets). The heterotrophic bacteria drive a part of the food
chain called the microbial loop, providing a recycling pathway that can return
dissolved organic material back to the microzooplankton. Without this microbial
loop, much of the organic material in the ocean would never make it into the higher
trophic levels.
The existence of large phytoplankton cells is dependent on differences in the rate at
which the grazers of different phytoplankton are able to respond to changes in the
biomass of their food source. The grazers of the diatoms, mesozooplankton such as
copepods and early-stage fish larvae, take time to respond to increases in diatom
biomass which allows the diatoms to briefly bloom in response to an influx of

m