Life in the shelf seas - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 161

inorganic carbon back into the water column, and potentially back to the atmosphere
to add to the pool of CO 2. Thus the bacteria help determine the air-sea flux of carbon.
The oxidation of organic carbon by bacteria to form DIC removes dissolved oxygen
from the surrounding seawater. In coastal regions influenced by riverine sources of
nutrients, leading to eutrophication, the subsequent reduction in dissolved oxygen
can have serious effects on marine life; we will show examples of this in Chapter 9 .
The microbial loop described above is the most general mechanism for returning
organic material back to a form suitable for uptake by the autotrophs. Some species
of phytoplankton are able to take up dissolved organic nitrogen (DON) and
phosphorus (DOP) directly. In regions where inorganic nitrogen has become
limiting to autotrophic growth, the largest pool of fixed nitrogen will be the
DON, and an adaptation to be able to utilise the DON directly is clearly advanta-
geous. The significance of this nutrient route in shelf seas is not clear; there are
some observations suggesting that DON in the form of urea and dissolved amino
acids could be important (Tett et al., 2003 ), particularly for the cyanobacteria (Heil
et al., 2007 ). The source of DON and DOP will dictate if its assimilation by
phytoplankton leads to new or regenerated production; for instance, DON arising
from the diazotrophs would fuel new production. While the role of dissolved
nutrients in shelf seas is not yet clear, there is growing evidence that the shelf seas
can be a source of dissolved nutrients that can be transported into the open ocean
where inorganic nutrients are severely limiting to phytoplankton production
(Torres-Valdes et al., 2009 ) .

Microzooplankton (size ∼ 2-100 m m)
The microzooplankton include single-celled heterotrophic flagellates, dinoflagellates,
tintinnids and ciliates ( Fig. 5.13a -c), along with the early stages of some of the
mesozooplankton (e.g. copepod larvae, called nauplii). These tiny grazers feed on
bacteria (both the cyanobacteria and the heterotrophic bacteria) and on the auto-
trophic phytoplankton. The smaller microzooplankton are particularly dependent on
the microbial loop as a source of organic material, and so act as a key step in the
transfer of DOM back up towards larger heterotrophs. An important point to note
about the microzooplankton is that their growth rates tend to be similar to those of
their autotrophic prey. As a result we rarely see blooms of small phytoplankton
because their predators are able to match their growth rates and so maintain a
grazing pressure sufficient to damp down any increases in small-celled phytoplankton
biomass. Generally in the ocean, the concentrations of autotrophic bacterial biomass
appear to be limited to 0.5 mg Chl m 3 as a result of this responsive grazing impact

(Chisholm, 1992 ; Armstrong, 1994 ).
One other group of grazers that needs to be mentioned are the mixotrophs. These
organisms, typically bacteria or microplankton, are able to utilise light, inorganic
nutrients and prey. They are able to shift the balance between autotrophic
and heterotrophic modes of growth depending on resource availability. Observations
typically show mixotrophs to form significant components of ecosystems where nutri-
ents are low (Stoecker et al., 1989 ; Lavrentyev et al., 1998 ) and in some circumstances

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