Effects of Changing Environmental Conditions on Lagoon Ecology - Coastal Lagoons: Ecosystem Processes and Modeling for Sustainable Use and Development - page 198

They
can be effectively grazed by heterotrophic flagellates and ciliates and then by other
zooplankton that in turn provide available food for larger pelagic organisms such as
fish larvae and juveniles. Although the energetic transfer efficiency of the microbial
loop is relatively low, because many trophic steps are involved, it remains as one of
the most characteristic food web structure in the pelagic environment of oligotrophic
lagoons

Bacteria in the water column are the base of the so-called microbial loop.

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based on recycled nutrients.

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5.2.1.2 Phytoplankton

Photosynthesis, the process allowing phytoplankton cells to grow, is regulated by
the adaptation of cells to varying environmental conditions at a certain range of
space and time scales ( see last row in Table 5.1) . The main environmental variables
affecting the physiological state of the algae are light, temperature, and nutrient
concentrations. Others, such as salinity, can be determinant for the presence of certain
species. The adaptative response of phytoplankton cells varies widely, depending on
their ecophysiology and on the environmental conditions of the area where they
have been growing.
For a specific lagoon habitat, some phytoplankton species would find better con-
ditions to grow on the basis of their ability to compete for resources at characteristic
ranges. As mentioned above, light is not usually a problem for phytoplankton in
oligotrophic waters, but lack of nutrients may constitute a serious limitation. Phy-
toplankton takes up nutrients from the water following the carrier-mediated transport
of Michaelis-Menten kinetics, in which nutrient uptake ( V ) is a hyperbolic function

of substrate concentration ( equivalent to the

concentration necessary to achieve half of the maximum rate of uptake (

S ), with the half-saturation constant K

s

V ):

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max

V = V S /( K + S ) (5.1)

max s

, making this parameter
characteristic for species from different areas, either oceanic or coastal (see
Chapter 4, Section 4.1.5 for details). If algal cells are under steady-state conditions
of nutrient limitation, then the Michaelis-Menten expression can be assumed to
reflect the growth kinetics in the form

K can vary, depending on temperature, light, and V

s max

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µ = µ S /( K + S ) (5.2)

max s

where µ and µ are the growth rate and maximum growth rate, respectively, and

max

K is now the half-saturation constant for growth which is very similar to the half-

saturation constant for nutrient uptake ( Figure 5.3) .
The steady-state condition of nutrient limitation assumed by this kinetics is not
generally fulfilled in oligotrophic waters as phytoplankton cells can store nutrients
in internal pools to be used when they are scarce. This phenomenon, known as

s

luxury

uptake , was described by Droop in his model for intracellular content of the

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limiting nutrient:

µ = µ [1-( Q / Q )] (5.3)

max o