Effects of Changing Environmental Conditions on Lagoon Ecology - Coastal Lagoons: Ecosystem Processes and Modeling for Sustainable Use and Development - page 200

The Michaelis-Menten kinetic parameters can be interpreted, with caution, as
ecological indicators of the physiological adaptation states of the phytoplankton. K s
values for NO 3 − and NH + uptake vary with different eutrophication levels, from higher
values in eutrophic water to lower values in oligotrophic water. 38 Some relationships
between K s and cell size also have been traced. 39,40 Comparatively, larger cells exhibit
lower surface to volume ratios per unit of biomass than smaller ones. Higher surface
to volume ratios indicate a relatively higher number of uptake sites in smaller cells,
thus providing some advantage in nutrient uptake in oligotrophic water. 38 The dom-
inance of small-celled phytoplankton in oligotrophic waters also might result from
the fact that the acquisition of nutrients by large cells can be limited by molecular
diffusion at very low nutrient concentrations. It has been estimated that for a non-
swimming osmotrophic cell, with density higher than seawater, the minimum con-
centration of limiting nutrient at which it can maintain a stable population is a fourth
power of cell radius. 41
Based on these principles, the oligotrophic lagoons are expected to show low
cell concentration, dominated by small sized and motile phytoplankton cells (such
as cryptophytes and prasinophytes), whereas larger algae (diatoms and dinoflagel-
lates) are expected to dominate in nutrient enriched water. 42
For modeling purposes, however, the high diversity and species richness of
phytoplankton make it unrealistic to know the growth kinetics for individual algae
species according to their adaptive stage. A rather reasonable and common alternative
to reduce this problem is to group the phytoplankton cells into functional groups
related to size (small flagellates, diatoms, dinoflagellates, etc.) rather than use strictly
taxonomic criterion classes.
Phytoplankton is usually classified by size into three groups: pico- (0.2-2

µ m

diameter), nano- (2-20 m). 43

As stated above, the standing stock of phytoplankton is influenced not only by
competition for resources (bottom-up control mechanisms) but also by grazing
processes (top-down control mechanism). Zooplankton (mainly crustaceans, such
as copepods and cladocerans, in marine and brackish water lagoons, respectively)
and fishes are generally the most important phytoplankton grazers. Competition for
resources, the bottom-up mechanism control of the food web, provides the phyto-
plankton with some characteristic species. These species are, at the same time,
available food for some particular type of zooplankton or higher consumers. Grazing
pressure is highly dependent on temperature, concentration, and size of the food, as
well as the consumer size. As a result, the biomass of phytoplankton assemblages
is a trade-off between nutrient competition (bottom-up control mechanisms) and
herbivory. 44

µ m), and micro-phytoplankton (>20 µ

5.2.1.3 Zooplankton

Zooplankton assemblages in coastal lagoons also can widely vary, depending on the
water features. Usually, as in the open sea, the most abundant taxonomic group is
copepods, grazing both on phytoplankton and microzooplankton. Although a distinc-
tion among herbivores, carnivores, and omnivores can be traced between copepods,
they can change their strategy depending on the availability of food. 45 In oligotrophic