Effects of Changing Environmental Conditions on Lagoon Ecology - Coastal Lagoons: Ecosystem Processes and Modeling for Sustainable Use and Development - page 204

of nutrients in water also favors the growth of epiphytes on seagrasses and macro-
algae, thus regulating their abundance and distribution. Nevertheless, some mac-
roalgae (as Caulerpa prolifera ) can actively avoid epiphytes by releasing toxic
substances, preventing its development. In addition, distribution of submerged
vegetation in shelter areas of the lagoons can also be efficiently regulated by
grazing, mainly due to fishes and waterfowl. 68

5.2.2.2 Microbial vs. Herbivorous Food Web

The planktonic food web configuration of coastal lagoons depends on the environ-
mental conditions. A continuum between the classical “herbivorous” food web and
the “microbial loop” web can be observed. The herbivorous food web occurs when
medium- to large-size microplankton can grow, thus transferring its production to
herbivorous zooplankton and to large invertebrates and fishes. In contrast, the pro-
duction of small (pico- and nano-) phytoplankton leads to “microbial food webs,”
which comprise phototrophic cells (eukaryotic algae and cyanobacteria) as well as
heterotrophic bacteria and protozoa. Both “herbivorous” and “microbial loop” food
webs would be at each extreme of the continuum.
According to Rassoulzadegan 69 the term microbial loop , coined by Azam et al., 70
designates the almost closed system of heterotrophic bacteria and zooflagellate
herbivores, in which the latter release dissolved organic matter (DOM) used as
substrate by the former.
On the other hand, the phytoplankton exude dissolved organic carbon that
stimulates the bacterial growth, thus fueling the microbial loop. At the same time,
bacteria remineralize nutrients that can be taken up by small-celled phytoplankton,
thus fueling the microbial food web. 40,71 Figure 5.5 represents the changes in the
planktonic food web with an increasing nutrient load in lagoons.
As described above, the planktonic food web structure in oligotrophic lagoons is
mainly based on both the detritical pathway and the subsequent microbial loop. Phy-
toplankton in these lagoons can be scarce, even in a very small amount, because of the
process explained above. Its growth is based on regenerated nutrients, mainly ammo-
nium, from both submerged vegetation and fauna. In contrast, in meso to eutrophic
lagoons, phytoplankton increases, and microbial and herbivorous food webs become
more important, with the major source of nitrogen being nitrate entering mainly from
agricultural run-off and/or urban and industrial sewage. 72 Phytoplankton assimilate
ammonium first (because of the easily obtainable energy compared to nitrate) and nitrate
is utilized only after ammonium has been consumed. There are several biochemical
reactions for this preferential assimilation of nitrogen forms involving both repression
of the enzyme responsible for nitrate uptake in the presence of ammonium and activation
of its synthesis process by exposure to nitrate and absence of ammonia 73 ( see Chapter
4 for details).
Availability of nutrients in the water also relieves the competitive disadvantage
of large phytoplankton cells against the small-celled ones, promoting the shift to
larger cells through the eutrophic gradient. Thus, whereas in oligotrophic water small
flagellates with low K s and µ max would tend to dominate, in more eutrophic water

larger cell diatoms with higher K s and

µ max would generally dominate. From these