Geoscience Reference
In-Depth Information
5.2.1
O
S
LIGOTROPHIC
TATE
Oligotrophic lagoons have low levels of nutrient concentrations in the water column.
The first consequence of this lack of nutrients is to restrict phytoplankton growth,
keeping water at high transparency levels. Light can easily reach the bottom, and is
not a limiting factor for benthic vegetation.
5.2.1.1
Submerged Vegetation and Related Energy Pathways
Under oligotrophic conditions, with very low concentrations of nutrients in the water
column, nutrients are mainly available at the sediment level. Therefore, the stimu-
lation of growth of aquatic plants that take up nutrients from roots vs
.
algae that
Seagrass can develop within
coastal lagoons for a long time (decades to centuries) based on slowly accumulating
nutrient pools which are efficiently recycled.
take up nutrients directly from water is enhanced.
18
This long-term development is also
supported by self-stabilizing mechanisms. Seagrass influences the water transpar-
ency, decreasing sediment resuspension by retention in the water-sediment interface.
Benthic microalgae also contribute to keep sediment oxygenated through photosyn-
thesis. Sediment mineralization ( see Chapter 4) usually supplies enough nutrients
to benthic micro algae to make them relatively independent of nutrient concentration
in the water.
19
Sediment maintained at high oxygenation levels provide a suitable
environment for both benthic filter feeders and detritivorous organisms. Low levels
of nutrients in water, moreover, prevent the presence of epiphytes on seagrasses that
can cause a detrimental effect on their growth by reducing light at the leaf level.
20,21
16,22
Benthic rooted vegetation seagrass is the main primary producer in oligotrophic
lagoons, providing food to many organisms such as benthic invertebrates and fishes.
However, the energy of most seagrasses becomes available to secondary producers
after being fragmented and processed through the detritical pathway.
The process
of decomposition of leaf litter usually starts with autolysis leaching out soluble
materials, such as dissolved organic matter (DOM) with bacteria colonizing frag-
mented material. Macrobenthic organisms, mainly debris-eating amphipods and
isopods, can also tear off pieces of plant material with its attached community of
microorganisms. Other macrobenthic organisms, such as herbivorous gastropods,
can enhance seagrass growth and production by grazing on epiphytes.
23
Popula-
tions of predators such as ciliates, nematodes, and some polychaetes can develop
and their feces may be re-colonized by microorganisms and reingested again, thus
reducing progressively the size of the debris.
24,25
26-28
Part of the dissolved organic matter is released to the water column and some
is aggregated into amorphous organic particles (approximately from a few
µ
m to
Many biotic and abiotic mechanisms are involved in the
aggregate formation, providing a microenvironment that facilitates growth of bacteria
and very small phytoplankton in nutrient-deficient waters.
500
µ
m in diameter).
29
Both environments, plant
debris and amorphous organic particles, can be colonized by bacteria, making them
available to larger consumers that are not effective in capturing free bacteria. Initial
colonization of plant debris by bacteria is subsequently completed by a community
of protozoa and ciliates feeding on them even though bacteria also can be released
to the water column.
30
 
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