Geoscience Reference
In-Depth Information
5.2.2
M ESOTROPHIC S TATE
Mesotrophic systems are characterized by a medium level nutrient concentration
in the water high enough to allow growth of macroalgae, together with phytoplank-
ton, as the major primary producers. Therefore, it is understood that nutrients at
this stage can still be assimilated by organisms, hence introducing major changes
in the community structure, but keeping the water at relatively high levels of
transparency. Main human-induced sources for mesotrophy are agricultural run-off
and urban or industrial sewage. However, rich nutrient river and groundwater inputs,
atmospheric deposition or the exchange of nutrient-enriched seawater from
upwelling areas outside the lagoon also can provide significant loads of nutrients.
The nutrient increase in the water column affects both the planktonic system and
the benthic system, by increasing competition between seagrass and macroalgae.
In this subsection, the general structure and function of mesotrophic lagoons is
summarized.
5.2.2.1
Competition between Rooted Vegetation
and Macroalgae
The increase of competition between seagrass and macroalgae on the lagoon bottom
is one of the primary effects derived from water nutrient enrichment. While seagrass
take up nutrients by their roots, macroalgae do it efficiently from water by their
fronds. Taking up nutrients from water provides macroalgae with a competitive
advantage over seagrass, allowing them to spread extensively on the lagoon bottom. 59
Seagrasses and slow-growing macroalgae have nutrient contents much lower than
those of phytoplankton. 60,61 It has been estimated that nitrogen and phosphorus
requirements of phytoplankton and macroalgae are about 50- and 100-fold higher,
and 8- and 1.5-fold higher, respectively, than those of seagrasses. 12
Occasionally water nutrient enrichment can stimulate blooms of opportunistic
species of green algae such as Enteromorpha , Cladophora, Caulerpa, Chaetomorpha
or Ulva , 62 but light also plays an important role in regulating vegetation. Seagrass,
as well as thick macroalgae, have low chlorophyll concentrations per unit plant
weight with light absorption per unit plant weight much lower than that of phy-
toplankton and fast-growing macroalgae. 12 In fact, successional sequence of sub-
merged vegetation during eutrophication is largely dependent on an associated shift
from nutrient to light limitation, with phytoplankton and free-floating macroalgae
being superior competitors under light limitation.
Moderate densities of macroalgae still keep some of the advantages of seagrass
meadows as refuge and shelter for fish larvae and juveniles and other planktonic
feeders, supply of food for many benthic organisms and sediment stabilization.
However, dense algae mats can induce catastrophic effects on the underlying
invertebrate fish and bird assemblages through deoxygenation at the sediment
level. 63-66 Some long-term studies on eutrophication 14,67 report an increase of
macroalgal biomass. In South Quay, in the Ythan Estuary, Scotland, it increased
from a few hundred g m −2 wet weight in the 1960s to about 1 kg m −2 in the 1970s
and reached more than 2 kg m −2 in the 1980s. 67 On the other hand, the availability
 
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