Effects of Changing Environmental Conditions on Lagoon Ecology - Coastal Lagoons: Ecosystem Processes and Modeling for Sustainable Use and Development - page 209

However, changes in the benthic meadows, with the substitution of seagrasses
by algae, can induce changes in species distribution and abundance. In the Mar
Menor Lagoon (Spain), the replacement of scant seagrass meadows of Cymodocea
nodosa by dense meadows of the macroalgae Caulerpa prolifera caused a displace-
ment of the benthic fish Gobius niger from the sandy bottoms to the Caulerpa
meadow, which provides a better refuge. 89 At the same time, changes induced at the
sediment-water interface by the dense covering of the Caulerpa meadows and by
the high annual supply of organic matter caused oxygen depletion, H 2 S release, and
changes in organoleptic characteristics of the organic matter. These changes have
been considered as one of the causes of the strong reduction in mugilid fisheries. 63

5.2.3 E UTROPHIC S TATE

Coastal lagoons are considered eutrophic when they have high levels of nutrients in
water and sustain large assemblages of phytoplankton as dominant primary produc-
ers. Hypereutrophy is considered the state in which phytoplankton assemblages
increase up to the self-shadow level, preventing light from reaching the bottom and
not allowing macroalgae to grow. This subsection outlines characteristics of the
eutrophic state.

5.2.3.1 Phytoplankton

The previously discussed ecological bases of phytoplankton ecology provide an expla-
nation of the expected changes in nutrient-enriched water. Several consequences for
the planktonic food webs, and hence for the whole ecology of the lagoon, have already
been outlined. Many lagoons in the eutrophic state meet the environmental conditions
required for phytoplankton to bloom: high nutrient loads and a relatively stable hydro-
logical regime. Phytoplankton blooms can reach very high numbers of cells up to a
point that they become light limited by self-shading. 90 In very nutrient-rich waters the
phytoplankton blooms sometimes are unable to completely absorb the amount of
dissolved nutrients. Large diatoms such as Chaetoceros or Coscinodiscus and
dinoflagellates such as Ceratium or Dinophysis , some of them toxic, frequently
bloom in eutrophic waters, and colonial cianobacteria are usually found in nutrient-
overenriched water.

5.2.3.2 Benthic Vegetation

As a result of nutrient loading, fast-growing macroalgae develop in large quantities
up to 1000-2000 g DW m −2 . 21 With such densities, the algae cover the bottom in
thick mats. The photosynthetic production in the lower part of the mats is limited
by light, where the respiratory oxygen demand easily exceeds the photosynthetic
oxygen production. When this situation lasts for several days, oxygen depletion and
H 2 S poisoning kill the algae. Growth-limiting nutrients, which previously were bound
in the macrophytes, become available for further growth of phytoplankton and oppor-
tunistic macroalgae. 91 Eutrophication does not stimulate the total primary production
per unit area but shifts the main productivity from the benthic to the planktonic
community. 92