Environmental Engineering Reference
In-Depth Information
blades. Epiphytes cover the blades with a layer of fuzz and
further reduce the amount of sunlight that can reach leaves
for photosynthesis.
Epiphytes can be controlled to some degree by small animals
like amphipods that graze on them. Grazing by these small
animals can be important in keeping seagrass beds healthy,
and the more diverse the grazers are the better they clean the
epiphytes off the blades. SAV declines usually involve sud-
den decreases in abundance rather than gradual changes, and
high salinity and temperature intensify the effects. Seagrasses
also suffer from blooms of sea lettuce, which causes reduc-
tion in their shoot density, leaf growth, and carbon content.
Seagrass and the detritus it generates provide food and shelter
for a variety of animals, and when its growth is reduced the
associated animal community declines. Sadly, there are only
a few cases of seagrass recovery following the reduction of
nutrient inputs.
What effects are seen in coral reefs?
Agricultural runoff with nutrients and sediments is trans-
ported to coral reefs by river discharge. Eutrophication is
especially harmful to coral reefs, where the nutrients stimu-
late benthic algae to grow over, cover, and smother the corals,
eventually leading to the replacement of the coral reef com-
munity with an algal community—especially when grazers
(e.g., sea urchins, parrotfish) are not plentiful. Only if the reef
has large populations of grazing herbivores to control the
algae can the corals survive and prevail in this competition.
All too often, these grazing herbivores are reduced due to
fishing, and reefs get covered with algae. Reefs off the Florida
Keys are degraded because of wastes from too many people,
while parts of the Great Barrier Reef in Australia suffer from
agricultural runoff. Degradation represents a major loss to
tourism, since divers prefer to be in areas with rich coral reef
environments.
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