NUTRIENTS - Marine Pollution: What Everyone Needs to Know - page 26

cannot move or move slowly may die in low-oxygen zones;
disease resistance can be compromised, and reproduction
and embryonic development can be impaired. Fish larvae
are poor swimmers, and become more vulnerable to preda-
tion. In general, animals attempt to cope with low oxygen by
reducing their activity in order to consume less oxygen. This
often means feeding for shorter periods of time and eating
less food. When bottom water is hypoxic, buried clams move
up closer to the surface of the sediments and are more eas-
ily eaten by blue crabs that are more tolerant of the low oxy-
gen and can make brief trips into the hypoxic zone. Marine
worms that have high tolerance for low oxygen show sub-
lethal effects—they have lower respiration and feeding rates,
and the fertilization and development of their embryos are
abnormal.
Sometimes in warm weather, crabs and other animals
swarm into shallow water and may actually crawl out of
oxygen-depleted water as they try to breathe. This phenom-
enon has been referred to as a jubilee. This may result in the
crabs surviving hypoxia long enough to be caught for dinner
by humans, who gave the name jubilee to this event (although
the crabs are clearly not happy).
What effects are seen in seagrasses?
Phytoplankton blooms make the water more turbid, reduc-
ing the light available to submerged aquatic vegetation
(seagrasses) on the bottom. Seagrasses are an important
component of the ecosystem, and they have been in decline.
They provide a nursery habitat and play an important role
in ecosystem structure and function. They are damaged by
both the shading of light and sulfide toxicity from eutrophi-
cation. The shading by dense phytoplankton blooms reduces
their ability to photosynthesize. Submerged aquatic veg-
etation (SAV) such as eel grass also declines because of the
growth of small algae (called epiphytes) attached on its grass