depauperate benthic fauna in the severely stressed
seasonal hypoxic/anoxic zone of the Louisiana inner
shelf in the northern Gulf of Mexico. Larger, longerlived burrowing infauna are replaced by short-lived,
smaller surface deposit-feeding polychaetes, and
certain typical marine invertebrates are absent from
the fauna, for example, pericaridean crustaceans,
bivalves, gastropods, and ophiuroids. Long-term
trends for the Skagerrak coast of western Sweden in
semi-enclosed fiordic areas experiencing increased
oxygen stress showed declines in the total abundance
and biomass of macroinfauna, abundance and biomass of mollusks, and abundance of suspension
feeders and carnivores. These changes in benthic
communities result in an impoverished diet for
bottom-feeding fish and crustaceans and contribute,
along with low dissolved oxygen, to altered sediment
biogeochemical cycles. In waters of Scandinavia and
the Baltic, there was a reduction of 3 million t in
benthic macrofaunal biomass during the worst years
of hypoxia occurrence. This loss, however, may be
partly compensated by the biomass increase that
occurred in well-flushed organically enriched coastal
areas not subject to hypoxia.
Where oxygen minimum zones impinge on continental margins or sea mounts, they have considerable effects on benthic assemblages. The benthic
fauna of OMZs consist mainly of smaller-sized
protozoan and meiofaunal organisms, with few or no
macrofauna or megafauna. The few eukaryotic organisms are nematodes and foraminiferans. Meiofauna appear to be more broadly tolerant of oxygen
depletion than are macrofauna. The numbers of
metazoan meiofaunal organisms, primarily nematodes, are not reduced in OMZs, presumably due to
abundant particulate food and reduced predation
pressure. In hypoxic waters of the northern Gulf of
Mexico, harpacticoid copepod meiofauna are reduced at low oxygen levels, but the nematodes
maintain their densities. Benthic macrofauna are
found in all hypoxic sediments of the northern Gulf
of Mexico, although the density is severely reduced
Demersal
invertebrates
Larger
infauna
Macroinfauna
2.0
0.05
Most
present
Most
present
Few mantis
and penaeid
shrimp
Absent
Present
Present and
burrowed
Present and
burrowed
Stressed
starfish,
brittle
stars)
Dead
Dead
Dead
Moribund
polychaetes
Nekton
Epibenthic
invertebrates
0.2
1.5
1.0
0.5
Absent
Stressed
(anemones,
gastropods)
Bacterial mats
Anoxic sediment, H 2 S in sediment and water
(mg O 2
l
−1
)
2.0
0.05
0.2
1.5
1.0
0.5
(mg O 2
l
−1
)
(Fish kills)
Figure 4 Progressive changes in fish and invertebrate fauna as oxygen concentration decreases from 2 mg l
À 1 to anoxia. From
Rabalais NN, Harper DE, Jr., and Tuner RE (2001) Responses of nekton and demersal and benthic fauna to decreasing oxygen
concentrations. In: Rabalais NN and Turner RE (eds.) Coastal and Estuarine Studies 58: Coastal Hypoxia: Consequences for Living
Resources and Ecosystems, pp. 115–128. Washington, DC: American Geophysical Union.
312 HYPOXIA
seasonal hypoxic/anoxic zone of the Louisiana inner
shelf in the northern Gulf of Mexico. Larger, longerlived burrowing infauna are replaced by short-lived,
smaller surface deposit-feeding polychaetes, and
certain typical marine invertebrates are absent from
the fauna, for example, pericaridean crustaceans,
bivalves, gastropods, and ophiuroids. Long-term
trends for the Skagerrak coast of western Sweden in
semi-enclosed fiordic areas experiencing increased
oxygen stress showed declines in the total abundance
and biomass of macroinfauna, abundance and biomass of mollusks, and abundance of suspension
feeders and carnivores. These changes in benthic
communities result in an impoverished diet for
bottom-feeding fish and crustaceans and contribute,
along with low dissolved oxygen, to altered sediment
biogeochemical cycles. In waters of Scandinavia and
the Baltic, there was a reduction of 3 million t in
benthic macrofaunal biomass during the worst years
of hypoxia occurrence. This loss, however, may be
partly compensated by the biomass increase that
occurred in well-flushed organically enriched coastal
areas not subject to hypoxia.
Where oxygen minimum zones impinge on continental margins or sea mounts, they have considerable effects on benthic assemblages. The benthic
fauna of OMZs consist mainly of smaller-sized
protozoan and meiofaunal organisms, with few or no
macrofauna or megafauna. The few eukaryotic organisms are nematodes and foraminiferans. Meiofauna appear to be more broadly tolerant of oxygen
depletion than are macrofauna. The numbers of
metazoan meiofaunal organisms, primarily nematodes, are not reduced in OMZs, presumably due to
abundant particulate food and reduced predation
pressure. In hypoxic waters of the northern Gulf of
Mexico, harpacticoid copepod meiofauna are reduced at low oxygen levels, but the nematodes
maintain their densities. Benthic macrofauna are
found in all hypoxic sediments of the northern Gulf
of Mexico, although the density is severely reduced
Demersal
invertebrates
Larger
infauna
Macroinfauna
2.0
0.05
Most
present
Most
present
Few mantis
and penaeid
shrimp
Absent
Present
Present and
burrowed
Present and
burrowed
Stressed
starfish,
brittle
stars)
Dead
Dead
Dead
Moribund
polychaetes
Nekton
Epibenthic
invertebrates
0.2
1.5
1.0
0.5
Absent
Stressed
(anemones,
gastropods)
Bacterial mats
Anoxic sediment, H 2 S in sediment and water
(mg O 2
l
−1
)
2.0
0.05
0.2
1.5
1.0
0.5
(mg O 2
l
−1
)
(Fish kills)
Figure 4 Progressive changes in fish and invertebrate fauna as oxygen concentration decreases from 2 mg l
À 1 to anoxia. From
Rabalais NN, Harper DE, Jr., and Tuner RE (2001) Responses of nekton and demersal and benthic fauna to decreasing oxygen
concentrations. In: Rabalais NN and Turner RE (eds.) Coastal and Estuarine Studies 58: Coastal Hypoxia: Consequences for Living
Resources and Ecosystems, pp. 115–128. Washington, DC: American Geophysical Union.
312 HYPOXIA