Environmental Engineering Reference
In-Depth Information
et al. unpubl. data). Wind-induced mixing turned over the water column and
introduced higher nutrients from the bottom into surface waters in shallow
nearshore Louisiana shelf waters for a temporary enhancement of production
[9]. The significance of bottom nutrient introduction to surface waters following
mixing events in the hypoxic zone is not known.
4. CONSEQUENCES TO LIVING RESOURCES
Hypoxia is one manifestation of a suite of symptoms that may result from eu-
trophication. The effects of eutrophication, including hypoxia, are well known
for some systems and include the loss of commercially important fisheries (e.g.,
Baltic and Black seas). The impacts of increased nutrient inputs and worsening
hypoxia on overall system productivity are not well known for the Louisiana
continental shelf food web. The hypoxic waters fall within an important com-
mercial and recreational fishery zone that accounts for 25 to 30 percent of the
annual coastal fisheries landings for the United States. The ability of organisms
to reside, or even survive, either at the bottom or within the water column is
affected as the depletion of oxygen progresses towards anoxia [31, 32]. When
oxygen levels fall below critical values, those organisms capable of swimming
(e.g., demersal fish, portunid crabs and shrimp) evacuate the area. Less motile
fauna experience stress or die as oxygen concentrations fall to zero. Larger,
longer-lived burrowing infauna are replaced by short-lived, smaller surface
deposit-feeding polychaetes, and several taxa are absent from the fauna, for ex-
ample, pericaridean crustaceans, bivalves, gastropods, and ophiuroids. 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 structure and sediment biogeochemical cycles.
Caddy [5] suggested that the fishery yield increases as nutrient loading
increases, but, as the ecosystem becomes increasingly eutrophied, there is a drop
in fishery yield. The benthos are the first resources to be reduced by increasing
frequency of seasonal hypoxia and eventually anoxia; bottom-feeding fishes and
crustaceans then decline. There is a negative relationship between the catch of
brown shrimp (the largest economic fishery in the northern Gulf of Mexico)
and the size of the mid-summer hypoxic zone [52]. The decadal average catch
per unit effort of brown shrimp declined during the last forty years in which
hypoxia was known to expand [14]. There are, however, changes in climate,
river discharge, salinity of the estuary during critical development periods,
acreage of nursery habitat and fishing effort that may also affect catch.
The point on the continuum of increasing nutrients versus fishery yields
remains vague as to where benefits are subsumed by environmental problems
that lead to decreased landings or reduced quality of production. There are
indications of a shift from a demersal dominated fish community to a pelagic
