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Human Impact
on the Oceans
Human
activity can alter marine environements or affect marine organisms
What
Where Effect
-
Harvesting
fish World-wide Changed size structure
and species composition of fish
-
Fishing
methods World-wide Trawling destroys
benthos; Dynamite fishing destroys reefs, kills unwanted animals
-
Discard
of by-catch World-wide Increased nutrient
supply to benthos, oxygen problems
-
Dam construction
Rivers, estuaries Loss of anadromous fish
-
Urban
development Coast, estuaries, mangroves, coral
reefs Loss of natural habitat; eutrophication/algal blooms; metal/other
pollution
-
Commercial
shipping World-wide Introduction of
alien species
-
Coral
mining Tropical reefs Destruction of
reefs
Impact of Fisheries
-
Greatest
and most severe human impact: annually catch >100 million tons of fish
and shellfish, constantly increasing due to technical development that
makes finding fish easier (fish finder, echosounder, satellite navigation,
etc.)
-
Almost
all fishgrounds (70%, FAO) are overfished: the catch-per-unit-effort decreases
but with less fish their value increases, keeping fisheries lucrative;
-
Changes
in fish size and species composition: smaller, non-commercial species replace
commercial species; commercial fish become smaller (40% decrease in size
of Atlantic swordfish between 1978 and 1989)
-
By-catch:
non-commercial animals with no use, dumped back into the sea; most severe
in shrimp fisheries: 4.2 kg fish for each kg of shrimp (GoM); can cause
changes in benthic communities and oxygen problems (degradation)
Increase in worldwide fisheries catch from 1949 to 1993
Change of fish composition on Georges Bank, Maine, between 1963 and
the breakdown of the commercial cod fisheries in 1992
Decrease of average weight of coho salmon in three estuaries in Puget
Sound, Washington State, USA.
Marine Pollutants
What
where effect
-
Petroleum,
oil,hydrocarbons Local
oil spills, Mass mortality benthos,birds;
world-wide Low-level
effects (unknown)
-
Plastics
BeachesFloating
debris, Entanglement of animals, Ingestion by animals
-
Pesticides
Local-point sources
Acute toxicity; World-wide Long-term
toxicity/harm
-
Heavy
metals Industrial
sources Mostly sublethal effects, reduced
fertility, growth abnormalities
-
Sewage
Local-point source, agricultural runoff,rivers
Eutrophication and altered community structure, Oxygen consumption, hypoxia,
Introduction of pathogens
-
Radioactive
waste Power plants,
dumping Considered below harmful levels
-
Thermal
effluents Power
plants Altered community structure
Oil Spills and Hydrocarbons
-
Sources:
tanker accidents, conventional shipping and tank cleaning at sea, waste
disposal, runoff, oil exploration, natural sources (cold seeps, Golf
of Mexico)
-
Tanker
accidents are the most known oil spills because
of vicinity to the coast and media coverage; however, they only contribute
a small percentage
-
Recovery
time after an oil spill is ca. 5-10 years,
dependent on temperature and wave action; 80% of hydrocarbons are degraded
bacterially or evaporated after 2 months in open sea spills, leaving heavy
hydrocarbons (long chains) behind
-
World-wide
average contamination in the ppb (parts per
billion) range is considered harmless because toxic effects occur in the
ppm (parts per million) range; long-term exposure effects are unknown,
though
Deadly Plastics
-
Size can
range from nylon drift nets to millimeter-sized plastic pellets
-
Not biodegradable,
plastic remains in the oceans for many years
-
Birds
and turtles mistake plastic for jellyfish and ingest them; 80% of shear-waters
in a North Pacific survey had plastics in their stomachs
-
Fish and
mammals entangle in „ghost nets“ and drifting plastic and suffocate
Pesticides and Heavy
Metals
-
Chlorinated
hydrocarbons: accummulate in tissue and are
enriched along the food chain; reduced fertility, abnormals growth, thin
egg-shell in birds. DDT, DDE (both now banned in USA and Europe): even
measured in Antarctic penguins; Dioxin and PCB: highly toxic, banned in
USA and Europe. TBT = tributyl tin: antifouling paints for ships, leaks
into water;
-
Heavy
metals: mercury, copper, cadmium, lead; accummulate
in food web, cell toxins, acute and chronic effects; „safe“ level depends
on consumption of contaminated seafood;
Sewage and Eutrophication
-
Sewage
disposal is the major form of coastal pollution
-
Release
of organic matter, heavy metals, pesticides, detergents, oil, nutrients
-
Nutrients
can cause algal blooms, among the harmful/toxic blooms, alter the phytoplankton
species composition (food chain effects!); can also promote growth of benthic
green algae and cyanobacteria, which overgrow and destroy natural benthis
communities
-
Pathogenic
bacteria and viruses can be introduced by sewage (e.g. Cholera epidemics
in India by bathing in the polluted Ganges River)
-
Oxygen
consumption increases by bacterial degradation of organic matter and sedimented
phytoplankton, causing hypoxia (anoxic conditions) and fish kills
-
Agricultural
runoff presents another major source of nutrient input into coastal seas,
leading to eutrophication effects
-
Aerial
deposition of ammonia from cattle or pig farms contributes to coastal eutrophication
A New Phytoplankton
Bloom
-
Kiel Bight,
Western Baltic Sea: Since 1983, annual, dense phytoplankton blooms discolored
the water brown in May, which normally happened to be the „clearwater phase“
with seasonally lowest phytoplankton stocks
-
Nutrient
balance of nitrate:silicate was altered by eutrophication: in the 1930‘s,
the phytoplankton spring bloom was ceased by nitrate exhaustion; in the
1980‘s, the spring bloom was ceased by silicate exhaustion, leaving plenty
of nitrate behind for non-diatom phytoplankton to grow
-
The phototrophic
silicoflagellate Dictyocha speculum adapted to the new conditions in growing
without its typical silica sceleton and formed dense, brownish blooms –
altered nutrient conditions fostered a new growth form
Dictyocha with
skeleton
Dictyocha in Kiel Bight, 1987: cells with (S) and without (N) skeleton.
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