|
Disappearing Diporeia
Are invasive zebra mussels responsible
for the demise of diporeia, one of Lake
Ontario's most important organisms? Since
the early 1990's, in many areas of the Great
Lakes, with the exception of Lake Superior,
populations of the tiny, shrimp-like amphipod,
about 1 centimetre in length, have either
disappeared or dramatically declined. In
the Kingston basin of Lake Ontario, diporeia
abundance has fallen to zero, from a previous
level of 14,000 per square metre.
|
Longer-Living
Lake Trout
Effective sea lamprey control,
improved water quality and three
decades of restocking programs
are beginning to pay dividends
for the lake trout population
of Lake Ontario. As the phosphorus
discharges from sewage treatment
plants have been reduced, and
levels of persistent toxic substances
have declined, lake trout introduced
to Lake Ontario have begun to
reproduce and survive beyond
the fry and larval stages. With
parasitic sea lamprey populations
also under control, Lake Ontario
biologists are optimistic that
the lake trout, once the most
valuable commercial species
in the Great Lakes, will stage
a comeback. Their resurgence,
however, will depend on control
of new threats from invasive
species such as the round goby,
the spiny water flea, and the
Asian carp.
|
|
Broken Link: As
an organism that formerly represented up
to 70% of the Great Lakes biomass of bottom-dwelling
invertebrates, the diporeia's decline has
caused a major food chain disruption. Nestled
into the upper layers of lake sediment,
the diporeia feed on settling algae from
the water column above. They in turn are
eaten by bottom-feeding prey fish such as
smelt and sculpin, which are then eaten
by piscivorous (fish-eating) fish such as
adult salmon and trout.
Decreases in the populations of slimy sculpin
and young lake trout in Lake Ontario have
been linked to the loss of diaporeia. In
Lake Michigan, desperate, diporeia-deficient
whitefish have even resorted to eating zebra
mussels, blocking their digestive tracts
with undigestable shell material and producing
a scrawny, inferior commercial catch.
Zebra Mussels Implicated:
Although the cause of the diporeia
decline has not been positively identified,
most scientific fingers are pointing toward
the invasive zebra mussel. Four theories
have been proposed to explain their effect
on diporeia populations:
1. Zebra mussels compete for the same food
- bacteria and algae -bit can multiply more
rapidly and filter greater quantities of
water.
2. Food supply is sufficient for both, but
the mussels can extract a nutrient essential
to the diporeia.
3. Mussels have introduced a pathogen that
is lethal to diporeia.
4. Mussels excrete a mucous-like substance
that is lethal to diporeia.
The precipitous decline in Great Lakes
diporeia populations comes just as many
fish species are rebounding, threatening
the success of several decades of water
quality improvement and restoration programs.
|
