Effects of Toxic Chemicals and Other Pollutants on Aquatic Ecosystems - Freshwater Ecology: Concepts and Environmental Applications - page 290

SUSPENDED SOLIDS

Turbidity and suspended solids are natural parts of all freshwater envi-
ronments. Some habitats are naturally highly turbid, but human activities
have increased levels of suspended solids in many habitats (Fig. 14.2). Agri-
cultural and urban runoff, watershed disturbance (e.g., logging, construc-
tion, and roads; Forman and Alexander, 1998), removal of riparian vegeta-
tion, and alteration of hydrodynamic regimes all can lead to anthropogenic
increases in total suspended solids.
Sediments can have different biological and physical effects depending on
the type of suspended solids (Table 14.5). High values of suspended solids can
lower primary productivity of systems by shading algae and macrophytes, at
times leading to almost complete removal. Suspended solids can also have
negative effects on aquatic animals by interfering with reproduction, respira-
tory O 2 transport, filter feeding, and habitat availability.
The negative impact of excessive sediments on stream biota has been
known for some time (Hynes, 1970; Waters,
1995). Such sediments lower incoming light
and primary production, increase scour,
harm sensitive invertebrate species, reduce
biodiversity, and lower the aesthetic value of
streams. Probably the strongest negative ef-
fect is filling gravel and cobble habitat
through deposition. Fine sediments clog
gravel and increase anoxia. This can harm
interstitial invertebrates. Spawning fishes
also use these habitats. Lowering flow
through gravels can cause O 2 levels to de-
crease below what is necessary for eggs to
develop successfully. Information is avail-
able on the susceptibility of salmonids to
sediments (Wilber, 1983). Other fish species
vary in their tolerance to suspended solids.
Light attenuation in lakes may comprise
a large part of the influence of suspended
solids on the biota. A highly turbid lake or
reservoir may have limited rates of primary
production. However, if there is sufficient
organic material in the suspended particles,
a productive food web based on microbial
utilization of the suspended particulates can
occur.

have decreased, but contaminated sediments
continue to cause problems.
The Great Lakes Water Quality Agreement
of 1978 listed 22 organic compounds that are
dumped into the lake as hazardous or poten-
tially hazardous. Of these, polychlorinated
biphenyls (PCBs), DDT, and dieldrin caused the
greatest concern. DDT use was restricted in
1970, and the concentrations in the smelt taken
from the lake decreased from 1.59 to 0.04 g
g 1 between 1967 and the late 1970s. Low lev-
els of DDT contamination continue because
DDT is sequestered in the sediments and
slowly reenters the food webs. Manufacture
and use of PCBs has been banned in the United
States since 1976; in 1978, PCBs were entering
Lake Erie at about 0.9 metric tons per year,
with the majority coming from atmospheric de-
position. A decade later fish in Lake Erie had
enough PCB content that consumption of more
than 5 kg of fish per year was deemed unsafe
(Burns, 1985). The recently introduced zebra
mussel now bioconcentrates PCBs and passes
them on to the waterfowl that consume them
(Mazak et al., 1997).
Human activities did not kill Lake Erie. How-
ever, the system is a good example of how mul-
tiple human impacts on a lake can decrease
the value for recreation, fisheries, and drinking
water. With careful stewardship, the water
quality in the lake will continue to improve, and
the lake will continue to survive.

THERMAL POLLUTION

Research has been conducted on the in-
fluence of cooling tower effluent (warm wa-
ter) on aquatic communities. In addition,
some reservoirs artificially warm downstream
waters when outflow is from the epilimnion