Why Study Continental Aquatic Systems? - Freshwater Ecology: Concepts and Environmental Applications - page 9

over $20 billion per year (Table 1.3). This includes only the actual cash or
trade value of the fish and crustaceans. In many countries, sport fishing
generates considerable economic activity.
For example, in the United States, $15.1 bil-
lion was spent on goods and services related
to freshwater angling in 1991 (U.S. Depart-
ment of the Interior and Bureau of the Cen-
sus, 1993). In addition, 63% of noncon-
sumptive outdoor recreation visits in the
United States included lake or streamside
destinations, presumably to view wildlife
and partake in activities associated with wa-
ter (U.S. Department of the Interior and Bu-
reau of the Census, 1993). Many of these
visits result in economic benefits to the vis-
ited areas. Maintaining water quality is
vital to healthy fisheries and healthy
economies. Pesticide-related fish kills in the
United States are estimated to cause $10-24
million per year in losses (Pimentel et al.,
1992). Finally, maintaining fish production
may be essential to ensuring adequate nutri-
tion in developing countries (Kent, 1987).
Thus, the value of fisheries exceeds that of
the fish. Managing fisheries clearly requires
knowledge of aquatic ecology. These fish-
eries and other water uses face multiple
threats from human activities.
Sediment, pesticide and herbicide
residues, fertilizer runoff, other nonpoint
runoff, sewage with pathogens and nutri-
ents, chemical spills, garbage dumping, ther-
mal pollution, acid precipitation, mine
drainage, urbanization, and habitat destruc-
tion are some of the threats to our water re-
sources. Understanding the implications of
each of these threats requires detailed un-
derstanding of the ecology of aquatic ecosys-
tems. The effects of such human activities on
ecosystems are linked across landscapes and
encompass wetlands, streams, groundwater,
and lakes (Covich, 1993). Management and
policy decisions can be ineffective if the link-
ages between the systems and across spatial
and temporal scales are not considered (Side-
bar 1.1). Effective action at the international,
federal, state, and local governmental levels,
as well as in the private sector, is necessary
to protect water and the organisms in it.
Success generally requires a whole-system

timated at $1 billion per year (Gillis, 1995). Costs
of modifying logging, agriculture, and dam con-
struction and operation probably exceed the
direct economic value of the fishery.
The second case concerns shrubland wa-
tersheds (fynbos) in South Africa that provide
water to large agricultural areas downstream
and considerable populations of people in ur-
ban centers and around their periphery (van
Wilgen et al., 1996). Introduced weed species
have invaded many of these shrubland drainage
basins (watersheds or catchments). The weeds
grow more densely than the native vegetation
and reduce runoff to streams. Also, about 20%
of the native plants in the region are endemic
and thus endangered by the weedy invaders.
Costs of weed management are balanced
against benefits from increased water runoff.
Costs associated with weed removal are offset
by a 29% increase in water yield from the man-
aged watersheds. Given that the costs of op-
erating a water supply system in the water-
shed do not vary significantly with the amount
of water yield, the projected costs of water are
$0.12 per m 3 with weed management and $0.14
per m 3 without it. Other sources of water (re-
cycled sewage and desalinated water) are be-
tween 1.8 and 6.7 times more expensive to use.
An added benefit to watershed weed control is
protection of the native plant species. Thus,
weed removal is economically viable.
The two cases illustrate how ecosystem
management requires understanding of hy-
drology and biology. In the case of the salmon,
vegetation removal (logging) is undesirable be-
cause it lowers water quality and reduces re-
productive success. In the South African
shrublands, removal of introduced weeds is
desirable because it increases water yield.
These examples demonstrate how economic
analyses and knowledge of factors controlling
water quality and supply can assist in policy
decisions. Knowledge of the ecology of the
systems is essential in making good decisions.