Aquatic ecosystems are exposed to the primary effects of local changes in temperature, sunshine, wind, and so forth and to a wide range of secondary effects, particularly from changes in hydrology and waterborne materials. Increased water temperatures, increased evaporation, and changes in inflows and flooding would change the thermal and chemical structures of rivers and lakes. These changes would directly affect the nutrient status of aquatic ecosystems; the survival, reproduction, and growth of organisms; the distribution and diversity of species; and overall ecosystem productivity (IPCC 1996, WG II, Sections 10.5, 10.6). Changes in rainfall produce disproportionate changes in runoff and in mean flows and levels of rivers and some lakes; these effects might be exacerbated if the climate were to become more variable, with more frequent flood and drought events. Sea-level rise is likely to have significant effects on lowland aquatic ecosystems near the coasts. (Coral reefs are discussed in Section 18.104.22.168.)
An increased frequency of more intense rainfalls would increase the intensity of runoff events and thus might contribute greater inputs of nutrients, organic material, agricultural waste, and sediment (IPCC 1996, WG II, Section 10.5). This could lead to reduced water quality, stress on some species, and potentially increased biological productivity forming nuisance growths. Waterways in the many intensively farmed areas of the region may be particularly vulnerable to increased pollution by fertilizers, animal waste, and agrochemicals, with greater potential for algal blooms and aquatic plant proliferation. Algal blooms and eutrophication already are a major problem in many of Australasia's inland waters (SOEC, 1996). Excessive plant growth in lowland streams can reduce drainage capacity and increase risks of flooding.
Sediment transport following heavy rainfall can smother extensive areas of estuarine habitat, resulting in loss of breeding habitat essential to many coastal fish species and affecting food supply for seabirds. Such an extreme event occurred in New Zealand's Whangapoua estuary in 1995. Any increase in extreme rainfall events and sedimentation would be likely to have major impacts on river, lake, estuarine, and coastal waters-particularly the Great Barrier Reef lagoon (Larcombe et al., 1996)-and lead to reduced aesthetic values and reduced recreational and tourist use.
In the lowland coastal rivers and floodplains of northern Australia, the possibly lower rainfall projected by the revised climate change scenarios (CSIRO, 1996a) would most likely lead to greatly decreased biodiversity because it has been found that poor wet seasons reduce the extent and duration of inundation-which in turn has dramatic impoverishing effects on the abundance and diversity of the biota. Drier conditions would likely lead to a decrease in the problem of insect-borne diseases, however. These floodplains may be particularly prone to the impacts of sea-level rise (Steering Committee of the Climate Change Study, 1995; Waterman, 1995), whereby storm surges, cyclonic floods, and seawater intrusions could devastate the freshwater biota. The possible effect of climate change on the present invasion of floodplains by aquatic and semi-aquatic weeds (Lonsdale, 1994; Miller and Wilson, 1995), or on future invasions, is unclear.
The fauna of small lowland tropical streams in northeastern Australia appear to be susceptible to depletion by floods and to have a relatively low rate of recovery (Rosser and Pearson, 1995), which suggests that an increase in the frequency and magnitude of extreme events may lower the diversity of lowland rainforest streams. In contrast, upland rainforest streams have a high diversity (Pearson et al., 1986) and appear to be relatively resilient (Benson and Pearson, 1987; Rosser and Pearson, 1995). A study in southeast Australia found that droughts deplete introduced trout but not the native galaxiids, resulting in an expansion of galaxiid populations downstream with the death of trout (Closs and Lake, 1996); thus, with increased drought the range of native fish in such small streams might increase.
The many ephemeral river and lake ecosystems in inland Australia (temporary
rivers and lakes that only flow and fill occasionally) (Boulton and Lake, 1988;
Lake, 1995) are attuned to high climate variability, but their resilience to
long-term change in the frequency and intensity of events is less certain. Information
on their biota is scanty. A survey of the small and intermittently flowing streams
of the George Gill Range in central Australia found an unexpected diversity
in species (Davis et al., 1993). Although increased temperature and droughts
may threaten the viability of fish populations in this region, such changes
may not greatly alter the invertebrate biota-which appear to be well adapted
to variability in water availability. Breeding cycles of water birds may be
affected (Hassall and Associates, 1997; see also Box 4-1).
A collaborative study by a consulting company, two New South Wales (NSW) government agencies, and the CSIRO has made a preliminary integrated assessment of the impacts of climate change on the management of the scarce water resources of the Macquarie River basin in northern NSW (Hassall and Associates, 1997). The catchment contains dryland agriculture (mainly wheat) and pastoralism, irrigated agriculture (mainly cotton), several small towns, and an episodically flooded wetlands area known as the "Macquarie Marshes," which is a major breeding area for birds. Over the past decade, agricultural and pasture production of sheep, beef, wool, wheat, and cotton contributed 92% to the regional economy.
The study considered the impacts of "low change" and "high change" climate
scenarios in the region by 2030, based on estimates from the CSIRO regional
climate model nested in the CSIRO slab-ocean GCM (experiment F1, Table
1-1), and IPCC (1996a) ranges of uncertainty in global warming. Spatially
and seasonally varying projected rainfall and temperature changes in the
catchment ranged from about 0 to -15%, and 0.4 to 1.2°C, respectively.
These were used in a catchment model to quantify possible changes in moisture,
runoff, and water supplies. Output was then used in the IQQM river management
model developed by the NSW Department of Land and Water Conservation.
Consequences for the pastoral, agricultural, and wider economy of the
region were then considered, using simple models of yield and income on
climate. Consideration of the need for "environmental flows" in the river
and to ensure wetland breeding habitat led to limitations on water diversions
for irrigation. Results of this study showed that mean annual runoff to
the Burrendong Dam (the main water-storage facility) was reduced by 11%
in the low-change scenario and by 30% in the high-change scenario, with
correspondingly less water available for irrigation under present river
management rules. Using optimistic assumptions regarding the beneficial
effects of increased CO2 concentrations on crops and pastures, the study
found aggregate losses to the agricultural economy in 2030 of 6% in the
low-change case and 23% in the high case. Beneficial effects of increased
CO2 on cotton approximately balanced the effects of climate change, but
this was less true for wheat. By far the biggest losses were for sheep,
beef, and wool, which constitute 63% of total agricultural production
in the region.
The NSW National Parks and Wildlife Service forecast that, if the rules were not changed, the loss of water supply would lead to reduced filling of the Macquarie Marshes and thus reduced or less frequent breeding of some bird species, with possible local or regional extinctions, depending on what changes occur in other breeding areas.
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