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Water Quality Occurrence 2.2 Natural Sources Organic matter budgets of small streams in forested landscapes are dominated by inputs of terrestrial material (Bilby and Bisson 1992; Delong and Brusven 1994; France 1995a,b). This dominance is caused by high input rates of material from allochthonous sources (i.e., of terrestrial origin) combined with suppression of autochthonous production (i.e., of aquatic origin) by shading from riparian vegetation. As a result, streams are typically heterotrophic in the headwaters. The importance of direct allochthonous inputs decreases downstream as increasing stream size reduces the influence of the riparian vegetation (Delong and Brusven 1994). The importance of allochthonous inputs to lakes varies with nearshore riparian slope, proximity of trees to the shoreline, extent of wetlands and peat lands in the drainage basin, proportional composition of deciduous leaf-fall, season, and the extent of bare ground surfaces exposed to precipitation (France and Peters 1995; France 1995b; Dillon and Molot 1997). The smaller the lake surface area, the less likely that fetch winds will develop that would remove leaf litter away from the water. Nevertheless, it is unlikely that lateral transport of leaf material will be as important for lakes as has been found for forested streams. France (1995b) found that the input rate for terrestrial leaf litter to small Canadian shield lakes was 2 g/m forested shoreline/year, an amount that represents 6% of the total annual allochthonous input of leaf-fall to these lakes. Other studies have suggested that terrestrial litter can contribute up to 15% of the total carbon supply to oligotrophic lakes (France and Peters 1995).
Anthropogenic activities can have a profound effect on the organic carbon budgets of aquatic systems (Schepers and Francis 1982; France 1995a,b; France and Peters 1995). Snowmelt, runoff from agricultural lands, municipal and industrial wastewater discharges, and stormwater overflows can lead to substantial increases in the levels of total and dissolved organic carbon in surface waters. Schepers and Francis (1982), for example, found that grazing livestock increased total organic carbon by 11% in runoff waters as a result of increased soil erosion and production of animal wastes. Of perhaps greatest concern in British Columbia, are forest management practices. Forest management practices can cause an initial increase in organic carbon as a result of increased soil erosion due to construction of infrastructure and harvesting of trees, discharge of coloured effluents, and leaching of wood debris, both in situ and from nearby storage sites (Butcher 1992; Taylor 1994; Nordin and Holmes 1992; Forsberg 1992). In the longer term, removal of riparian vegetation through timber harvesting reverses the relative importance of allochthonous organic sources by stimulating primary production (e.g., by removal of shading) and decreasing inputs of terrestrial organic matter (e.g., less leaf litter)(McLeod et al. 1983). In a comparison of two headwater tributaries to the Deschutes River, Washington, Bilby and Bisson (1992) found that allochthonous organic matter dominated inputs (300 g/m2/year) in the tributary bordered by old-growth coniferous forest compared to autochthonous inputs (100 g/m2/year). In the tributary in the clear-cut area, autochthonous inputs were 175 g/m2/year and allochthonous inputs were only 60 g/m2/year. Greater autochthonous production and decreased inputs of terrestrial materials have important consequences for consumer trophic levels in aquatic systems because autochthonous organic material (e.g., algae and algal-based detritus) generally has higher levels of primary amines, proteins, and carbohydrates and is more digestible than most incoming terrestrial material (McDowell and Likens 1988). Further, the food quality of the terrestrial organic material in areas that have been logged or turned over to agricultural crops is considered lower than in areas associated with mature riparian vegetation (Delong and Brusven 1994).
The existing data indicate that median total and dissolved organic carbon levels in British Columbia lakes and rivers are generally less than 5 mg/L except for waters that have high natural sources. For example, the median total organic carbon concentration ranged from 26-105 mg/L in Del Burns Bog. Similarly, sites that are located near anthropogenic sources such as Dawson Creek near the city discharge and dump (median range TOC=21-25.5 mg/L) may have elevated total organic carbon concentrations. Median total organic carbon levels in bogs and swamps or near anthropogenic sources are often greater than 20 mg/L. The data indicate that DOC is the dominate fraction of TOC in most British Columbia waters, and that organic carbon levels are generally correlated with true colour. The available data on total and dissolved organic carbon levels worldwide indicate that, as in British Columbia, the mean TOC and DOC levels are typically below 5 mg/L. For
some regularly sampled locations, there is evidence of considerable
temporal variation. For example, in Adirondacks streams, mean
seasonal DOC levels are typically between 1.5 and 5 mg/L; the
annual ranges, however, indicate that DOC levels vary from
lows of <0.7 mg/L to highs of >7.5 mg/L (Baldigo and
Murdoch 1997). Numerous studies have noted that total and dissolved
organic carbon levels can exhibit short-term spikes following,
for example, storms or litter-fall (e.g., Sugai and Burrell
1984; France 1995a,b). Figure 1 shows that levels in the Elk
River and in two lakes in British Columbia (Kootenay Lake,
Koocanusa Lake) vary greatly over time. Available data shows
two to four-fold variations in organic carbon levels are frequent
over time frames of several months (e.g., Gordon River at TR4,
Flora Lake at Nitinat, Little Nitinat River at Rock Cut).
Several
recent studies of receiving waters near pulp and paper mills
in British Columbia indicate that many mills are not discharging
the large quantities of total or dissolved organic carbon that
had occurred in the past (see BC Environment 1998a). For example,
concentrations of total organic carbon were similar (1.3 to
1.8 mg/L) in receiving waters from all near-field, far-field
and reference sites in the Campbell River near the Elk Falls
Pulp and Paper mill (Hatfield Consultants 1997a). Similar results
were found for receiving waters near the Western Pulp, Port
Alice operation (TOC = 1.8 to 4 mg/L, the highest level being
observed at the reference site) (Hatfield Consultants 1997b),
and near the Crofton Pulp and Paper Mill (Hatfield Consultants
1994). The observed improvement in effluent quality may be
due to process changes that took place at many mills following
enactment of the Canadian Pulp and Paper Effluent Regulations
under the Fisheries Act in May 1992.
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