No abstract

The impact of clay discharges on benthic invertebrates was investigated by comparison of communities upstream and downstream of alluvial gold mining on 6 streams on the West Coast of the South Island, New Zealand. Mean turbidity was increased by 7-154 NTU above background (mean 1.3-8.2 NTU) by the mine discharges during the 2 months before sampling. Patterns of increase in suspended solids (strongly correlated with turbidity, r = 0.95) were similar. Invertebrate densities were significantly lower at all downstream sites, ranging from 9 to 45% (median 26%) of densities at matched upstream sites. Downstream densities as a proportion of those upstream were negatively correlated with the logarithm of the turbidity loading (r = -0.82, P< 0.05). The densities of the common taxa were also generally lower downstream of mining. Taxonomic richness was significantly lower at downstream sites in the four streams receiving higher turbidity loads (mean turbidity increase = 23-154 NTU). Lower epilithon biomass and productivity, and degraded food quality at the downstream sites probably explain the lowered invertebrate densities. At some sites, reduced bed permeability and interstitial dissolved oxygen, and avoidance reactions of invertebrates (i.e., increased drift), may have also contributed to lower invertebrate densities.

Placer gold-mining on the West Coast of the South Island of New Zealand provided a field test-bed for investigating the impacts of fine inorganic suspensoids (clays) on streams not subjected to other abuses. The suspensions of clays (40% between 0.55 and 1 pm in diameter) seeping into the streams from gold mines were colloidally stable. The clay particles attenuated light in the streamwater with near maximum efficiency leading to severe degradation of stream optical quality. Turbidity increased from a median of 2.4 NTU upstream often to > 100 NTU (median 15 NTU) downstream. The stream waters, which were strongly-coloured by humic substances, were changed from a dark organge colour to a bright 'muddy' appearance downstream of mining, and visual clarity was reduced from a few metres to as low as 0.03 m (median 0.33 m). The clay discharges decreased light penetration into the stream water such that irradiance averaged over a 12 hr photoperiod at the bed (typically about 0.3 m depth in runs at baseflow) fellfromabout340~Em~2s~'upstreamtoaslowas80~Em~2s~1 (median 190pEm

No abstract

1. The West Coast of the South Island, New Zealand (Westland) is a region of mountains, forests, high rainfall, and a history of exploitation. The Southern Alps rise to over 3000m in the east of the region, and a narrow coastal plain supports some agriculture and the main centres of population. 2. Stream waters in the Southern Alps are characterized by low concentrations of major ions, and most can be described as calcium-sodium-bicarbonate waters. Brown waters with low pH and high concentrations of dissolved organic carbon are common at low and intermediate altitudes.3. Many mountain streams and rivers provide physically harsh environments for aquatic biota with their rapidJy changing flows and frequent spates. Hydrological factors and low nutrient concentrations limit periphyton standing crops, and biomass of coarse detritus is often iow. Invertebrate populations are usually dominated by insect larvae that feed primarily on FROM and stone surface biofilms. 4. Features of the macroinvertebrate stream fauna on the West Coast are the wide range of physicochemical conditions tolerated by many common species, and the numerical dominance of the mayfly Deleatidium (Leptophlebiidae) in many streams. The Plecoptera also exhibit high diversity relative to other parts of the country, and an unusual trend towards terrestrialism is shown by larvae of Gripopterygjdae. 5. The West Coast has a long history of coal and gold mining, forestry and fanning, activities that have had negative impacts on stream communities and water quality. We discuss some ecological and management issues associated with present day mining practices, and a proposal to take large volumes of alpine stream water for export.