EFFECTS OF CLAY DISCHARGES ON STREAMS .2. BENTHIC INVERTEBRATES

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.