Physico-chemical conditions and benthic community dynamics in four South Westland streams
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Physico-chemical characteristics and benthic ecology of four small, stony, brownwater streams in South Westland, New Zealand were investigated between November 1984 and November 1986. Streams were in a high rainfall region (ca. 5 m per annum) and had low pH (4.5-6.5), alkalinity and conductivity but relatively high DOC (4-9 mg.l- 1). Discharge responded rapidly to frequent rainfall events, increasing DOC and lowering pH, alkalinity and conductivity. Epilithic biomass on tiles was always low (TOC< 200 ug.cm- 2). Non-algal biomass increased rapidly and was subsequently colonized by bacteria, fungi and algae. Algal composition differed among streams, but not seasonally. Field experiments demonstrated that permanent inundation affected biomass and composition and that invertebrate grazers could reduce biomass. Amount of detritus (> 0.25 mm) in the streambeds was low « 60 gAFDW.m- 2), patchily distributed and occurred non-seasonally. Eighty-seven invertebrate taxa (including 27 Trichoptera, 18 Plecoptera, 15 Diptera) were recorded. The mayfly De7eatidium and the family Chironomidae numerically dominated all streams throughout the year, whereas over 2/3 of the other taxa were rare « 1% of numbers). Faunal densities were moderately low (400-1300 m- 2) as was production of six major insect taxa calculated by the size frequency method (SFM) and Hopkins' method (HM). Although HM has been used infrequently, it gave similar results to SFM and requires less data analysis. All six taxa had poorly synchronized life histories (difficult to interpret; apparently all univoltine). Low drift rates were dominated by terrestrial insects, De7eatidium and Chironomidae. Drift densities were negatively correlated with discharge, which suggests that insects may actively avoid drifting when flow increases. Of the four, Noone Creek had the greatest epilithic algal diversity, invertebrate richness and flow continuity (ie. regularly channelled flow). It is hypothesized that this latter factor may account for the high biotic dixersity. Stream faunas could be described as "determinate" assemblages (Lake & Barmuta 1986) whose populations were highly resistant and resilient to physical disturbances.