Dynamics of benthic invertebrate communities in a northern New Zealand kauri forest stream ecosystem

Abstract

The structure and function of benthic invertebrate communities in the Waitakere River, a kauri forest stream ecosystem, was studied in association with physicochemical parameters from May 1973 until March 1976. Emphasis was placed upon life histories of the dominant species, so that changes in community structure along the stream could be characterised. Analysis of vegetation along the forested watercourse showed a predominance of podocarps and kauri (Agathis australis) in the canopy, with tree ferns dominating in the subcanopy. As the stream flowed into open farmed areas, algal blooms, which were mainly present in summer in some forested sites, became more widespread and persistant. These appeared to influence the composition of the invertebrate communities. Physicochemical analyses of water from several sites in the Waitakere River and its tributaries (from January to December 1974, and in July 1975 and February 1976) provided no evidence that algal blooms could be associated with nutrients derived from agriculture. In the classification of Williams (1964) the waters were "calcium poor" (generally <8.0 g/m3) and intermediate between "soft" and "medium hard" (alkalinity 15-44 g/m3). However, they were comparatively rich in silica (14-35 g/m3), sodium (17-25 g/m3) and chloride (25-39 g/m3). The association between physicochemical variables was tested by correlation matrix analysis. Two distinct groups were formed. Group A variables (temperature, sodium, conductivity, alkalinity, hardness, chloride and potassium) were positively correlated with each other, but negatively correlated with precipitation, whereas group B variables (manganese, iron and sulphate) were positively correlated with turbidity. A summer groundwater influence appeared to provide maximum values for group A variables, and increased turbidity during high rainfall (winter) produced maximum values for group B variables. The invertebrate fauna was surveyed throughout the watershed, and studied intensively at four sites (two in open farmland and two in forest) for 12 months. Separation of most invertebrate groups to species level provided 144 taxa, almost three times the number in previously published faunal lists for New Zealand streams, but comparable with those obtained in the Northern Hemisphere. Aspects of the life histories of the following 19 taxa were investigated: Potamopyrgus antipodarum (Gastropoda : Hydrobiidae), Paracalliope fluviatilis, (Amphipoda : Eusiridae), Coloburiscus humeralis (Ephemeroptera : Siphlonuridae), Zephlebia nodularis, Atalophlebioides sepia, Deleatidium lillii, D. myzobranchia, Deleatidium sp C, Deleatidium sp E (Ephemeroptera : Leptophlebiidae), Zelandobius furcillatus (Plecoptera : Gripopterygidae), Hydora nitida (Coleoptera : Elmidae), Maoridiamesa harrisi,? Austrocladius sp., Paratanytarsus agameta (Diptera : Chironomidae), Austrosimulium australense (Diptera : Simuliidae), Aoteapsyche colonica (Trichoptera : Hydropsychidae), Oxyethira albiceps (Trichoptera : Hydroptilidae), Pycnocentrodes spp and Olinga feredayi (Trichoptera : Sericostomatidae). Most of these had poorly synchronised, indistinct life cycles which could not be classified using the system of Hynes (1970) for temperate Northern Hemi sphere streams The most diverse communities occurred in the rapidly flowing, cool tributaries of the Waitakere River, dominated by insects with long life cycles. Downstream the communities were dominated by non-insect species, or insect species with rapid life cycles and a seasonal occurrence. Many of these appeared to be associated with algal blooms. A black polythene artificial canopy (44 m2) was placed over an area of the lower Waitakere River prone to algal blooms, to investigate the effect of algal removal induced by shading. Groups which declined in abundance under the canopy were Chironomidae and Hydroptilidae, while drift filtering species (Aoteapsyche colonica and Austrosimulium australense) increased in number under the canopy. Other taxa (Pycnocentrodes spp., Potamopyrgus antipodarum and Elmidae larvae) remained equally abundant in shaded and non-shaded areas. The possible contribution of species colonizing such "new" habitats through drift and adult flight was investigated. Invertebrates of the Waitakere River and its tributaries differed from those of temperate Northern Hemisphere streams in taxonomic composition, trophic structure (relative lack of "shredding" species), life history patterns and the presence of spatial and temporal overlap of related species (members of the genus Deleatidium). It is suggested that most of these characteristics occur throughout New Zealand, and are related to the types of detritus entering New Zealand streams, and the effects of long geological isolation.