An investigation of carbon pathways in New Zealand streams
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
An investigation of carbon pathways in New Zealand streams was carried out using a multi-faceted, experimental approach. A series of research projects, presented as individual papers, was conducted with special emphasis on consumer processing. Breakdown of mountain beech (Nothofagus solandri var. cliffortiodes) enclosed in coarse-mesh (3 mm) and fine-mesh (0.2 mm) bags was studied in two physically contrasting streams. Weight losses were most rapid in coarse-mesh bags held in Middle Bush Stream, a relatively stable stream with a large shredder population. The rate of leaf breakdown in coarsemesh bags in Craigieburn Cutting Stream was similar to that In fine-mesh bags buried in both streams. Leaves in bags buried in the beds of both streams lost weight more slowly than their surface counterparts and had slightly lower protein levels and respiration rates. Larvae of the caddisfly, Zelandopsyche ingens, were primarily responsible for the rapid disappearance of beech leaves In Middle Bush Stream. Cage experiments demonstrated that larvae grew and survived equally well in both streams and the absence of Z. ingens from the Craigieburn Cutting Stream benthos was attributed to the inability of the stream to retain coarse particulate organic matter inputs. Retention appears to be the key to shredder distribution and abundance and hence is a major factor controlling litter breakdown. The development, structure and utilisation of stone surface organic layers was investigated at two field sites in dark and natural lighting conditions. In the dark at the forest site an organic layer consisting of slime, fine particles, bacteria and fungi developed and attained maximum biomass (0.08 mg.cm- 2 ) in about two months. In the dark at the spring site where water was low in dissolved and particulate organic matter (DOC level <0.5 g.m- 3 ) no organic layer developed whereas under conditions of natural lighting at both sites, layers were dominated by diatoms and/or filamentous algae. Laboratory experiments demonstrated the importance of dissolved organic matter (DOM) as a prerequisite for layer formation. Uptake by micro-organisms accounted for most of the reduction in levels of dissolved organic carbon (DOC) recorded in recirculating stream channels. Radiotracer experiments ( 14C and 144Ce) showed that several common stream invertebrates could feed on "heterotrophic layers" (assimilation efficiencies 18 to 74%) and suggested that the non-autotrophic components of stone surface organic layers are likely to playa significant role in carbon transfer to the benthos, particularly in small, shaded streams. Utilisation of allochthonous and autochthonous inputs by benthic invertebrates was investigated using stable carbon isotope analysis. The faunas of small, forested streams depended primarily on allochthonous sources of carbon whereas variable utilisation of allochthonous and autochthonous materials was shown by species from a grassland stream. Relatively depleted 13C values obtained for the mayfly Deleatidium taken from forested sites suggest that its larvae assimilate algae selectively. A shift to greater dependence on autochthonous energy sources was shown by invertebrates from small recently clear-cut catchments (1-4 years) in response to canopy removal and flushing of forest-derived organic materials. Although only of a preliminary nature, my results suggest that stable carbon isotope analysis is a useful technique for providing insights into trophic linkages within stream communities. Finally, the influence of physical factors and forest type on the distribution of benthic invertebrate faunas in 43 New Zealand streams was investigated using a systematised survey technique. Several common taxa were numerically dominant at most sites regardless of forest type and both distribution of shredders and faunal diversity were related to stream stability. Implications of these findings for forest-stream management are discussed. In general, New Zealand stream ecosystems are viewed as being strongly dominated by physical factors which determine and limit the framework within. which biological communities can function. Integration of the mass-balance and intra-system approaches to carbon processing is suggested as the logical way to promote understanding of carbon flows to, from and within stream ecosystems.