R. Bruce Williamson

Land use effects on habitat, water quality, periphyton, and benthic invertebrates in Waikato, New Zealand, hill‐country streams

Abstract Water quality, habitat, and biota were compared during spring amongst c. 100 m reaches on 11 streams draining pasture, native (podocarp‐broadleaf) forest, and exotic pine forest established on pasture 15 years previously. Differences were greatest between the pasture and native forest streams. Only 1–3% of incident light reached native and pine forest streams whereas 30% reached pasture streams. Pasture streams had 2.2°C higher mean temperature than the native streams, and 5‐fold higher nitrate, 30‐fold higher algal biomass, and 11‐fold higher gross photosynthesis. Native streams were 60% wider than pasture, with pine streams intermediate. Pine and pasture streams had 3‐fold higher suspended solids and fine sediment stored in the streambed than native streams. Woody debris volume was 17‐fold greater in pine than pasture streams, with native streams intermediate. Invertebrate taxa richness did not differ between land uses. Community composition differed most between pasture and native forest, with...

Effects of riparian grazing and channelisation on streams in Southland, New Zealand. 2. Benthic invertebrates

Abstract A survey of benthic invertebrate faunas in riparian‐protected, riparian‐grazed, and channelised reaches of five Southland streams with catchment sizes of 3–37 km2 was carried out. It was part of a wider investigation to assess the effects of riparian grazing and channelisation on stream habitat and biota. In small streams (catchment areas 3–10 km2 ; widths 1–4 m), channelisation or intensive grazing by cattle greatly reduced shading by riparian vegetation, resulting in substantial increases in daily maximum temperatures during summer. Channelisation also caused gross changes in channel morphology and intensive grazing of a reach with moist streamside soils was associated with increased bed sedimentation and bank damage. Marked changes in invertebrate communities were associated wilh these habitat modifications. In general, taxa favoured by cool water and low periphyton abundance (e.g., Plecoptera, Paraleptamphopus caeruleus, Deleatidium sp., and Helicopsyche albescens) decreased in density, where...

Factors affecting the distribution of benthic macrofauna in estuaries contaminated by urban runoff

Contaminants derived from urban runoff have been shown to accumulate in estuarine sediments, reaching concentrations potentially capable of causing biological effects. Demonstration of effects, however, is difficult due to strong natural environmental gradients and the effects of past or present point-sources of contamination. We used multivariate methods to test two hypotheses relating to the effects of urban-derived contaminants on estuarine benthic communities. First, that patterns of distribution and abundance of benthic invertebrates in two urbanised estuaries would be different from those in two non-urbanised estuaries. Second, that the distributions of benthic invertebrates within and among the four estuaries would be related to those of urban-derived contaminants. Concentrations of contaminants were larger in estuaries with urbanised catchments and concentrations of Cu, Pb, Zn and DDT in some samples exceeded those at which biological effects may be expected to appear. Tests of differences in composition of benthic communities among estuaries showed that the two urban estuaries were not significantly different, but that they differed from both rural estuaries, which also differed from each other. Distributions of benthic invertebrates were significantly related to those of environmental variables, and were ordinated along axes that correlated with both natural environmental variables (nature of the sediment, position in estuary) and contaminants. Differences in faunas between the urban and non-urban estuaries were not, however, clear-cut and nor were relationships between faunal assemblages and environmental variables (including contaminants) consistent between two times of sampling.

Reconstruction of urban stormwater contamination of an estuary using catchment history and sediment profile dating

Cores were collected from the length of Pakuranga estuary, a small urban estuary in Auckland, New Zealand, to determine sedimentation and contaminant history, and in particular the impact of urbanization. Catchment sediment loads for the most recent history (1953–1995), including urbanization since 1960, were reconstructed using the landcover history and soil erosion modeling. Pollen and14C dating and pre-urban landcover history were used to reconstruct early estuary sedimentation (i.e., post-3000 yr BP to 1960). Heavy metal concentrations, particle size,137Cs, pollen, and catchment sediment loads were all needed to disentangle the complex estuarine response to urbanization.137Cs profiles did not reflect the historical fallout pattern, but deposition of137Cs-labelled eroded catchment soil, coinciding with peaks in urban construction. Temporal variations in stormwater137Cs concentrations are likely due to varying contributions from137Cs-rich topsoil and137Cs-poor subsoils. A similar pattern was observed in heavy-metal concentrations and attributed to street runoff rather than topsoil being diluted by metal-poor subsoils. Dating of the sediment profiles showed that during urbanization sedimentation rates in the tidal creek and estuary were higher than sedimentation rates associated with past agricultural landuse and the original forest landcover. Urbanization has brought about substantial environmental changes in the upper estuary through continued infilling of shallow, intertidal areas, contamination by heavy metals to levels of ecological concern, sediment textural changes, and rapid mangrove colonization of formerly bare intertidal sediments.

Heavy metal and suspended sediment fluxes from a contaminated, intertidal inlet (Manukau Harbour, New Zealand)

Abstract The magnitude of mobilization of Cd, Cu, Fe, Mn, Pb, Zn and suspended particulate matter (SPM) was measured in a 6.5 km2 contaminated inlet on Manukau Harbour, New Zealand, to assess the importance of this process in the recovery of contaminated sediments from pollution and as a source of secondary pollution to the water column. Mobilization of contaminated surface sediments is the main source of contamination to the water column, and the magnitude of mobilization suggests that this process is very important in contaminant cycling in the inlet sediments. During two surveys, the mass fluxes of SPM and metals were greater during the flood than the ebb tide showing that any mobilized contaminants were largely returned to the inlet, consistent with it acting as a sink for sediments and associated metals. Therefore, pollution recovery by the process of mobilization and dispersal will be slow.

Trace metal partitioning in bioturbated, contaminated, surficial sediments from Mangere Inlet, New Zealand

Abstract Diagenesis and its effects on the partitioning of trace metals was examined in intertidal muddy sediments in polluted Mangere Inlet, Manukau Harbour, New Zealand. Two cores were sectioned down to 26 cm, and analysed for compounds of Fe, Mn, S, and organic matter. Trace metals were analysed by sequential extraction (non‐residual metals), 0.5M HCl, and XRF (total metals). The sediments were bioturbated, with strong mottling of sediment colour in both cores and intact macropores in one core. Bioturbation appeared to affect sediment chemistry and to cause differences between the cores. There were changes in trace metal partitioning in the non‐residual phases with depth, but the changes were relatively small, in line with the changes in major sediment phases. None of the trace metals showed an increase in the lithogenic fraction with depth and we concluded that immobilisation of trace metals by incorporation into residual phases is not an important mechanism in the surface sediments.

Analysis of Acid Volatile Sulphide and Pyrites Using Microdiffusion

Abstract The microdiffusion method published by Brouwer and Murphy[1] for analysis of acid volatile sulphide (AVS-chiefly FeS) has been extended to both AVS and FeS2in a two step process. For the first step (AVS analysis), excellent recoveries (∼100%) are obtained for Na2S, FeS, PbS, CdS, and ZnS provided diffusion times are long enough. Recovery from other insoluble metal sulphides (Ag2S and HgS) were low, while for CuS were variable. In the second step, recoveries were ∼100% for ground, crystalline FeS2. This work re-emphasises the importance of adding a sufficiently strong reducing agent during AVS determination in sediments to prevent oxidation of S2− by Fe3+ produced in the acid dissolution of ferric phases. The method is amenable to adoption to different apparatus provided recoveries and reaction times are checked with suitable standards.