An underlying assumption of laboratory‐based toxicity tests is that the sensitivity of organisms in the laboratory (in vitro) is comparable to that in the field (in situ). We tested this assumption by exposing estuarine amphipods (Chaetocorophium cf. lucasi) to a concentration series of cadmium‐spiked sediments in vitro and in situ for 10 d. In situ exposures were conducted within plastic‐mesh cages on an intertidal mudflat. To characterize exposure, we measured interstitial water cadmium concentrations (IWCd), acid volatile sulfide (AVS), and simultaneously extracted Cd (SEMCd) at the beginning and end of the exposures. Between day 0 and day 10, AVS decreased in both in vitro and in situ exposures, while IWCd levels declined less in vitro (median 27%) than in situ (median 76%). Despite more extreme conditions of temperature (10–36°C) and salinity (18–22%o) in situ, in vitro and in situ exposures showed comparable survival responses based on SEMCd/AVS (LC50 [95% CI]: 1.6 [1.46–1.78] and 1.8 [1.76–1.83], respectively), with the onset of marked mortality above a SEMCd/AVS value of about one and minimal survival (<5%) above a value of two. Based on IWCd concentrations, however, sensitivity was significantly greater in vitro (LC50 = 0.41 μg Cd/L [0.171–0.959], in situ LC50 = 1.6 μg Cd/L [1.15–2.16]). We concluded that, in our tests, amphipod sensitivity in vitro was equal to or greater than its sensitivity in situ.

Macrobenthic community structure was assessed on intertidal sandflats, apredominant feature of Manukau Harbour. Thirty-six replicate core samples were collected from each of six 9000 m 2 sites, during October 1987. The distribution of individuals amongst taxa was similar at the six sites, but the numerical dominance of the most common taxa changed appreciably from site to site. A shift from polychaete-to bivalve-dominated communities was apparent between sites. Examination of a hypothesised relationship between sediment grain size and trophic structure of the macrobenthos highlights the difficulties in using this relationship to monitor environmental change. Identification of core taxa by DECORANA ordination and TWINSPAN classification is considered a useful mechanism in defining taxa likely to play major roles in influencing community structure and function.

Crabs are among the larger and more active burrowers in intertidal sedin~ents in New Zealand, as in many other parts of the world. Abundances of mud crabs Helice crassa and their burrows were compared among sites differing in the nature of the11 sediments. Differences in burrow archltecture among sites were quantified using casts of burrows made in situ. The effects of bioturbation on sediment geochemistry were also determined with respect to redox potential and the concentrations of 2 chemical phases, acid volatile sulphides (AVS) and iron pyrites (FeS,), that influence the bioavailability of heavy metals. The objective of this study was to identify effects of differences in the extent and nature of burrowing activity of the crab H. crassa among different sediment types on sediment geochemistry, particularly with respect to the bioavailability of heavy metals. Burrows were more abundant at muddy sites (average 22 to 59 burrows 20 cm diameter core-') than at sandy sites (average 12 burrows core-'). Although not necessarily deeper at muddy sites, burrows were more complex in structure. Average volumes of casts were larger at muddy than sandy sites by a factor of up to 14.8 and at the most densely-burrowed site, burrows represented 14,?? of the volume of the surrounding sediment. The architecture of the burrows of H. crassa is discussed in the context of published models of burrow structure and function for other crustacea. Patterns of differences in chemical variables among sites were not clear cut. Redox profiles generally showed decreases with depth in the sediment and, among the muddy sites, potentials were hlghest and most variable at the site with greatest burrowing activity. Concentrations of AVS and FeS, were lower in sandy sediments than in muddy ones. Among muddy sites, the concentration of AVS was lowest at the site with the greatest amount of burrowing, consistent with introduction of oxygen to the sediment and the consequent oxidation of AVS. Concentrations of FeSz showed a pattern that suggested that burrowing introduces FeS2 from deeper layers, where concentrations are higher, into shallower, bioturbated layers of the sediment, offsetting effects of oxidation due to burrowing activity. Concentrations of FeS, were highest at the site with most burrowing activity KEY WORDS: Crab .

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Populations of the freshwater mussel Hyridella menziesi were compared from six lakes and one river site along the Waikato River system, North Island, New Zealand. Estimated densities ranged from several hundred per square metre in Lake Taupo to less than one per square metre in Lake Karapiro. Lengthfrequencies were similar at all sites, with unimodal distributions and a notable absence of any individuals less than 20 mm long. The extent of shell erosion varied from sites where most mussels had less than 1% of the right valve affected (i.e., Ohakuri & Karapiro), to Lake Aratiatia where most shells had erosion over 1-20% of their surface. Taupo and Ohakuri mussels showed an unusually high incidence (over 50%) of internal shell abnormalities (including dulling, deformities and protuberances). Canonical discrimination based on shell length, height, width and weight clearly separated Taupo and Ohakuri mussels from all other sites, and ANCOVA on morphological characteristics confirmed significant differences between sites. Physical condition indices (based on dry flesh weight per shell weight and shell volume) showed an increasing downstream trend, but this was not significantly correlated with food availability (i.e., chlorophyll a concentration) because of unexpectedly low condition in lakes Ohakuri and Karapiro. Length v. age relationships contained considerable scatter and did not show between-site differences. Measured differences in external erosion, internal shell abnormalities and morphology were attributed to physical, chemical and biological factors. Use of mussels for environmental monitoring must take these 'background' differences into account.

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Sediment physicochemical characteristics, benthic community structure, and toxicity were measured at reference and contaminated intertidal mudflats around the North Island of New Zealand. Chronic whole-sediment toxicity tests were conducted with the estuarine amphipod Chaetocorophium lucasi and the marine bivalve Macomona liliana, and pore-water toxicity tests were conducted with embryos of the echinoid Fellaster zelandiae. Although concentrations of organic chemicals and heavy metals were up to several orders of magnitude higher at the sites considered to be contaminated, levels of contamination were relatively low compared to internationally based sediment quality guidelines. Although no pronounced difference was found in benthic community structure between reference and contaminated sites, multivariate analysis indicated that natural sediment characteristics (total organic carbon, acid-volatile sulfide, ammonium) and factors related to contamination (dichlorodiphenyltrichloroethane, sediment toxicity) may have been affecting community structure. Although benthic effects caused by present levels of contamination are not yet dramatic, subtle changes in community structure related to pollution may be occurring. The two whole-sediment and the pore-water toxicity tests presented different response patterns. Growth of C. lucasi and M. liliana was a less sensitive endpoint than survival. None of the three toxicity tests responded more strongly to the contaminated than to the reference sites, that is, neither naturalsediment and pore-water characteristics nor unmeasured contaminants affected the test organisms. It is possible that sediment collection and handling may have induced chemical changes, confounding interpretation of toxicity tests.