John W. Oldman

USING HABITAT SUITABILITY INDEX AND PARTICLE DISPERSION MODELS FOR EARLY DETECTION OF MARINE INVADERS

Eradication and control of invasive species are often possible only if populations are detected when they are small and localized. To be efficient, detection surveys should be targeted at locations where there is the greatest risk of incursions. We examine the utility of habitat suitability index (HSI) and particle dispersion models for targeting sampling for marine pests. Habitat suitability index models are a simple way to identify suitable habitat when species distribution data are lacking. We compared the performance of HSI models with statistical models derived from independent data from New Zealand on the distribution of two nonindigenous bivalves: Theora lubrica and Musculista senhousia. Logistic regression models developed using the HSI scores as predictors of the presence/absence of Theora and Musculista explained 26.7% and 6.2% of the deviance in the data, respectively. Odds ratios for the HSI scores were greater than unity, indicating that they were genuine predictors of the presence/ absence of each species. The fit and predictive accuracy of each logistic model were improved when simulated patterns of dispersion from the nearest port were added as a predictor variable. Nevertheless, the combined model explained, at best, 46.5% of the deviance in the distribution of Theora and correctly predicted 56% of true presences and 50% of all cases. Omission errors were between 6% and 16%. Although statistical distribution models built directly from environmental predictors always outperformed the equivalent HSI models, the gain in model fit and accuracy was modest. High residual deviance in both types of model suggests that the distributions realized by Theora and Musculista in the field data were influenced by factors not explicitly modeled as explanatory variables and by error in the environmental data used to project suitable habitat for the species. Our results highlight the difficulty of accurately predicting the distribution of invasive marine species that exhibit low habitat occupancy and patchy distributions in time and space. Although the HSI and statistical models had utility as predictors of the likely distribution of nonindigenous marine species, the level of spatial accuracy achieved with them may be well below expectations for sensitive surveillance programs.

Features of 3‐dimensional barotropic and baroclinic circulation in the Hauraki Gulf, New Zealand

Abstract Predominant features of barotropic and baroclinic circulation and mixing in the Hauraki Gulf on New Zealands north‐east coast are described using measurements and 3‐dimensional numerical model simulations. Circulation in the Hauraki Gulf is strongly 3‐dimensional with a primary dynamical balance between surface wind stress and the associated pressure gradients against the land. This leads to persistent up/downwelling and surface manifestations in sea surface temperature patterns which are shown to vary systematically and markedly with wind direction and stratification intensity. A high degree of correspondence between a baroclinic numerical model and measured temperature and nitrate concentrations indicated that many of the observed spatial patterns could be largely explained by the interaction of the wind and tidal circulation with the unique morphology of the Gulf. After strong southeasterly winds, local responses operated in conjunction with the regional “capping” mechanism described by Sharpies (1997) of downwind surface water intrusion from the shelf into the Gulf. However, the morphology acts to enhance local upwelling causing bottom waters to be injected into the surface layers which disrupts the “cap”. The headlands and islands play an additional vertical mixing role by presenting bathymetric variability leading to the formation of upwelling jets in the core of eddies forming during ebb and flood tides. By introducing bottom waters into the upper water column and acting to over‐turn the water body, up/downwelling is an important mechanism for mixing and biological productivity which could vary systematically within the Gulf in response to seasonal and interannual variability in the upwelling patterns. Persistent south‐east winds above a threshold of 10–12 m s‐1 were found to initiate breakdown of seasonal temperature stratification in the Gulf, with complete breakdown after 3 days during a cyclone with 8–23 m s‐l winds. Vertical eddy diffusivity increased from 0.0015 to 0.04 m2 s‐1 as the cyclone strengthened.

Wave mechanisms responsible for grain sorting and non-uniform ripple distribution across two moderate-energy, sandy continental shelves

Abstract Mechanisms responsible for observed ripple geometry and grain size trends at two open coast sites are considered. The field sites are two moderate-energy sandy shelves on the inner continental shelf out to 50 m depth at East Gippsland in southeastern Australia and at Pakiri in northeast New Zealand. Both are characterised by bands of medium sediments inshore and offshore, separated at 20–45 m depth by a zone with significantly coarser grain size (0.9 mm cf. 0.3 mm). Video observations of the seabed at 126 sites on the East Gippsland shelf revealed that the band was also characterised by larger wavelength ripples (1.0 m cf. 0.3 m). The size of the band stretching over an average of 20 km from 20–45 m depth and its presence at both field sites suggests that the observed characteristics are a long-term response to physical forcing, possibly in near-equilibrium with modern processes. Numerical modelling demonstrated that grain sorting on the shelf is initiated by the presence of maxima in predicted ripple activity and seabed roughness around 35 m depth. The local grain size evidently increases due to winnowing of the fines. Positive feedback, through dependence of ripple size on grain size, causes further lengthening of ripple wavelength, until the pattern has evolved to become highly pronounced on the shelf and self-sustaining, but limited by reducing sediment mobility as grain size increases.

Sediment facies and pathways of sand transport about a large deep water headland, Cape Rodney, New Zealand

Abstract Cape Rodney is a large headland that protrudes 3–4 km into deep water in the Hauraki Gulf and separates the Mangawhai‐Pakiri and Omaha littoral cells. Detailed swath mapping of seabed sediments around Cape Rodney was carried out using by side‐scan sonar and ground‐truthed by SCUBA, grab sampling, and video. Despite the barrier imposed by the headland two pathways of sand transport around the headland, separated by the topographic high of Leigh Reef, have been identified. One lies close to the headland, where sand from the beach and nearshore of the Mangawhai‐Pakiri embayment is driven by waves and currents along a 500‐m‐wide pathway in c. 20–25 m depth around the headland to the vicinity of Leigh Harbour. The other lies in 50 m water‐depth seawards of Leigh Reef. Here fine sand, sourced from the nearshore of the Mangawhai‐Pakiri embayment and driven offshore from the tip of the headland, is transported back and forth by tidal currents in 50 m water depth on the floor of the Jellicoe Channel. The sand bodies along both these pathways are thin and so sand leakage from the Mangawhai‐Pakiri embayment is thought to be small. Transport at these depths is dependent on both tide and wave generated currents and episodic occurring during storm events. The sediment facies associated with little sand transport about a headland in deep water is one of thin and discontinuous and patchy sand cover between rocky areas and over coarser megarippled substrate. Ocean swell, tidally driven phase eddies that spin up on both sides of the headland, and bathymetry all play a role in shaping those facies.

Predicting suitability of cockle Austrovenus stutchburyi restoration sites using hydrodynamic models of larval dispersal

Abstract An important aspect of shellfish restoration projects is to evaluate whether potential sites are likely to be recolonised naturally once disturbances are removed and habitats are restored. Similarly, if active translocation or reseeding of juveniles or adults is undertaken, it is important to understand whether these introduced populations will be self‐maintaining in future, and whether any seed they produce are likely to be retained within the restored area. We used a combined hydrodynamic and particle tracking model to predict larval dispersion patterns for the common cockle Austrovenus stutchburyi under different hydrodynamic scenarios for seven release locations in Whangarei Harbour, New Zealand. Our results implied that sites varied substantially in their potential for self‐seeding and for exporting seed to other locations. For sites with more restricted dispersal, the model predicted that most larvae originating at these sites would settle inside the release region (68–94% for passive particle simulations), whereas relatively few larvae originating from the other release sites settled at these sites. In contrast, model larvae released from sites exhibiting high connectivity dispersed to all sub‐regions in the harbour, and export outside of the model region was high. Forthcoming field validation of these model predictions will result in better integration of hydrodynamic connectivity in whole estuary restoration programs.

Dynamics of a 3‐dimensional, baroclinic, headland eddy

Abstract Wave, current, and temperature measurements from the north side of Cape Rodney, northeastern New Zealand, over a 6‐week field programme, reveal the detailed internal dynamics of a baroclinic eddy. The measurements include water levels, currents, and water temperatures through the water column and seabed descriptions. A detailed calibration of a 3‐dimensional numerical model leads to further examination of the flow and density structure within the eddy, in plan and cross‐section. The eddy stretches c. 5 km along the northern side of the headland and is of the order of 1.5–2.0 km wide. The eddy currents are strongest near the bed and against the coast. The eddy is characteristically complex and vertical eddies readily form in parts of the water column during different phases of the tide, although these internal eddies rarely penetrate through the full depth. The dynamics near the surface and near the bed are significantly different and the structure of the eddy is often partitioned vertically and is strongly affected by wind.