Terry M. Hume

Bedload and water-column transport and colonization processes by post-settlement benthic macrofauna: Does infaunal density matter?

Copyright (c) 1997 Elsevier Science B.V. All rights reserved. In this study, we document dispersal and colonization by post-settlement benthic macrofauna on a large (250×500 m) area of intertidal sandflat at Wiroa Island in the Manukau Harbour, New Zealand, over a 2-week period in February 1994. We examine the effects of variation in the natural density of large (>15 mm) Macomona liliana Iredale (a tellinid bivalve) on these processes. Post-settlement transport was measured using cylindrical bedload and water-column traps positioned at 22 experimental sites within the study site. Macrofaunal colonization was documented using small pans of defaunated sediment deployed at each of the experimental sites. Greatest macrofaunal colonization of pans occurred in areas of greatest sediment reworking and deposition, suggesting transport and deposition of post-settlement stages may be passive processes. For some species [e.g. Austrovenus stutchburyi (Wood)] there was a significant positive relationship between their abundance and the weight of sediment in bedload traps, as well as wind conditions over the study period. This is consistent with passive transport processes, but does not exclude the possibility of active dispersal processes also being important. Differences in the hydrodynamic conditions (e.g. increased wind generated wave activity) over the 2-week study may have contributed to changes in abundance on the 2 sampling dates. For other species (e.g. Macomona liliana) the absence of significant positive relationships between abundance in either bedload or water-column traps and the weight of sediment retained in traps, as well as between ambient densities and the net flux of individuals across sites, is consistent with active dispersal and colonization. There was a negative relationship between background density of large Macomona liliana and numbers of smaller conspecifics (≤4 mm) and other bivalves [Austrovenus stutchburyi, Cyclomactra ovata (Gray) and Nucula hartvigiana Pfeiffer] colonizing pans of defaunated sediment. The results suggest that 10 to 100s m scale variation in the natural densities of an abundant siphonal surface deposit feeder may have a significant influence on dispersal and colonization and, thus, the spatial patterns of macrofauna on the tidal flat.

Oceanographic processes and sediment mixing on a sand flat in an enclosed sea, Manukau Harbour, New Zealand

Abstract Studies of oceanographic and sedimentary processes on intertidal sand flats in an enclosed sea were undertaken to gain a better understanding of the factors controlling the mixing and dispersal of sediment bound contaminants. Field investigations included a 90 day process experiment during which wind, waves, tidal currents, tides, depth of disturbance and sand flat morphology were measured, and 27 months of sand flat profile monitoring. Sediment entrainment by strong spring tidal currents is restricted to the middle and lower regions of the sand flat which are inundated during the peak tidal flows. The upper 2–3 cm of sediment is re-worked across the middle and upper sand flat by mild storm wave events ( H s = 70 cm ), which occurred four times during the 90 day experiment. Numerous ridges and runnels in the upper sand flats are wave-formed features and are maintained by the lack of currents of sufficient magnitude to re-work the features. The ridge and runnel morphology is testimony to large infrequent storm events which re-work the sediment to depths of 20 cm. Such storms are an important mechanism for the release of contaminants and were recorded on 3 occasions in the 27 month profile record.

Characterisation of physical environmental factors on an intertidal sandflat, Manukau Harbour, New Zealand

Abstract Physical environmental factors, including sediment characteristics, inundation time, tidal currents and wind waves, likely to influence the structure of the benthic community at meso-scales (1–100 m) were characterised for a sandflat off Wiroa Island (Manukau Harbour, New Zealand). In a 500×250 m study site, sediment characteristics and bed topography were mostly homogenous apart from patches of low-relief ridges and runnels. Field measurements and hydrodynamic modelling portray a complex picture of sediment or particulate transport on the intertidal flat, involving interactions between the larger scale tidal processes and the smaller scale wave dynamics (1–4 s; 1–15 m). Peak tidal currents in isolation are incapable of eroding bottom sediments, but in combination with near-bed orbital currents generated by only very small wind waves, sediment transport can be initiated. Work done on the bed integrated over an entire tidal cycle by prevailing wind waves is greatest on the elevated and flatter slopes of the study site, where waves shoal over a wider surf zone and water depths remain shallow enough for wave-orbital currents to disturb the bed. The study also provided physical descriptors quantifying static and hydrodynamic (tidal and wave) factors which were used in companion studies on ecological spatial modelling of bivalve distributions and micro-scale sediment reworking and transport.

The sandflat habitat: scaling from experiments to conclusions

a , a a a b * S.F. Thrush , R.D. Pridmore , R.G. Bell , V.J. Cummings , P.K. Dayton , c d a a e a R. Ford , J. Grant , M.O. Green , J.E. Hewitt , A.H. Hines , T.M. Hume , f g h a i S.M. Lawrie , P. Legendre , B.H. McArdle , D. Morrisey , D.C. Schneider , a j k a S.J. Turner , R.A. Walters , R.B. Whitlatch , M.R. Wilkinson National Institute of Water and Atmospheric Research, P.O. Box 11-115, Hamilton, New Zealand Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA 92093-0201, USA Dept. Marine Science, University of Otago, P.O. Box 56, Dunedin, New Zealand Dept. of Oceanography, Dalhousie University, Halifax, Canada B3H 4JI Smithsonian Environmental Research Centre, P.O. Box 28, Edgewater, MD 21037, USA Culterty Field Station, University of Aberdeen, Newburgh, AB40AA, Scotland g ́ ́ ́ Departement de Sciences Biologiques, Universite de Montreal, C.P. 6128, succursale Centre-ville, ́ ́ Montreal, Quebec H3C 3J7, Canada Biostatistics Unit, School of Biological Sciences, University of Auckland, Private Bag 92019, Auckland, New Zealand Ocean Sciences Centre, Memorial University of Newfoundland, St John’s, Canada ALC5S7 United States Geological Survey, 1201 Pacific Ave, Suite 600, Tacoma, WA 98402, USA Dept Marine Sciences, University of Connecticut, Avery Point, Groton, CT 06340-6097, USA

Patterns of sediment reworking and transport over small spatial scales on an intertidal sandflat, Manukau Harbour, New Zealand

Copyright (c) 1997 Elsevier Science B.V. All rights reserved. Measurements of physical sediment reworking and transport were conducted at 22 experimental sites within a 250×500 m study site on a sandflat at Wiroa Island (Manukau Harbour, New Zealand), in order to examine spatial patterns of sediment transport, and its relationship to passive advection of benthic fauna (Turner et al., 1997). Sediment reworking and transport were measured four times during February 1994 as replacement of dyed sand in pans of sediment buried in the intertidal zone, change in total height of the sediment column in the pans, and as deposition in tube traps with openings flush with the bed (bedload traps) and at 15 cm above the bed (water-column traps). Sediment reworking replaced about 2-3 mm of sand per day, with increasing cumulative transport to a depth of 20 mm during the study period. In addition, there were site-specific differences among sampling dates. Spatial structure in sediment reworking was analyzed by trend surface analysis. Depending on date, variance in reworking was influenced by location within the study site, tidal shear stress (model generated), and elevation on the sandflat. Analysis of residuals demonstrated that sediment reworking at times contained inherent spatial structure after accounting for the effects of other explanatory variables. Bedload trap rates in the final sampling period accounted for most of the variance in deposition indicated by sediment height. Sediment reworking and transport are variable over scales of 10 1 -10 2 m, as well as over a period of days, such that measurements determined in single point studies cannot necessarily be extrapolated over larger spatial scales. Patterns of sediment reworking and transport patterns provide a template against which to compare patterns of faunal transport. However, the linkage will be most apparent when 1) sediment reworking and transport are substantial in magnitude, 2) there is significant XY spatial structure to the pattern of sediment transport at the scale of the study, and 3) the fauna of interest are at least potentially transported as bedload (e.g. shelled forms).

Structure of the internal boundary layer over a patch of pinnid bivalves ( Atrina zelandica ) in an estuary

Measurements of tidal-current boundary-layer flow over an experimental 2-m by 2-m patch of pinnid bivalves (Atrina zelandica) in a northern New Zealand estuary are presented. Previous work demonstrated a link between mesoscale (order 100 m) patchiness of the benthic biota and time-averaged boundary-layer dynamics. The aim in this new experiment was to describe the three-dimensional structure of turbulence at the patch scale (order 1 m). Flow over three densities of Atrina was investigated: 340 individuals per 4 m 2 , 50 individuals per 4 m 2 and zero individuals. An internal boundary layer (IBL) grows downstream from the leading edge of the patch at the base of the ambient boundary layer. One meter in from the leading edge, the top of the IBL was ∼12 cm above the bed for the high-density patch and ∼6 cm for the low-density patch. Flow in the IBL was three-dimensional in that vertical and transverse mean velocities were nonzero, secondary Reynolds stresses were nonzero and comparable with the primary stress, and velocity spectra deviated from scaling relationships for two-dimensional flow. Thus, the observed IBL was still in its infancy, i.e., it consisted of a roughness sublayer only as the distance from the leading edge of the patch was not enough for development of a second, overlying logarithmic layer. In summary, the IBL that envelops the Atrina patch is a region of lower mean longitudinal velocities but more energetic turbulence relative to the ambient boundary layer. The former translates into shelter, which some organisms might take advantage of, and the latter translates into increased vertical exchange across the top of the IBL, which might enhance fluxes of nutrients, colonists and suspended sediments, and might have implications for deposition and resuspension of organically rich biodeposits. The results extend our knowledge of turbulence over patches of suspension feeders at the 1-m scale and therefore provide information needed to improve depiction of flow in models of suspension-feeder-flow interactions.

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.

Use of organochlorine contaminants to measure sedimentation rates in estuaries: a case study from the Manukau Harbour

Abstract Organochlorine contaminants are widely distributed in sediments of the Manukau Harbour. The first appearance of DDT in cores is a potential stratigraphic marker because it can have appeared only since extensive applications of DDT on New Zealand pastures for grass grub control began in c. 1950. The technique permits calculation of average annual sedimentation rates, but precision is limited by bioturbation. A nett sedimentation rate of about 5 mm yr-1 was determined from measurements of DDT for muddy tidal flats in Drury Creek. This compared well with a rate of 5 mm yr-1 based upon a pollen dating technique. The past widespread use of DDT may enable comparisons to be made of very recent sedimentation rates in other New Zealand estuaries. Other contaminants identified in the Manukau Harbour sediments include dieldrin, lindane, PCBs and chlordane.

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.

Coastal oceanography and sedimentology in New Zealand, 1967–91

Abstract This paper reviews research that has taken place on physical oceanography and sedimentology on New Zealands estuaries and the inner shelf since c. 1967. It includes estuarine sedimentation, tidal inlets, beach morphodynamics, nearshore and inner shelf sedimentation, tides and coastal currents, numerical modelling, short‐period waves, tsunamis, and storm surges. An extensive reference list covering both published and unpublished material is included. Formal teaching and research programmes dealing with coastal landforms and the processes that shape them were only introduced to New Zealand universities in 1964; the history of the New Zealand Journal of Marine and Freshwater Research parallels and chronicles the development of physical coastal science in New Zealand, most of which has been accomplished in last 25 years.