Malcolm O. Green

THE GENERALITY OF FIELD EXPERIMENTS: INTERACTIONS BETWEEN LOCAL AND BROAD‐SCALE PROCESSES

Linking the results of localized field experiments to generalities about the role of specific processes is essential in ecology. Comparative studies conducted at multiple locations enable the general importance of processes to be assessed. However, spatial or temporal variation in the strength of local ecological relationships frequently makes it difficult to draw general conclusions, as increasing the extent of a study is likely to increase the physical and biological heterogeneity. To unravel the influence of differences in wave climate on local ecological interactions among adult and juvenile sandflat bivalves, an experiment was conducted at three sites in each of three harbors in the North Island of New Zealand. The selected sites covered a range of wind-wave exposures but were all mid-intertidal sandflats with macrobenthic communities dominated by bivalves. Four treat- ments were used: additions of 0, 15, 45, and 110 adults of the tellinid bivalve Macomona liliana. At each site a DOBIE wave gauge was used to provide time-series data on wave orbital speed at the seabed over the 35-d duration of the experiment. Significant experiment x location interactions indicated site-dependent variation in the strength and direction of treatment effects. However, multiple regression models based on site environmental char- acteristics were very successful in explaining differences between sites in the strength of experimental treatment effects (66-99% of the variance explained). We used the cube of the orbital wave velocity at the seabed (U3) as an index of energy dissipation by waves. Both the site average and maximum U3 were important variables explaining location- dependent treatment effects. Average U} increased the strength of the negative effects of experimental additions of adult Macomona on juvenile bivalves, presumably by increasing the opportunity for juvenile bivalves to be resuspended by small waves and transported away from areas with high adult densities. Maximum U3 decreased the strength of the experimental effects, probably by increasing the purely passive transport of juveniles with sediment bedload and thus obliterating patterns in the distribution of juvenile bivalves relative to adult Macomona. Hence, different aspects of the wave climate influenced adult- juvenile interactions in different ways. Ambient density of adult Macomona around the experimental sites was also an important factor. This multisite experiment confirmed the importance of local adult-juvenile interactions and suggested ways in which these inter- actions are influenced by local environmental characteristics. Where the influence of such broad-scale variables can be identified, linking broad-scale information to small-scale stud- ies that identify mechanisms can increase the generality of ecological experiments.

Review of wave‐driven sediment resuspension and transport in estuaries

Waves are fundamentally important to the physical and biological functioning of estuaries. Understanding and predicting contaminant transport, development of sedimentary structures, geomorphological response to changes in external forcings such as rising sea level, and response of estuarine ecosystems to contaminant stressors require understanding of the relative roles of wave- and current-driven sediment transport. We review wave-driven sediment resuspension and transport in estuaries, including generation of bed shear stress by waves, initiation of sediment motion by waves, and the ways waves modulate, add to, and interact with sediment transport driven by currents. A key characteristic of the wave-induced force on the seabed is extreme spatial and temporal variations; simple analytical models are revealing of the way such patterns develop. Statistical methods have been widely applied to predict wave resuspension of intertidal-flat bed sediments, and physically based predictors of resuspension developed from open-coast studies appear to also apply to short-period estuarine waves. There is ample experimental evidence to conclude that over the long term, waves erode and tidal currents accrete intertidal flats. Waves indirectly add to the formation of fluid mud by adding to the estuarine pool of fine sediment, and waves may fluidize subtidal seabeds, changing bed erodibility. Models have been used to explore the dynamic balance between sediment transport by waves and by currents and have revealed the key control of waves on estuarine morphology. Estuarine intertidal flats are excellent natural laboratories that offer opportunities for working on a number of fundamental problems in sediment transport.

Control of estuarine sediment dynamics by interactions between currents and waves at several scales

Abstract The effect of interactions between continuous (tidal currents) and intermittent (waves) processes on sediment dynamics and transport is addressed by presenting detailed field measurements of waves, boundary-layer currents and suspended sediment from an estuarine channel and an adjacent intertidal sandflat in Manukau Harbour, New Zealand. The aim is to determine in what ways it is necessary to couple waves and currents in numerical models, and thereby put limits on the fundamental structure of process-based estuarine sediment transport models. Waves were important on the intertidal flat: turbidity switched on and off with the appearance and disappearance of waves; wave groups dominated entrainment of bed sediment; a wave-current boundary-layer model explained measured bed shear stress and hydraulic roughness; and the measured near-bed time-averaged suspended-sediment concentration was mostly well predicted by a pure-wave model. Both the waves themselves and wave-related processes varied markedly over the tidal cycle. The variation in the former (principally changes in wave height) was related to changes in fetch caused by the harbour-wide emergence and submergence of intertidal regions. The variation in the latter was related to changes in water depth relative to the wavelength of the waves, which controlled the penetration to the bed of wave-orbital currents. In addition to the variation over the tidal cycle in the ‘intensity’ of wave processes, there was also a change in ‘kind’, which occurred with the arrival at the measurement site of the ‘turbid fringe’, which is the narrow, highly turbid edge of the estuarine water body. The relationship between suspended-sediment concentration and wave-orbital velocity in the turbid fringe was radically different to the relationship in the estuarine water body proper, which suggests a change in dynamics, perhaps related to breaking waves. A ‘hybrid’ modelling approach is required, i.e. one that treats discrete events but resolves tidal-cycle-scale variation within the event. There is a need to resolve the variation in the wave train over the tidal cycle and the penetration to the bed of wave-orbital motions, both of which could only be done adequately within an estuary tidal model. In contrast to the situation on the intertidal flat where waves intermittently entrained sediment, sediment transport in the channel was continuous, driven by tidal currents. To predict sediment flux in the channel we need to know the upstream sediment-transporting capacity of the flow (including that contributed by waves), the character of the bed sediment, and the sediment-settling time scale. These factors confounded even the simplest notion of flood and ebb dominance, which frequently has been applied to understand estuarine morphodynamics.

Seabed drag coefficient over natural beds of horse mussels (Atrina zelandica)

Measurements of seabed drag coefficient, C 100 , were made under tidal currents at four sites in Mahurangi Harbour, New Zealand. At the first three sites the dominant roughness element was the pinnid bivalve, Atrina zelandica (horse mussel). At the fourth site, which was devoid of horse mussels but covered in cockle shells, patches of seaweed and crab burrows, C 100 was smallest (0.0055), but still twice as large as the value typically applied to abiotic, flat, cohesionless seabeds (0.0025). The mean drag coefficient plus-or-minus standard error at the three sites with horse mussels was: 0.0082 ± 0.0010 (site 1); 0.0096 ± 0.0009 (site 2); 0.0115 ± 0.0016 (site 3). There were no clear differences amongst sites 1, 2 and 3 in terms of the attributes of individual horse mussels (e.g. shell height, width or orientation), which could have been used to explain the ranking of the drag coefficients. There were, however, differences amongst the three sites in terms of spatial distribution of individual bivalves. The site with the highest density of horse mussels, site 1, had the lowest drag coefficient and an areal concentration (λ) of horse mussels higher than typical values cited for the critical concentration (λ c ) for the onset of skimming flow over various idealized, three-dimensional roughness elements. At sites 2 and 3, the drag coefficient was given by: Formula math. which was valid for λ < λ c , where K is von Karmans constant, k is the horse mussel height (i.e., protrusion above the seabed), m 100 and λ c 0.2. The stable eddies that are hypothesized to lodge between roughness elements at concentrations greater than λ c may influence benthic community dynamics.

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

Suspended-sediment reference concentration under waves: field observations and critical analysis of two predictive models

Two commonly adopted but fundamentally different approaches for predicting time-averaged suspended-sediment reference concentration (CREF) under waves are tested against field measurements and compared with each other. The first model relates CREF to the cube of the non-dimensional skin friction, whereas the second model adopts a more complex function of excess skin friction incorporating the empirical constant γ0. The dataset is from the zone of wave shoaling seaward of an open-coast surfzone and includes measurements of waves, currents, suspended sediment and bedforms. Estimates of CREF are derived from acoustic backscatter data, and the seabed and suspension process are described from video footage. When waves were energetic, the bed was deformed into large hummocks; during less energetic conditions, the bed was rippled. The time-averaged concentration profiles over the ripples were consistent with settling flux balanced by pure gradient diffusion and a sediment diffusivity that is constant with elevation above the bed. CREF in that case is shown to apply at z=0, where z is the elevation above the bed. Over the hummocks, there was a sheet flow at the base of the suspension and CREF is shown to apply at z=1 cm. The concentration profiles over the hummocks implied sediment diffusivity that varied linearly with elevation within ∼10 cm of the bed and constant sediment diffusivity above that level. For both rippled and hummocky beds, γ0 derived from the field data was found to be sensitive to the value assumed for critical stress for initiation of sediment motion, which could explain the range of values reported in the literature for γ0. γ0 was also found to vary in a complex way with skin friction, which suggests that the reference-concentration model based on excess skin friction is not correctly formulated. Nevertheless, two functions for γ0 (one applying to rippled beds and the other to hummocky beds) were contrived to make the model fit the data. The model based on non-dimensional skin friction was found to be a good predictor of CREF when a correction was made for flow contraction over ripples. The correction was not required for the hummocky bed, where sediment was being entrained in a thin sheet flow layer. The model based on non-dimensional skin friction correctly portrayed the relationship between flow and sediment response without contrivance and therefore should be the favoured approach in predicting reference concentration.

Feedbacks between bivalve density, flow, and suspended sediment concentration on patch stable states.

We explore the role of biophysical feedbacks occurring at the patch scale (spatial scale of tens of meters) that influence bivalve physiological condition and affect patch stability by developing a numerical model for the pinnid bivalve, Atrina zelandica, in cohesive sediments. Simulated feedbacks involve bivalve density, flow conditions (assumed to be primarily influenced by local water depth and peak current speed), suspended sediment concentration (evaluated through a balance between background concentration, deposition, and erosion), and changes in the physiology of Atrina derived from empirical study. The model demonstrates that high bivalve density can lead to skimming flow and to a concomitant decrease in resuspension that will affect suspended sediment concentration over the patch directly feeding back on bivalve physiology. Consequently, for a given flow and background suspended sediment load, the stability of a patch directly depends on the size and density of bivalves in the patch. Although under a range of conditions patch stability is ensured independently of bivalve density, simulations clearly indicate that sudden changes in bivalve density or suspended sediment concentration can substantially affect patch structure and lead to different stable states. The model highlights the role of interactions between organisms, flow, and broader scale environmental conditions in providing a mechanistic explanation for the patchy occurrence of benthic suspension feeders.

Sorted bed forms as self‐organized patterns: 1. Model development

[1]xa0Here we present a modified version of the exploratory numerical model originally presented by Murray and Thieler (2004) and use it to further investigate the development of sorted bed forms on the inner continental shelf. The new version of the model is based on widely accepted parameterizations for hydrodynamics and sediment transport that are more detailed and empirically tested than the parameterizations used in the original model. The shallow water wave equations are replaced by the general equations, and changes in water depth related to morphological development are accounted for. This model reproduces the main results of the previous one, supporting the hypothesis that sorted bed forms are a self-organized pattern and develop as a result of a coupling between bed composition and sediment flux. The sensitivity of the model predictions, evaluated in terms of the geometry of the sorted bed forms and their growth rate, with respect to external forcing variables and internal model parameterizations, shows increasing (decreasing) sorted bed form spacing (height) with increasing wave height and period. The opposite occurs for increasing water depths. More complicated behavior is observed when the ratio fine-to-coarse sediment diameter (or the percentage of fine and coarse material) is varied. The shape of the suspended sediment and current profiles and the formulation used to predict the geometry of wave-generated ripples can significantly affect (or even inhibit) growth rates of sorted bed forms. Results from a sensitivity analysis show that there are deficiencies in the description of small-scale processes that limit the ability to quantitatively predict sorted bed form evolution, highlighting gaps in our knowledge of sediment transport over mixed grain beds that need to be filled.

Processes driving estuary infilling by marine sands on an embayed coast

Abstract Detailed measurements of waves, currents and sediment transport from an intertidal sandflat at the mouth of a partially filled New Zealand estuary in an embayment with low littoral drift are presented with a view to establishing how marine sands are transported into the estuary. Waves mobilised sediments during episodic events. Early in events, when waves were energetic, sediments were in motion throughout the tidal cycle. Under abating waves, sediment motion became intermittent, finally becoming restricted to periods around low tide, when wave-orbital motions were able to penetrate to the bed. Three components of the total sediment flux were estimated from the data and analysed. (1) A wind-driven current, which presumably has a complicated spatial expression in the vicinity of the estuary mouth, controlled the net (i.e. integrated over event) direction of flux of suspended sediment entrained by waves and advected by currents. There was no evidence of non-symmetry over the tidal cycle in the suspension process that might drive a net sediment flux; however, there is some suggestion that lag in growth and decay of the estuary wave field over the tidal cycle systematically enhances offshore transport. (2) Net bedload transport by waves and currents was directed onshore, which was guaranteed by two mechanisms. Firstly, increasing wave-orbital-current skewness as the tide ebbed turned bedload transport onshore late in the ebb tide against the current and, secondly, bedload peaked early in the flood tide when skewed waves were reinforced by the flooding current. (3) Correlation between seaward-directed forced mean flow and enhanced suspension under groups of high waves resulted in persistent offshore flux of suspended sediment at infragravity frequencies. Suspended-sediment flux carried by gravity waves was directed seawards on flood tides (and counter to the waves) and onshore on ebb tides (and with the waves), which was explained in terms of a simple vortex-ejection model. We conclude that sediment transport on the sandflat at the mouth of this partially filled estuary is governed by subtle interactions between waves and currents that vary over the tidal cycle. Some interactions, such as those that drive bedload transport, promote estuary infilling by marine sands, and others, such as those that drive suspended-sediment flux at infragravity frequencies, do the opposite.