Derek G. Goring

SUBSIDY AND STRESS RESPONSES OF STREAM PERIPHYTON TO GRADIENTS IN WATER VELOCITY AS A FUNCTION OF COMMUNITY GROWTH FORM

Previous studies have shown major differences in the way biomass of stream periphyton is controlled by spatial variations in velocity. We hypothesize that these differences may be the result of different growth forms within the community. Some dense and coherent growth forms (e.g. mucilaginous diatom/cyanobacterial mats) may be resistant to diffusion and also resistant to dislodgment by shear stress. Higher velocities applied to such communities could therefore be expected to enhance biomass accrual by increasing rates of mass transfer, but without greatly increasing losses through sloughing. Conversely, other growth forms (e.g. long filamentous green algae) have an open matrix, and high rates of diffusion into the mats can potentially occur even at low velocity. However, as velocities increase, high skin friction and form drag should lead to higher rates of sloughing. The overall result of these processes should be that maximum biomass occurs at low velocities. This “subsidy‐stress” hypothesis was tested twice with each of three different periphytal growth forms: a coherent, mucilaginous, diatom community; a moderately coherent, stalked/ short, filamentous diatom community; and an open‐weave, long, filamentous green algal community. A monotonic increase in chl a biomass occurred as a function of near‐bed velocities for the first of the two mucilaginous diatom communities investigated. No biomass‐velocity relationship was found, however, with the second mucilaginous community, probably because the waters were highly enriched and mass transfer driven by molecular diffusion was probably high throughout the velocity gradient. Biomass was moderate at low velocities, peaked at near‐bed velocities from 0.18 to 0.2 m·s−1 (∼0.40–0.45 m·s−1 mean column velocity), and then decreased at higher velocities in both of the stalked/ short filament communities of diatoms analyzed. With the long filamentous green algal communities, a monotonic reduction in biomass occurred as a function of increases in velocity. Proliferations greater than 100 mg·m−2 chl a occurred at low near‐bed velocities (i.e. <0.2 m·s−1), after which biomass declined nearly exponentially as a function of increasing velocity to less than 10 mg·m−2 chl a at velocities greater than 0.4 m·s−1. These biomass‐velocity trends support our hypothesis that community growth form determines periphytal responses to spatial variations in velocity within stream reaches.

On gravel-bed roughness characterization

The random field approach for gravel-bed roughness characterization, which is based on the presentation of bed elevations as a three-dimensional random field, is justified as an alternative to the characteristic particle size approach. We first show that the bed elevation distribution is close to Gaussian and then investigate gravel-bed roughness using the second-order structure function. The latter reveals two distinct regions: a scaling region at small spatial lags and a saturation region at large scales. The scaling exponent H (a form of Hurst exponent) appears to be isotropic and universal for both manually created “unworked” gravel beds (H=0.5) and natural water-worked gravel beds (H=0.79). However, the gravel-bed roughness, in general, is not isotropic and should be characterized by three independent characteristic scales. A simple model of gravel-bed roughness based on the structure function parameterization is developed and compared with the characteristic particle size approach.

Ocean tides around New Zealand

Abstract The distribution of amplitude and phase for eight ocean tidal constituents (M2, S2, N2, K2, K1, O1, P1, Q1) is presented as tidal maps for the New Zealand area. The distribution was calculated using a barotropic tidal model driven by TOPEX/ Poseidon data on the outer ocean boundaries. The maps exhibit the known features of the tides in this area such as a complete rotation of the semi‐diurnal tides around New Zealand and the reduced spring‐neap variations on the east coast. They also point out several new features for which there are few or no observations, such as diurnal trapped waves and shelf waves. A comparison of the model results with observations shows that sea level errors are within 0.1 m in amplitude and 10° in phase for the largest constituents at all locations, including sites where the data are of low quality and where the geometry is not adequately resolved. For locations where the geometry is adequately represented and the observations are of high quality, sea level errors are within 0.02 m in amplitude and 7° in phase. These results represent the most accurate and highest resolution calculations of tides and currents yet attained for this area.

Some observations of the effects of micro-organisms growing on the bed of an open channel on the turbulence properties

In this paper we report the results of an experimental study of periphyton–flow interactions conducted in a specially designed outdoor hydraulic flume. ‘Periphyton’ is a collective term for the micro-organisms which grow on stream beds, and includes algae, bacteria, and fungi, with algae usually the dominant and most conspicuous component. The main goals of the study are to identify potential effects of periphyton–flow interactions as well as the potential mechanisms of mass transfer in the near-bed region, which could influence periphyton growth and losses. The main results of the study may be summarized as follows. A linear velocity distribution in the interfacial sublayer (i.e. below the roughness tops), and a logarithmic distribution above the roughness tops appeared to be reasonable approximations for both flow types, with and without periphyton on the bed. However, the appearance of periphyton on a rough bed shifts the origin of the bed upwards, increases the roughness length z o by 16–21%, and reduces the ratio of the mean velocity at the level of roughness tops to the shear velocity by ≈30%. In general, below the roughness tops the periphyton suppresses the mean velocities, the turbulent stresses, turbulence intensities, and vertical turbulent fluxes of the turbulent energy and turbulent shear stresses. It was found that in flows without periphyton large-scale eddies successfully penetrate the interfacial sublayer. However, tufts of periphyton on the tops of the roughness elements significantly weaken the penetration processes leading to spatial de-correlation in the velocity field within the interfacial sublayer. The appearance of periphyton on the bed does not change appreciably the velocity spectra above the roughness tops but reduces the total spectral energy and generates a wide spectral peak in the interfacial sublayer. Most probably, this peak is formed by penetration of sweep events into the interfacial sublayer, ‘filtered’ by the periphyton tufts. Thus, sweep events may be the main mechanism responsible for the delivery of nutrients from the outer region to the biologically active interfacial sublayer. The potential effects of flow properties on the periphyton community are also discussed.

Ice Shelf Elevation Changes due to Atmospheric Pressure Variations

The inverse barometer effect (IBE) is the isostatic response of ocean surface height to changes in atmospheric pressure (Pair) at a rate of about 1 cm per hecto-Pascal. The IBE is a significant contributor to variability of ice shelf surface elevation (ηice), as we demonstrate with simultaneous Global Positioning System measurements of ηice and local measurements of Pair from the Amery, Brunt, and Ross Ice Shelves in Antarctica. We find that an IBE correction is justified for frequencies (ω) covering the “weather-band”, 0.03<ω<0.5 cycles per day. The IBE correction reduces the standard deviation of the weather band signal of ηice from ~9 cm to ~3 cm. With this correction, the largest remaining high-frequency error signal in ηice is the inaccuracy of the present generation of Antarctic tide models, estimated to be of order 10 cm for most of Antarctica.

Diffusion of saltating particles in unidirectional water flow over a rough granular bed

We show that the motion of saltating particles on a flat rough bed in unidirectional water flow is diffusive and comprises three ranges (local, intermediate, and global) of spatial and temporal scales with different scaling behaviour and diffusion properties. Our computer simulations suggest that the ratio of the travelling particle diameter to the prevailing diameter of static bed particles (or the height of bed roughness) is one of the key parameters controlling particle diffusion.

On stream periphyton‐turbulence interactions

Abstract A set of experiments was carried out to determine what effects periphyton communities could have on near‐bed hydraulic fields. We analysed velocity distribution, skewness and kurtosis coefficients, Reynolds stresses, relative turbulence intensity, coefficient of eddy diffusivity, velocity spectra, and turbulence scales at two flows with, and without, diatom‐dominated periphyton on the bed. We found that turbulence was affected by the periphyton mat selectively. The largest influence occurred for the velocity distribution, Reynolds stress, coefficient of eddy diffusivity, and velocity cross‐spectra. Changes as a result of the periphyton mat were revealed in the large‐scale turbulence structure. The periphyton affected not only the region near the bottom but also the entire logarithmic layer. The periphyton mat increased roughness length by a factor of ≍5 whereas the integral resistance to flow (reciprocal Chezy coefficient and Mannings roughness coefficient) was increased by 20–25%. The microscal...

Observed and modelled tidal currents in the New Zealand region

Abstract Tidal currents derived from current meter measurements are compared with the output from a barotropic tidal model of the New Zealand region. For the semi‐diurnal constituents there was very good agreement for the M2 tide and good agreement for the S2 tide. For the diurnal constituents (Kl, Ol) it was found that as the amplitude of the constituents decreased so did both the model/observation agreement and the accuracy of the observed tidal ellipse parameters. Consequently it was not possible to decide whether differences arose through shortcomings in the model or in the data. However, the overall performance of the model as a prognostic tool for ocean tidal current simulation appears to be good.

El Niño and decadal effects on sea‐level variability in northern New Zealand: A wavelet analysis

Abstract Sea‐level data from two sites in northern New Zealand, along with the Southern Oscillation Index (SOI), are analysed for interannual and decadal variability using wavelets. The analysis shows, using statistically significant wavelet power, there is a significant relationship between mean sea level (MSL) and SOI. However, the relationship is highly variable, both in magnitude and in the range of time‐scales over which it occurs. This non‐stationarity necessitates the use of techniques such as wavelets for analysis. An interdecadal response in MSL around northern New Zealand has been isolated, with shifts occurring in 1950 and the late 1970s. This behaviour in MSL appears to coincide with shifts in the Pacific Decadal Oscillation, thought previously to be largely centred in the North Pacific. A strong correlation between SOI and sea surface temperature (SST) is also demonstrated. This relationship appears to be stable in magnitude (a large change in SOI produces a large change in SST) and to occur ...

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.