David R. Plew

Hydrodynamic implications of large offshore mussel farms

Around 3000 Ha of New Zealands coastal waters are presently allocated to mussel farming of the Greenshell mussel Perna canaliculus mainly in small inshore farms of 30 000 to 80 000 m/sup 2/ each. The farms are constructed from blocks of parallel moored long-lines, with loops of spat-impregnated line hanging vertically in the upper 10-15 m of the water column (provided sufficient water depth). When fully laden, the mussel loops can present a rough diameter of 0.20 m or more to the flow. Industry expansion is likely to be in the form of larger offshore farms. Some of the larger proposed farms will have more than 1000 km of mussel line. This presents a potentially significant drag to currents and waves. In this paper, observations are used from one of the first large offshore farms constructed in New Zealand, in conjunction with scaling estimates of energy loss, to investigate the effect of the farm on waves, currents, and stratification. The farm, consisting of 230 long-lines arranged into 20 blocks, measured 650 m /spl times/ 2450 m. The hanging mussel loops at this site reached an average depth of 8 m, and water depth at midtide varied across the site from 10-12 m. Transects were taken through the farm using both conductivity, temperature, depth (CTD), and Acoustic Doppler Profiler (ADP) instruments. A second ADP was moored inside the farm providing velocity time series. The average current was reduced within the farm by 36%-63%. The moored ADP detected an undercurrent beneath the farm with velocities nearly twice that within the farm. Sharp changes in stratification coincide with the start of fully stocked mussel long-lines. Furthermore, other than at the very upstream end of the farm, stratification appears to have prevented significant vertical mixing between the fluid within the farm and that flowing under the farm. Wave energy was low during the 7-d deployment, with significant wave heights peaking at 0.25 m. The attenuation of transitional waves through the farm is examined by comparing simultaneous pressure sensor measurements from offshore and inshore of the farm. The observations show that wave energy attenuation was frequency dependent. Observed wave energy attenuation across the 650 m wide farm was approximately 5%, 10%, and 17% at wave frequencies of 0.1, 0.2, and 0.25 Hz, respectively.

Interactions between the mat-forming alga Didymosphenia geminata and its hydrodynamic environment

Benthic autotrophs in oligotrophic rivers must adapt to and modify their hydrodynamic environment to balance the conflicting requirements of minimal drag (to minimize detachment risks) and maximal exposure to turbulent flow (to maximize nutrient acquisition). We explored flow–organism interactions using the benthic, freshwater alga Didymosphenia geminata. D. geminata forms large mats in swift, oligotrophic alluvial rivers. The physical properties that allow D. geminata to resist detachment and proliferate under these harsh conditions are unknown. We transplanted cobbles with attached D. geminata mats from a riverbed to a flume and used velocimetry and microelectrode profiling to measure hydrodynamic and transport conditions above and within the mats over a wide range of flows. We then removed the mats from the cobbles and repeated the velocimetry measurements. Experiment results indicated that D. geminata mats reduce form-induced stresses and near-bed turbulent velocity fluctuations, which may reduce the risk of detachment. D. geminata mats also increase turbulent shear stress just above mat surfaces, which may enhance water column–mat solute exchange. High friction associated with flow at mat surfaces leads to very low velocities and predominantly diffusive transport within mats, which may in turn favor the retention of solutes derived from organic matter within and below mats. Enhanced mass transfer at mat surfaces and effective solute retention in mat matrices suggest a mechanism by which D. geminata cells acquire nutrients from different sources: advection-dominated transport of water-column nutrients to cells at mat surfaces, and diffusion-dominated transport from decomposing organic matter within mats, with minimal advective losses.

Aquatic Macrophytes Alter Metabolism and Nutrient Cycling in Lowland Streams

Macrophytes influence the physical, chemical, and biological characteristics of lowland streams, so may be critically important in stream management. We investigated the role of macrophytes in regulating metabolism and nutrient cycling in three lowland, agricultural streams. We measured stream metabolism over the growing season and following experimental macrophyte removal, and used short-term nutrient additions of phosphate (P) and ammonium to assess macrophyte influences on nutrient uptake. Primary production was closely correlated with macrophyte cover across all streams and dates, and decreased greatly with macrophyte removal, whereas ecosystem respiration was not correlated with macrophyte cover and was not altered by macrophyte removal. Phosphate uptake velocity was negatively related to primary production, suggesting that macrophyte activity actually slowed P uptake. Ammonium uptake was not correlated with macrophyte cover or metabolism metrics. Stream nitrate concentrations typically exceeded concentrations of incoming groundwater, suggesting little net nitrate retention in these macrophyte-dominated streams. Phosphorous demand by macrophytes was 10-fold lower than observed uptake rates, indicating that macrophyte P demand was much lower than that of other stream biota. Nitrogen demand by macrophytes was nearly equal to ammonium uptake and was not sufficient to affect the high nitrate flux. These results indicate that macrophytes drive ecosystem metabolism but have limited influence on water column nutrient concentrations because macrophyte demand is much lower than the supply available from the water column. Thus macrophytes in our streams had a large impact on stream trophic state, but offered little potential to influence nutrient removal via management.

Fish swimming speed variability at constant flow: Galaxias maculatus

Abstract Digital videography was used to study the swimming performance of the adult fish Galaxias maculatus in a flume at fixed flows. Instantaneous fish swimming velocities varied about the mean flume velocity and these variations in swimming speed were found to increase as velocity in the flume increased. Standard deviations in swimming velocity measured in the longitudinal direction increased from SD 0.183 m s–1 at U0 = 0.40 m s–1, SD 0.191 at U0 = 0.48 m s–1 to SD 0.237 m s–1 at U0 = 0.60 m s–1. Increases in standard deviations were also seen in cross‐stream and vertical swimming velocities. Similarly, variability in acceleration increased as flume velocity was increased with SD 1.850 m s–2 at U0 = 0.40 m s‐1, SD 2.492 m s–2 at U0 = 0.48 m s–1, and SD 3.125 m s–2 at U0 = 0.60 m s–1 recorded for the longitudinal acceleration component. Maximum swimming velocities of 2.05 m s–1 and accelerations of 25.4 m s–2 were recorded.

Parameterizing suspended canopy effects in a three-dimensional hydrodynamic model

ABSTRACT This paper presents an amendment of an existing three-dimensional hydro-environmental model (Environmental Fluid Dynamics Code) to incorporate effects of suspended canopies on the vertical flow structure. Five different modelling approaches are investigated, encompassing hydrodynamic form drag imparted by the suspended canopy, an amended two-equation turbulence scheme representing turbulence generated locally by elements within the canopy, and three separate formulations for vertical profiles of drag coefficients. Data from laboratory experiments with rigid cylinders are used to validate the calculations of velocity and shear stresses. The results show that the most accurate reproduction of the canopy flow was obtained using a vertically varying drag coefficient along with a two-equation turbulence closure scheme that includes additional turbulence production and dissipation terms. The numerical model reproduced velocity profiles accurately, but the shear stresses are slightly overestimated.

Changes to flow and turbulence caused by different concentrations of fish in a circular tank

ABSTRACT The effects of juvenile Atlantic salmon (Salmo salar) on flow and turbulence in a circular tank were investigated. Three fish sizes were studied (37.5 g, 82.5 g and 218 g) with between 268,050 and 318,000 fish in a 15 m diameter by 4 m deep tank (mean stocking densities of 15.3 kg m–3, 35.6 kg m–3, and 79.4 kg m–3). Flow in the enclosed tank was driven by the inflow from the water supply system and a degassing system. Velocities were measured using acoustic Doppler velocimetry with and without fish present. Dissolved oxygen was also measured, and the turbulent transport of dissolved oxygen calculated from eddy correlation. The average water velocity was reduced by 15% at low and medium stocking densities, and 57% at high stocking density. Turbulent kinetic energy, turbulence intensity, and turbulence dissipation rates were higher with fish than without. Fish altered the distributions of mean velocity, turbulence and oxygen, and increased the turbulent transport of oxygen. Vertical distributions of turbulence were consistent with echo-sounder derived fish distributions.

Drag of Clean and Fouled Net Panels – Measurements and Parameterization of Fouling

Biofouling is a serious problem in marine aquaculture and it has a number of negative impacts including increased forces on aquaculture structures and reduced water exchange across nets. This in turn affects the behavior of fish cages in waves and currents and has an impact on the water volume and quality inside net pens. Even though these negative effects are acknowledged by the research community and governmental institutions, there is limited knowledge about fouling related effects on the flow past nets, and more detailed investigations distinguishing between different fouling types have been called for. This study evaluates the effect of hydroids, an important fouling organism in Norwegian aquaculture, on the forces acting on net panels. Drag forces on clean and fouled nets were measured in a flume tank, and net solidity including effect of fouling were determined using image analysis. The relationship between net solidity and drag was assessed, and it was found that a solidity increase due to hydroids caused less additional drag than a similar increase caused by change in clean net parameters. For solidities tested in this study, the difference in drag force increase could be as high as 43% between fouled and clean nets with same solidity. The relationship between solidity and drag force is well described by exponential functions for clean as well as for fouled nets. A method is proposed to parameterize the effect of fouling in terms of an increase in net solidity. This allows existing numerical methods developed for clean nets to be used to model the effects of biofouling on nets. Measurements with other types of fouling can be added to build a database on effects of the accumulation of different fouling organisms on aquaculture nets.

A national-scale GIS-based system for modelling impacts of land use on water quality

Management of freshwater quality requires modelling tools for rapid evaluation of land use and management scenarios. This paper presents the CLUES (Catchment Land Use and Environmental Sustainability) model to address this need. CLUES provides steady state, spatially distributed, integrated catchment models tightly coupled to GIS software to predict mean annual loads of total nitrogen, total phosphorus, sediments and E.coli, and concentration of nutrients throughout New Zealand (268,000km2) with a subcatchment resolution of 0.5km2. CLUES also estimates potential nutrient concentrations for estuaries and provides key farm socio-economic indicators. The model includes a user interface for study area selection, scenario creation, data geo-visualisation, and export of results. It is pre-populated with spatial data and parameter values for New Zealand. Evaluation of the model and a summary of applications demonstrate the tractability and utility of national-scale rapid scenario assessment tools within a GIS framework. Display Omitted Stream water quality prediction at mean annual time-scale throughout New Zealand at kilometre-scale resolution.Rapid scenario generation and evaluation of land use change and management measures (mitigation factors, intensification).Includes links to an estuary nutrient component and simple socio-economic indicators.Has a growing number of applications for catchment, regional and national scenario assessment.

Using Simple Dilution Models to Predict New Zealand Estuarine Water Quality

A tool based on simple dilution models is developed to predict potential nutrient concentrations and flushing times for New Zealand estuaries. Potential nutrient concentrations are the concentrations that would occur in the absence of nutrient uptake or losses through biogeochemical processes, and so represent the pressure on a system due to nutrient loading. The dilution modelling approach gives a single time- and space-averaged concentration as a function of flow and nutrient input, with the capability to include seasonal nutrient and flow differences. This tool is intended to be used to identify estuaries likely to be highly sensitive to current nutrient loads based on their physical attributes, or to quickly compare the effects of different land-use scenarios on estuaries. The dilution modelling approach is applied both to a case study of a single New Zealand estuary, and used in a New Zealand-wide assessment of 415 estuaries. For the NZ-wide assessment, annual nutrient loads to each estuary were obtained from a GIS-based land-use model. Comparison with measured data shows that the predicted potential nitrate concentrations are significantly correlated with, but higher than, measured nitrate values from water quality sampling time series. This is consistent with expectations given that the measured concentrations include the effects of nitrogen uptake and loss. The estuary dilution modelling approach is currently incorporated into the GIS-land use model, and is also available as a web-app for assessing eutrophication susceptibility of New Zealand estuaries.

Calibration of a 3D hydrodynamic aquaculture model

Aquaculture structures comprising suspended canopies extending from the free surface to a point above the sediment are represented as porous blockages in a hydrodynamic surface-water model. These canopies alter flow conditions and material transport within and around the structure, especially between the bottom of the canopy and the sediment bed. These altered flow conditions can have significant implications for sediment, nutrient, and waste transport in the environments in which aquaculture structures are placed. This study uses an augmented version of the Environmental Fluid Dynamic Code to model flow conditions in and around a suspended canopy calibrated against data obtained from a series of flume tests. The parameter estimation code, PEST, was used to calibrate various model parameters including horizontal momentum diffusivity, vertical eddy viscosity, turbulence closure constants, and depth-dependent drag coefficients to ensure that model simulations match the experimental (calibration) data comprising vertical velocity and turbulent kinetic energy profiles. The model was then validated against vertical profiles of turbulent kinetic energy production rate, which were not used during the calibration process. Ultimately, trends of increasing average drag coefficient with decreasing canopy blockage ratio and increasing average flume flow speed were observed. The calibrated canopy-turbulence parameters may yield improved predictions of the hydrodynamic and material transport conditions resulting from the various aquaculture structures and flow systems. In turn, these predictions will help develop methods to minimize environmental impacts and to increase production from aquaculture farms.