Ian Ashton

Measurement of the Effect of Power Absorption in the Lee of a Wave Energy Converter

Monitoring the effect of floating wave energy converter (WEC) devices on the surrounding wave field will be an important tool for monitoring impacts on the local wave climate and coastlines. Measurement will be hampered by the natural variability of ocean waves and the complex response of WEC devices, causing temporal and spatial variability in the effects. Measurements taken during wave tank tests at MARINTEK are used to analyse the effectiveness of point wave measurements at resolving the influence of an array of WEC on the local wave conditions. The variability of waves is measured in front and in the lee of a device, using spectral analysis to identify changes to the incident wave field due to the operating WEC. The power capture and radiation damping are analysed in order to predict the measured changes. Differences in the wave field across the device are clearly observable in the frequency domain. However, they do not unanimously show a reduction in wave energy in the lee of a device and are not well predicted by measured power capture.Copyright

A Sensitivity Analysis of the Impact of Rain on Regional and Global Sea-Air Fluxes of CO2

The global oceans are considered a major sink of atmospheric carbon dioxide (CO2). Rain is known to alter the physical and chemical conditions at the sea surface, and thus influence the transfer of CO2 between the ocean and atmosphere. It can influence gas exchange through enhanced gas transfer velocity, the direct export of carbon from the atmosphere to the ocean, by altering the sea skin temperature, and through surface layer dilution. However, to date, very few studies quantifying these effects on global net sea-air fluxes exist. Here, we include terms for the enhanced gas transfer velocity and the direct export of carbon in calculations of the global net sea-air fluxes, using a 7-year time series of monthly global climate quality satellite remote sensing observations, model and in-situ data. The use of a non-linear relationship between the effects of rain and wind significantly reduces the estimated impact of rain-induced surface turbulence on the rate of sea-air gas transfer, when compared to a linear relationship. Nevertheless, globally, the rain enhanced gas transfer and rain induced direct export increase the estimated annual oceanic integrated net sink of CO2 by up to 6%. Regionally, the variations can be larger, with rain increasing the estimated annual net sink in the Pacific Ocean by up to 15% and altering monthly net flux by > ± 50%. Based on these analyses, the impacts of rain should be included in the uncertainty analysis of studies that estimate net sea-air fluxes of CO2 as the rain can have a considerable impact, dependent upon the region and timescale.

Design and operation of a low-cost and compact autonomous buoy system for use in coastal aquaculture and water quality monitoring

The need to ensure future food security and issues of varying estuarine water quality is driving the expansion of aquaculture into near-shore coastal waters. It is prudent to fully evaluate new or proposed aquaculture sites, prior to any substantial financial investment in infrastructure and staffing. Measurements of water temperature, salinity and dissolved oxygen can be used to gain insight into the physical, chemical and biological water quality conditions within a farm site, towards identifying its suitability for farming, both for the stock species of interest and for assessing the potential risk from harmful or toxic algae. The latter can cause closure of shellfish harvesting. Unfortunately, commercial scientific monitoring systems can be cost prohibitive for small organisations and companies to purchase and operate. Here we describe the design, construction and deployment of a low cost (<£ 5000) monitoring buoy suitable for use within a near-shore aquaculture farm or bathing waters. The mooring includes a suite of sensors designed for supporting and understanding variations in near-shore physical, chemical and biological water quality. The system has been designed so that it can be operated and maintained by non-scientific staff, whilst still providing good quality scientific data. Data collected from two deployments totalling 14 months, one in a coastal bay location, another in an estuary, have illustrated the robust design and provided insight into the suitability of these sites for aquaculture and the potential occurrence of a toxin causing algae (Dinophysis spp.). The instruments maintained good accuracy during the deployments when compared to independent in situ measurements (e.g. RMSE 0.13–0.16 °C, bias 0.03–0.08 °C) enabling stratification and biological features to be identified, along with confirming that the waters were suitable for mussel (Mytilus spp.) and lobster (Homarus gammarus) aquaculture, whilst sites showed conditions agreeable for Dinophysis spp.

Reduced air–sea CO 2 exchange in the Atlantic Ocean due to biological surfactants

Ocean CO2 uptake accounts for 20–40% of the post-industrial sink for anthropogenic CO2. The uptake rate is the product of the CO2 interfacial concentration gradient and its transfer velocity, which is controlled by spatial and temporal variability in near-surface turbulence. This variability complicates CO2 flux estimates and in large part reflects variable sea surface microlayer enrichments in biologically derived surfactants that cause turbulence suppression. Here we present a direct estimate of this surfactant effect on CO2 exchange at the ocean basin scale, with derived relationships between its transfer velocity determined experimentally and total surfactant activity for Atlantic Ocean surface seawaters. We found up to 32% reduction in CO2 exchange relative to surfactant-free water. Applying a relationship between sea surface temperature and total surfactant activity to our results gives monthly estimates of spatially resolved ‘surfactant suppression’ of CO2 exchange. Large areas of reduced CO2 uptake resulted, notably around 20° N, and the magnitude of the Atlantic Ocean CO2 sink for 2014 was decreased by 9%. This direct quantification of the surfactant effect on CO2 uptake at the ocean basin scale offers a framework for further refining estimates of air–sea gas exchange up to the global scale.Biologically produced surfactants in the sea surface microlayer reduce air–sea exchange of CO2 in the North Atlantic Ocean, according to tank and ocean measurements.