Technical Development of the
Australian National Reference Stations
By David Hughes
Coastal Moorings Team Leader
Dr. Tim Lynch
Australian National Mooring Facility Leader
Commonwealth Scientific and Industrial Research
The Australian marine and climate research community was consulted over several years in order to create a multidisciplinary observing system. The formalization of this process began in 2007 with the creation of the Integrated Marine Observing System (IMOS).
IMOS is organized into nodes and facilities, which respectively steer and implement marine research infrastructure. These include a blue-water node, focusing primarily on research off Australia’s continental shelf, and five regional nodes distributed around the country. The deployment of a wide range of IMOS sensors and sampling technology, as well as the subsequent delivery of data to researchers, is coordinated by 11 facilities.
The Australian National Mooring Network (ANMN) is one of these facilities and is further organizationally divided into a number of subfacilities, one of which is the National Reference Stations (NRS), an operational continental-scale network of coastal study sites.
Each site, which is linked to centralized processing and information systems, includes both in-situ sensors and water sampling to measure the physical and biological properties of the surrounding water column. The geographical scope of this research is sparsely distributed around the entire coastline of Australia.
The network now consists of nine sites. These build on three long-term monitoring sites at Maria Island (off Tasmania), Port Hacking (off Sydney in New South Wales) and Rottnest Island (off Perth in Western Australia) that have been sampled since the 1940s. The historical water sampling from these establishment sites has been extended to all nine sites to collect biogeochemical data and truth the on-site instrumentation.
Management of the NRS is divided between four Australian marine institutes: the Australian Institute of Marine Science (AIMS), a federal government science institute; the Sydney Institute of Marine Science (SIMS), a consortium of universities and state government bodies; the South Australian Research and Development Institute (SARDI), a state-based research group; and the Commonwealth Science and Industrial Research Organisation (CSIRO), a federal government science organization.
CSIRO, in addition to managing four of the stations, has four laboratories for the analysis of zooplankton, phytoplankton, hydrochemistry and carbon samples collected at the NRS.
Reference Station Instrumentation
Technical deployment of the various NRS moorings has continuously evolved. Elements of the network have been designed to better suit temperate or tropical waters, sand to rock substrate, sheltered to exposed water and spatial isolation from servicing infrastructure. These conditions have also influenced the frequency of mooring services required.
The core instrumentation of the NRS locations are Water Quality Monitors (WQM) designed by WET Labs Inc. (Philomath, Oregon) and Sea-Bird Electronics Inc. (Bellevue, Washington). This instrument has six data streams: conductivity, temperature, depth, dissolved oxygen, turbidity and fluorescent proxies for chlorophyll.
Capability of the mooring infrastructure has steadily increased with the addition of extra instruments to a number of the NRS sites. These include the Batelle (Columbus, Ohio) pCO2 sensors, Aanderaa Data Instruments (Bergen, Norway) Oxygen Optodes and Sea-Bird Electronics’ 16plus SEACAT sensors and SAMI2-pH sensors, which will be used to enhance the study of ocean acidification and acoustic Doppler current profilers (ADCPs) for local determination of current. Near-real-time telemetry has also been integrated into several sites, allowing constant monitoring of data streams. This has enabled a number of positive technical flow-on effects, including the opportunity to monitor sensor health on the moorings and the ability to insure data against instrument failure. As telemetered data are captured, this avoids the potential for six months or more of stored data being lost in the event of an instrument flooding or mooring failure.
Future instrument additions include the WET Labs’ ECO Triplet B, which will be used to collect data in order to verify satellite remote-sensed ocean color products.
Project Areas Details
Maria Island. Maria Island is managed by CSIRO. Regular physical water sampling has been carried out on this site since the 1940s and is now carried out monthly. This station was instrumented in April 2008 with two WQMs, one at 20 meters and one close to the bottom in 90 meters of water. This subsurface mooring is also used as a test bed for new instrumentation. In April 2009, a separate surface mooring was added, and the subsurface mooring was modified with the addition of acoustic modems to each of the WQMs, allowing the system to provide near-real-time telemetry of the WQM data. A weather station, a near-sea-surface temperature sensor, a motion reference unit (to measure significant wave height) and an acoustic modem were incorporated into the surface float along with independent GPS tracker to act as a beacon if the surface float broke free or the main electronics failed. In December 2010, a GPS module was added to the float and integrated into the main data stream due to issues with the GPS tracker reliability. Once the new unit is confirmed to be working, the tracker unit will be decommissioned. All of these data streams are now telemetered.
In April 2010, wireless capability was added to the surface float. Up to this point, a tether cable was used to communicate with the float to carry out final checks such as acoustic path tests to the subsurface mooring. This tether complicated the deployment operation and added additional operational risks. To reduce this risk, a self-contained wireless module, molded in ultraviolet-stable urethane, was developed to plug into the top of the float. This system also enables float diagnostics or firmware upgrades to be carried out without having to recover the mooring or be physically connected to the float. At the same time, an acidification mooring was established on site. This mooring carries an oxygen optode, a Batelle pCO2 sensor and a Sea-Bird 16plus sensor to provide an independent telemetry stream.
A standalone ADCP lander frame is currently being completed in which an ADCP mounted in a gimballed frame will be deployed in July of this year. The prototype unit was deployed at the North Stradbroke Island NRS last December.
The various moorings have been structurally improved in response to identified wear and failure inspections during rotation. One of these modifications was an increase in the number of bungee tethers on the surface float from one to two and eventually to three bungee tethers, increasing the mooring’s stiffness and mitigating against a single-bungee failure. The subsurface float was also made more buoyant so the acoustic release frame still floats to the surface when released, even if heavily fouled. Meanwhile, the anchor weight increased from 300 to 500 kilograms to counterbalance the extra flotation. Finally, the weather station mast height was raised to decrease failures due to large waves. This last change also improved accuracy of wind speed indications. A semiflexible couplings for the weather station mast base has also been developed and deployed, alleviating fatigue cracking of the aluminum mast assembly.
Experimentation in a number of areas allowed the surface mooring servicing period to double from three months to approximately six months. Judicious selection of mounting materials, the addition of cathodic protection through zinc blocks and careful instrument positioning within stainless steel frames has also alleviated crevice corrosion found in earlier deployments of the subsurface mooring. The testing of various anti-fouling coatings has identified optimum types to be used in different locations on the mooring. Additionally, revisions of system firmware code have allowed power optimizations to decrease the draw on the battery.
It is also hoped that telemetry may be expanded to the ADCPs. In addition to providing near-real-time current data, ADCP telemetry would be useful in determining fouling levels on the subsurface mooring infrastructure. This can be achieved by monitoring the pressure levels on the WQMs in concert with the current data from the ADCPs. For a given current, a fouled mooring presents more drag and will be laid over more than a clean mooring. Understanding this relationship between current speed, fouling and mooring layover will allow a better targeted servicing regime.
Kangaroo Island. This mooring is managed by SARDI and is located in 110 meters of water. It consists of two Falmouth Scientific Inc. (Cataumet, Massachusetts) NXIC units with an Aanderaa optode, and WET Labs fluorometry and turbidity sensors sampling hourly. They are mounted at depths of 40 and 100 meters. A chain of 12 Aquatec Group Ltd. (Hampshire, England) pressure-temperature data loggers, sampling at 15-minute intervals, are placed at approximately five-meter intervals between them.
Water sampling and mooring rotation at the NRS is carried out approximately eight times a year as part of a wider five- to six-day field survey, and the water samples are analyzed for nutrients and phytoplankton species and abundance. Moored sensor instrumentation began in August 2008 and sampling in October 2008. To continue this article please click here.
David Hughes managed his own electronics technology design company for 17 years until 2008, when he joined CSIRO. He manages the coastal moorings team and develops electronic systems for moorings infrastructure.
Dr. Tim Lynch is the manager of the ocean sensor deployment group at CSIRO. With a background in marine protected areas, wildlife and fisheries biology, Lynch has been managing multidisciplinary marine research programs for 14 years. His areas of interest as a program leader have now expanded to include marine technology, physical and chemical oceanography as well as plankton biology.