13 April 2004

Tuesday, 13 April 2004

Get more go on moorings

Sometimes the God of the Sea, Tangaroa, plays a vital role in deciding the milestones and outcomes of oceanographic voyages. Yesterday was a case in point with strong westerly winds and a heavy, rough sea, which prevented us from recovering and re-deploying one of NIWA’s two deep-ocean biophysical time-series moorings. These have been deployed in the open ocean east of New Zealand over the last 3.5 years. This was the first time we haven’t been able to service them which is remarkable considering the sometimes hostile environment we are working in. The northern mooring is located about 300 km southeast of Napier in subtropical waters, while the southern mooring is situated on the southern slope of the Bounty Trough about 700 km due east of Dunedin in the subantarctic zone.

Both of the steel wire moorings stretch from near the sea-surface to the seafloor at water depths of about 3 km. They comprise various instruments placed at specific depths to collect information on biological, chemical and physical parameters throughout the water column. The instruments can be deployed on the moorings for periods of months to years. Near the sea surface, fluorometers and light meters provide data on plant biomass and productivity, and temperature and salinity loggers record changes in the physical properties of the water over time. Current meters measure the speed and direction of currents and help us understand the dynamics of water masses in the area. These data provide valuable information on how physical and biological processes are coupled in surface waters and, specifically, help us to determine the seasonality of marine algal (phytoplankton) production in the oceans around New Zealand. Generally, production peaks in spring in subtropical waters and in summer in subantarctic waters.

Once nutrient levels are depleted to an extent that phytoplankton production rates cannot be sustained, the particles sink out of the surface ocean. These sinking particles are captured by a sediment trap deployed on the mooring at a depth of 1500 metres. This instrument collects particles in a large funnel under which a set of bottles are rotated sequentially to allow a time-series of particle “rain” rates to be estimated.

Combined with information of particle production rates in surface waters, as determined from the near-surface fluorometers, these sediment trap data allow us to determine how quickly organic material is decomposed during its descent through the water column. This information means that we can construct inventories of important elements, like carbon and silica, in the ocean and make estimates of how much carbon is being sequestered into the deep ocean and is available as food for deep-ocean mid-water column and benthic communities, including economically important animals, such as deep-sea fish.

We are now heading north with the expectation of a more successful recovery and re-deployment of the northern mooring.

Scott Nodder (NIWA)

Approaching storm.

The two mooring sites lie in quite different ocean types: sub-Antarctic water in the south, and subtropical water in the north.

Instruments on the moorings.

Research subject: Oceans