Miles Furnas reported on recent physical observations and phytoplankton dynamics.

He found temperatures in the region rose from about 23 C in Oct., 1997 to 30 C in Feb., ’98. Observations of the Chlorophyll standing crop showed that very small phytoplankton dominated the communities offshelf. Just prior to a Tropical Cyclone very large diatom populations were observed initially. A shift away from diatoms coincided with significant changes in the water mass on the shelf, with some hint of dynamic mesoscale changes on time scales of a few days to a few weeks.

Carbon production was observed to attain 5 gm C/ m^-2/day. This indicates a very productive system comparable to what can be observed in the upwelling systems off Peru, South America.

The larger diatoms are eaten mainly by copepods, while the smaller ones are consumed by bacteria.

Population doublings of 1 or 2 times per day were observed, while nutrients are very low. This implies that the system is running off recycled nutrients. It also suggests that any external nutrient sources will be rapidly taken up. In deed the flora suggests that such external sources do exist.

As a general rule, higher standing crops were observed offshore than on the shelf, while high productivity extends over vast area of the NW shelf.

David McKinnon:

David gave a synopsis of his work on secondary production and zooplankton studies. This work falls between the fisheries work being done by Mark Meekan and Peter Doherty on fishes, pre-settlement reef fish and larvae and the phytoplankton work of Miles Furnas.

The work concerns mainly the ‘packaging’ of food items for larval fishes. The major zooplankton group being studied is marine copepods, which are the most abundant mesozoic animals on the planet.

The studies focus on both microbial and classical food webs with Nitrogen in the form of Ammonia being recycled via the microbial loop and Nitrate being supplied via upwelling systems to diatoms, clupeids, euphausiids and whales. In particular, diatoms and copepods dominate the coastal systems.

Microplankton are particularly important in the food web.

Field studies are focussed on the across-shelf transect mentioned by Peter Doherty with 8 stations A (onshore, in the entrance to Exmouth Gulf) through H on the upper slope. Past and ongoing plankton studies in the North Western Australia include work by CSIRO in the Eastern Indian Ocean, a community study of Shark Bay and AIMS studies of Exmouth Gulf. The latter show that seagrasses and microalgae cannot account for the primary production needed to support the extant prawn fishery.

At Station E the presence of a blue water organism (Oncea) indicates elements of both coastal and offshore systems are present on the shelf. Based on observations before and after a Tropical Cyclone, abundances went up and the result appeared to be more closely related to a shore-based community.

Alan Pearce asked if this community could have come from an alongshore counter current, but David responded that the community looked more like it had been derived from Exmouth Gulf.

Other work has focussed on egg production rates.

What do we know?

• Dynamics are variable
• The system is quite productive
• The most likely source of new Nitrogen in from the deep ocean.
• There is an accumulation of plankton within Exmouth Gulf.

What we need to know?

• Physical factors contributing to Nitrogen inputs
• Forces moving large packets of water
• Fate of pelagic biomass within the Gulf.

Tenshi surveyed the work he has being doing in Exmouth Gulf. He also observed that the system is very productive with large mammals such as whales and dolphins being supported in significant numbers. He also described work undertaken in 1996 on the FRANKLIN and tabled relevant reprints He emphasised the need to understand the variability in Exmouth Gulf and in particular to appreciate the role of horizontal advection and related physical processes.

Presentation and Discussion Notes:

John Parslow talked about his work in developing appropriate algorithms for remote sensing of ocean colour. This work is partly integrated into the efforts of the Australian Ocean Colour Working Group (AOCWG).

The major function of the algorithms is to remove atmospheric effects and estimate the water leaving radiance. To achieve this, both empirical and semi-empirical relationships are investigated.

Many new Ocean Colour missions are expected in the future, most of these being polar orbiting (and hence sun synchronous? –ed). In particular, he discussed access to the SeaWIFS data sets currently being acquired.

He suggested that Plankton are a good tracer for eddy motions, (particularly where SST gradients are too small to provide a useful signal –ed).

He showed 9 km pixel size data around Australia to illustrate this. More eddy structure is evident in the oligotrophic regions. The response is almost logarithmic which leads to good structure in low Chlorophyll situations.

Research needs

Validation:

• Establish accuracy of standard global products
• Fixed measurements for testing algorithms
• More robust atmospheric corrections.

Coastal Waters:

• Distinguishing effects of turbidity, humus, chlorophyll and bottom reflections.
• These combine to mislead the investigator seeking to characterise the offshore Chlorophyll structure.
• The proposed MERIS mission will focus effort on handling ocean colour in coastal waters.
• It will do this by providing better spatial and spectral resolution.
• Specific Absorption Coefficients vary in surface waters from different locations and communities (eg tropical compared with temperate).

Applications:

Biogeochemical models (eg, Ocean General Circulation models)
Fisheries oceanography
Surface chlorophyll dynamics
Feature tracking
Mixed layer heat balance

William Skirving gave an overview of work being done on thermal infra-red remote sensing. He briefly surveyed the available remote sensing platforms:

AVHRR is generally considered to have an accuracy of order 1 C, but this can be significantly degraded in the moist atmosphere of the tropics, particularly during summer.

GMS has 8-bit resolution data in contrast to AVHRR which has 10-bits per pixel, and so the accuracy is limited by quantisation.

ERS ATSR and ENVISAT-1 AATSR have dual scanning angles which lead to better atmospheric attenuation estimates, and hence improved accuracy.

NOAA AVHRR with its 1 km pixel resolution at Nadir, matches the ocean colour sensor resolutions and has an accuracy of 0.5 – 2 deg C.

Problems being addressed in his work include the atmospheric attenuation effects and changes in emissivity due to sea state. Conventional radiometry assumes this is uniform. He estimates that for a 50 deg incidence angle, which is often used in shipboard radiometry, ignoring the effects of ship motion and error as great as 3 C can result from ignoring this effect. However, he found that the effect can be modelled quite accurately to the extent that temperatures can be resolved to 0.1 C.

GMS 6 that will include the Gbar sensor from the GOES series will be launched early in year 2000. The resolution will be 10 bits, with a 4km pixel size for temperature and thermal images will be acquired hourly. It will have an accuracy of 0.5 to 2 deg C. Cloud winds can also be derived from this system that should lead to more accurate SST estimates.

William also briefly discussed the implementation of systems for feature tracking (eg Emery et al.) that have been gradually undergoing refinement. These may be used to estimate surface drift currents under appropriate conditions.