Research > Core research projects > CORE PROJECT DESCRIPTION Research: Core research projects Project one: Biophysical Oceanography off Western Australia: Dynamics across the Continental Shelf and Slope DOWNLOAD FULL PROJECT DESCRIPTION [PDF 3.5mb] Investigators CSIRO Marine Research, Floreat, WA: Tony Koslow (Project Leader), Alan Pearce, Nick Mortimer, Joanna Strzelecki, Peter Fearns, Christine Hanson CSIRO Marine Research, Hobart, TAS: Lesley Clementson, Rudy Kloser, Tim Ryan University of Western Australia: Harriet Paterson, Stéphane Pesant, Anya Waite Murdoch University: Barbara Muhling, Lynnath Beckley Curtin University: Leon Majewski Department of Environment: Ashrafi Begum Executive summary The aim of the SRFME Biophysical Oceanography project is to characterize the continental shelf/slope pelagic ecosystem off southwestern WA: its productivity and dynamics, and the physical, chemical and biological factors driving variability along dominant spatial and temporal scales. To achieve this, we undertook monthly sampling from 2002 – 2004 along an onshore-offshore transect off Two Rocks from nearshore to the outer continental shelf (100 m water depth), which we extended quarterly to offshore waters (1000 m depth). Cruise sampling was combined with satellite observations of sea-surface temperature (SST), ocean colour and altimetry, and subsurface measurements of currents and temperature from moorings. The program involved six core research components: 1) The physical structure and nutrient dynamics within the water column 2) Phytoplankton community composition, biomass and productivity 3) Microzooplankton communities and their grazing dynamics 4) Mesozooplankton communities and their grazing dynamics 5) Ichthyoplankton community composition and ecology 6) Spatial structure of zooplankton and micronekton communities Standard sampling protocols included: meteorological (wind, air temperature) observations from coastal stations; vertical CTD (conductivity-temperature-depth) casts with concurrent measurement of in situ fluorescence, dissolved oxygen and subsurface irradiance; discrete water column samples for analysis of salinity, dissolved nutrients (nitrate+nitrite, ammonium, phosphate and silicate), chlorophyll a, HPLC pigments, particulate organic carbon, phytoplankton and microzooplankton species composition and abundance, primary production, and microzooplankton grazing; sediment trap deployments for measurement of vertical carbon fluxes; bongo net samples for mesozooplankton biomass, species composition, grazing rates and secondary production; low-frequency acoustic transects, and high-frequency acoustic vertical profiles in combination with targeted water column zooplankton sampling. Summer conditions on the shelf and offshore were characterized by a shallow upper mixed layer, with a strong thermocline and well stratified water column. Surface waters were nitrate-depleted and generally contained low phytoplankton biomass levels (< 0.2 mg m-3), overlying a deep chlorophyll maximum (DCM) located between the 0.1% and 1.0% light levels. The DCM was frequently associated with a deep nitracline (≥ 100 m water depth). In contrast, in late autumn and winter, the upper mixed layer deepened and stratification generally weakened, leading to shoaling of the nitracline and DCM layer and increased phytoplankton biomass. Satellite observations indicate that the late autumn/early winter bloom is a coherent feature from approximately the Abrolhos Islands to Cape Leeuwin and coincides with intensification of the Leeuwin Current, leading us to hypothesize that enhanced meander and eddy activity may stimulate upwelling or convective mixing. Phytoplankton biomass and production integrated over the water column was higher offshore, although maximum volumetric chlorophyll concentrations were generally observed inshore. Annual phytoplankton production over the study period was 46 g C.m-2.yr -1 inshore and about 115 g C.m -2.yr -1 on the shelf and offshore – relatively oligotrophic for a coastal environment. Not unexpectedly, more biomass and production was in the small size phytoplankton size fraction (< 5 um). Distinct phytoplankton assemblages were observed on the inner shelf and further offshore, and between summer and winter. The outer shelf and offshore stations were characterised by high prochlorophyte and unicellular cyanobacteria populations, while small flagellates and periodic diatom blooms dominated inshore waters. Small haptophytes were ubiquitous. Zooplankton biomass was also generally greatest in late autumn and winter. The assemblages differed significantly in nearshore and shelf/offshore waters and between winter and other seasons, following patterns among species groups observed elsewhere in coastal waters. Experiments have been carried out to assess zooplankton secondary production based on copepod egg production, and the results are being compared with a new enzyme assay and simple models based primarily on body size and temperature. A three-frequency (70, 120 and 200 kHz) acoustic system was used to sample along the onshore-offshore transect during quarterly cruises. Mixing frequencies appears promising as a means to separate major groups, and to assess relationships between topography and water mass features and the broad-scale distribution of large zooplankton and nekton. A 6-frequency, high frequency (256 kHz – 3 megaHz) acoustic system (TAPS) was used in conjunction with an in situ pump sampler to assess the vertical distribution of zooplankton. Initial results are promising, showing good agreement between acoustic and pump sample profiles of zooplankton abundance. The project also integrates two postgraduate projects, reported elsewhere, characterizing microzooplankton and ichthyoplankton assemblages in the region; the role of microzooplankton grazing is also being assessed. The Biophysical Project is working closely with biogeochemical modelers to develop a regional biophysical model that nests the region’s hydrodynamics and its nutrient-phytoplankton-zooplankton dynamics. Over the next year, analyses will be completed and results written up for publication in refereed scientific journals. Plans are being made for a follow-up project to develop regional oceanographic monitoring of the biological environment, based on developing algorithms relating satellite ocean colour, SST and altimetry to primary productivity and possibly nutrients. A proposal has been submitted for ship-time on the RV Southern Surveyor for a synoptic cruise in late autumn/early winter to test hypotheses linking Leeuwin Current and phytoplankton bloom dynamics; the cruise would also examine regional coherence in oceanographic processes during this dynamic time of year. Plans are also being made to develop a project off Ningaloo Reef to examine nutrient-plankton dynamics and the exchange between the reef and pelagic environments. back to top [core research projects]

 

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