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Exploring the slope/offshore Great Australian Bight (GAB) pelagic habitat paradox

Rudy KloserCSIRO

Rudy KloserTim RyanAndy RevillAdrian FlynnCaroline SuttonLisa Gershwin

Paul van RuthMark DoubellNicole PattenRyan DownieAnthony Richardson

Faculty of Edit this on the Slide Master The University of Adelaide

Pelagic Habitat – context and focus on offshore central GAB - southern sub-tropical convergence zone –Longhurst (2010) – moderate satellite derived NPP


Pelagic ChallengeTo study the microbial, planktonic and micronekton communities of the central and eastern GAB to see when and where deep-water pelagic food sources are plentiful, and what this means for the wider community.

NutrientsSize classesDiversityBiomassProduction Energy transfer

Trophic connections


At the start of the project: Central GAB had proposed year round down welling - low production?– hypothesis to test?

Expected lower biomassSmaller size rangesPicoplankton high

Expected higher biomasLarger size rangePicoplankton low


Historic and new acoustic data shows a paradox of high pelagic fish biomass in the proposed low production central GAB –

How supported?

Simple echo gram data useful to highlightlarge amount of schooling fish in predicted low production downwelling region


Seismic vessel data provided insights into epi-mesopelagic schools observed in the region –5 month survey 2014-15 – IMOS BASOOP


Continuous plankton recorder (cpr) 2010 to 2016 IMOS CPR• Provides large spatial scale and

within and between year temporal sampling – 294 segments.

• Highlights seasonal patterns with autumn -winter peaks and yearly trends.

• Key functional species differences between regions

• Offshore “larvaceans” provide a production shunt from the more microbial food web.

• Abundance of euphausiids highest in offshore waters


Mid-trophic ecosystem dynamics-zooplankton and micronekton – nets, acoustics and optics


The micronekton – fish, crustaceans, squids and gelatinous ~2-20 cm


Standardised micronektonnet biomass

Note high gelatinous biomass in offshore waters, highest in the central region


Regional biomass comparison.

Density of micronekton in the epipelagic and mesopelagic depths (g.1000m-3)

Southern Tasmania4.60 ± 5.52

Great Australian Bight2.45 ± 3.53

Southern Tasman Abyssal Basin2.64 ± 1.86

3.06 ± 1.90

Western Tasmania2.18 ± 3.04

Northern Tasman2.73 ± 1.77 Biogeography (fish)

Affinities with subtropical convergence and south Tasman Sea


Novel sampling - Oblique camera provides a gelatinous census a major component of the mesopelagic ecosystem

Sample volume ~ 1 m^3


Differences in food webs - first use of Amino Acid Compound Specific Isotope Analysis

Chikaraishi et al. (2009, L&O Methods 7:740-750)

Glutamate

Phenylalanine

βx/y

Βx/y = 3.4

∆x

∆x = 8.0

∆y

∆y = 0.4

TEF = 7.6

Average source δ15N maintained

Clear differences in euphasiid source nitrogen between east and central GAB

Central indicative of atmospheric nitrogen fixed source

East indicative of upwelled/higher latitude water


New knowledge of structure and functioning of the system – based on project


• This was the first detailed study of the central GAB offshore upper-slope and oceanic pelagic ecosystem that characterised the biota from sizes of bacteria to tuna.

• Acoustic data showed a large number of fish schools in the central GAB offshore that implies a production source to support them.

• New production pathways were uncovered that highlighted the role of biological process (nutrient recycling and nitrogen fixation)

• New tools and methods developed of acoustics, optics, and compound specific isotope analysis

• Provides a robust baseline and archive of historic data (acoustics CPR IMOS) for future studies and developed new methods for future monitoring

Key findings and impacts

AcknowledgementsCSIRO O&A Engineering and technology programIMOS BASOOPPGS and CSIRO MNF

Faculty of Edit this on the Title Master

Open Water Research

Paul van Ruth Rudy Kloser SARDI CSIRO

Rudy KloserTim RyanAndy RevillAdrian FlynnCaroline SuttonLisa Gershwin

Paul van RuthMark DoubellNicole PattenRyan DownieAnthony Richardson


• Results revealed intermittent high productivity in the eastern Great Australian Bight, driven by sporadic but intense summer upwelling which brings nutrients into well-lit surface waters, and underpins an efficient “classic” food web.

• Importantly this study showed that this eastern production was highest at depth and not evident from satellite remote sensing.

• We uncovered a pelagic habitat paradox of the central slope Bight whereby large numbers of micronekton were found to occur in what was thought to be a nutrient poor region. The discovery of a new, biologically driven nutrient pathway in the region goes some way to resolving this paradox.

• Knowledge of production pathways and its variability is important to understand food availability to other consumers in the system and to manage human uses of the region.

• Key Findings and Implications


Unique seismic vessel data collected over 5 months with 12 and 38 kHz– collected through BASOOP IMOS

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