ArticlePublisher preview available

Shelf–oceanic dynamics of surface environmental parameters in the Kangaroo Island–Bonney Coast region

Marine and Freshwater Research

November 2020
72(5):679-692

DOI:10.1071/MF20100

Authors:

Dahlia Foo

Australian National University

Clive R McMahon

Sydney Institute of Marine Science

Mark A Hindell

University of Tasmania

Simon D Goldsworthy

South Australian Research and Development Institute

The shelf and oceanic waters of the Kangaroo Island–Bonney Coast region are important foraging habitats for top marine predators in the ecosystem; however, the dynamics between the two distinct water types have not been investigated. This study examined the spatial and temporal variability of oceanographic parameters in the southern waters of Australia (36–43°S, 136–141°E) associated with the Bonney Upwelling (shelf) and subtropical front (STF; oceanic). Using satellite data from 1997 to 2016, we found that productive oceanic waters were associated with the STF and eddy activity; they were generally furthest from the shelf break in spring–summer (upwelling season on the shelf) and closest to the shelf break in winter–autumn (downwelling season on the shelf). Inter-annual variabilities of chlorophyll-a concentration (Chl-a), sea-surface temperature and sea surface-height anomaly were generally higher in summer than in winter for both shelf and oceanic waters. El Niño–Southern Oscillation, Southern Annular Mode and Indian Ocean Dipole were cross-correlated with anomalous shelf and oceanic Chl-a at various lagged times (range = 15–0 months). This study provides a regional perspective of the spatial and temporal oceanographic variability in southern Australian waters, which may help with understanding apex-predator ecology in the ecosystem.

Map showing the Great Australian Bight region in southern Australia. The red box represents the study region. Kangaroo Island is just above the red box, which is represented in Fig. 2. Major coastal currents in the region during winter are shown. FC, Flinders Current; LC, Leeuwin Current. The mean position of the subtropical front is represented by the dashed white line.

…

Monthly climatology plots for (a) chlorophyll-a anomaly (log10(Chl-a), mg m3), (b) sea surface-height anomaly (SSHA, m), and (c) sea surface-temperature anomaly (SSTA, 8C). Sea surface-current anomaly (represented by arrows, with length indicating the magnitude) are also shown in (a) Climatology means were calculated from either monthly or 5-daily (sea-surface currents) time series from 1997 to 2016. The dashed line represents the 2000-m isobath. The shaded area represents the subtropical front (14–14.58C SST in January to April; 12– 12.58C SST in May to December), and the contour lines in (b) and (c) represent the standard deviation for the entire time series. BC; Bonney Coast where the Bonney Upwelling occurs.

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Average monthly cumulative alongshore wind stress. Positive values show the cumulative effect of long periods of upwelling-favourable winds. The whiskers of the boxplots extending from the box represent the greatest and lowest values within 1.5 times of the inter-quartile range, and black points represent values outside that range. The upper and lower boundaries of the box represent the 75th and 25th percentiles. The black line within the box represents the median. The dashed line represents the zero y-intercept to distinguish between upwelling-favourable and non- upwelling-favourable winds stress.

…

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Shelf–oceanic dynamics of surface environmental

parameters in the Kangaroo Island–Bonney Coast region

Dahlia Foo

,Clive McMahon

,Mark Hindell

and Simon Goldsworthy

Institute for Marine and Antarctic Studies, University of Tasmania, 20 Castray Esplanade,

Battery Point, Tas. 7004, Australia.

Sydney Institute of Marine Science, 19 Chowder Bay Road, Mosman 2088 NSW, Australia.

South Australian Research and Development Institute (Aquatic Sciences), 2 Hamra Avenue,

West Beach, SA 5024, Australia.

Corresponding author. Email: dahlia.foo@utas.edu.au

Abstract. The shelf and oceanic waters of the Kangaroo Island–Bonney Coast region are important foraging habitats for

top marine predators in the ecosystem; however, the dynamics between the two distinct water types have not been

investigated. This study examined the spatial and temporal variability of oceanographic parameters in the southern waters

of Australia (36–438S, 136–1418E) associated with the Bonney Upwelling (shelf) and subtropical front (STF; oceanic).

Using satellite data from 1997 to 2016, we found that productive oceanic waters were associated with the STF and eddy

activity; they were generally furthest from the shelf break in spring–summer (upwelling season on the shelf) and closest to

the shelf break in winter–autumn (downwelling season on the shelf). Inter-annual variabilities of chlorophyll-a

concentration (Chl-a), sea-surface temperature and sea surface-height anomaly were generally higher in summer than

in winter for both shelf and oceanic waters. El Nin

˜o–Southern Oscillation, Southern Annular Mode and Indian Ocean

Dipole were cross-correlated with anomalous shelf and oceanic Chl-aat various lagged times (range ¼15–0 months). This

study provides a regional perspective of the spatial and temporal oceanographic variability in southern Australian waters,

which may help with understanding apex-predator ecology in the ecosystem.

Keywords: ecology, marine mammals, oceanography, pelagic zone.

Received 4 April 2020, accepted 15 October 2020, published online 26 November 2020

Introduction

The World’s continental shelves, slope edges and mesoscale oce-

anic features, such as eddies and fronts, are important sources of

food for top marine predators (Bost et al. 2009;Rogers et al. 2015).

As the aquatic environment is highly dynamic, the productivity of

these features vary seasonally (Behrenfeld et al. 2001;Bender et al.

2016) and inter-annually (Demarcq et al. 2003;Legaard and

Thomas 2007). This variability in the productivity of the local

environment may influence the distribution and abundance of mid-

trophic species, the foraging success of marine predators (Blanchet

et al. 2015) and, ultimately, their fitness (Oosthuizen et al. 2016).

For example, little penguins from south-eastern Australia increase

their foraging effort with lower sea-surface temperature in the local

region (Berlincourt and Arnould 2015). Several species of seals

concentrate foraging in areas with a greater sea surface-

temperature variability, a potential proxy for long-term produc-

tivity (Bradshaw et al. 2004). Therefore, understanding how

environmental conditions change at various temporal scales is an

important step in understanding how the physical and biological

processes underpin the prey base that supports marine predators.

The Bonney Upwelling is one of the most prominent and

predictable upwelling centres in south-eastern Australia and it is

part of the eastern Great Australian Bight (GAB) ecosystem

(Fig. 1,2). The Bonney Upwelling and other upwelling centres

in the eastern GAB are important drivers of phytoplankton

growth, feeding marine animals in the region (Butler et al.

2002). Several marine species in the region, such as seabirds

(Angel et al. 2015;Berlincourt and Arnould 2015), fishes

(Rogers et al. 2015), whales (Butler et al. 2002) and seals

(Page et al. 2006;Lowther and Goldsworthy 2011), are known

to feed at or near the Bonney Upwelling area; it is also a

productive fishing ground for rock lobster (Butler et al. 2002;

Goldsworthy et al. 2013).

The Bonney Upwelling occurs on the narrow shelves of the

Bonney Coast where its seasonal upwelling cycle begins in the

austral summer and extends to late autumn (November–April).

Enhanced primary production from upwelling is greatest in

March (Nieblas et al. 2009). The Bonney Upwelling is predomi-

nantly a wind-driven system (Butler et al. 2002) where the

upwelling season is characterised by westward shelf currents,

and south-easterly coastal winds along the Bonney Coast (Robe,

South Australia to Portland, Victoria; Fig. 2a;Middleton and Bye

2007). Thus, the Bonney Upwelling plume usually extends north-

west towards the local waters south of Kangaroo Island.

CSIRO PUBLISHING

Marine and Freshwater Research, 2021, 72, 679–692

https://doi.org/10.1071/MF20100

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Shelf–oceanic dynamics of surface environmental parameters in the Kangaroo Island–Bonney Coast region

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