The Bonney Upwelling

 

BONNEY UPWELLING

A clear explanation of this impressive natural phenomenon is provided at the bluewhalestudy site:
     "Driven by wind, the process of upwelling draws deep, nutrient-rich, cold water upwards toward the ocean's surface, replacing the warmer, usually nutrient-depleted, surface water. The nutrients in upwelled water are derived from marine organisms (both plant and animal) dying and sinking to the ocean floor. These nutrients are most abundant near coasts and river outlets, but may be conveyed by currents great distances along the ocean floor to be upwelled far from their source.
     When upwelled nutrients meet sunlight near the surface, minute phytoplankton (plant-like cells) 'bloom', turning the ocean green and providing a vital food source for a range of animals from krill (a type of zooplankton) to small schooling fish. These feed larger animals including rock lobster, giant crabs, fish (including commercial species), squid, seabirds, seals, dolphins and whales.
     Upwelling events may last from hours to weeks and are followed by 'relaxation' periods as winds calm or blow from other directions. The right balance between upwelling and relaxation events is crucial to primary (phytoplankton) production."

The bluewhalestudy site goes on to describe the local manifestation of this process:
      "The Bonney Upwelling, part of the Great Southern Australian Coastal Upwelling System, stretches from Portland, Victoria, past Robe toward Kangaroo Island, South Australia ... [and] is driven by spring-summer winds that blow from the south-east. These winds drive ocean currents to the north-west along the coast and displace surface water offshore. This displaced warm surface water is replaced with cold Antarctic Intermediate Water that has travelled across the floor of the Southern Ocean and onto the shallower continental shelf. The shelf is narrow offshore of the Bonney Coast (about 20 km from shore to continental slope) and each season, from November to May, deep water is funnelled through a series of submarine canyons."

An accompanying sea-temperature map illustrates the Bonney system in its present geographical context:


"The Bonney Upwelling is the large blue 'plume' in the centre of the image, marking the most intense surface upwelling in the large, continuous, cool-water upwelling system reaching from western Tasmania and Bass Strait to the eastern Great Australian Bight. This map has been generated by converting ocean surface temperatures measured by an orbiting satellite with infra-red sensor, into colours that make intuitive sense: the coolest water is shown as dark blue, the warmest as red. Surface upwelling is seen off western Tasmania, along the coast from western Bass Strait to past Robe, S.A., off the western end of Kangaroo Island, and off the west coast of Eyre Peninsula."


BONNEY CANYON

This canyon is one of many deep-sea conduits delivering cold, nutrient-rich water from the depths of the Abyssal Plain (3-5,000 metres) to the continental shelf (2-500 metres) along the Bonney Coast. It was the subject of a partial study (from 100 to 1500 metres) by scientists aboard RV Southern Surveyor in February 2008. An associated contour-line map places the canyon within the context of other 'slope-confined' canyons (abutting but not breaching the 'shelf-break' line that marks the transition to shallower water). 


Bonney Canyon is neither the longest nor the most defined of the submarine structures that cut into the side (or slope) of the continental shelf. Significantly deeper and wider 'shelf-incised' canyons are shown in the south-east, closer to the S.A. / Victoria border and Cape Bridgewater. Bonney is therefore typical of canyons that are not paleo-related to riverine outflows from land, such as the Murray to the northwest or the Glenelg near the border.

Journal articles published after the investigative voyage in 2008 (DR Currie et al, 2012, KE Conlan et al, 2015) provide similar information on the canyon's physical dimensions:
     "Muti-beam soundings of the Bonney Canyon reveal a diffuse entrance at the shelf-break (~200 m depth) and a well defined headwall (or abrupt fall) on the upper slope (~800 m depth). Below the headwall, the canyon is narrow (< 3km wide) and deeply incised (> 1km deep) with steep sidewalls (gradient > 1:1). The floor of the canyon is terraced and bears numerous scallop-shaped scars that are indicative of erosion and slumping.    
    Marked depth-related differences in near-bottom water temperatures were observed around the Bonney Canyon. A band of cool water (9.5-12.1⁰C) characterised the continental shelf inshore from the head of the canyon. Beyond the shelf-break, near-bed water temperatures gradually declined with increasing depth, and reached a uniform minimum (2.7⁰C) both on and either side of the central canyon axis at 1500 m depth."


OTWAY CANYONS

Much commentary on the Bonney Upwelling assumes that it is effectively limited to the coastline from Cape Nelson (Portland, Vic)) westward to Cape Jaffa (Robe, SA). However, two freshly published papers (J Kampf, 2024, D Harishidayat et al, 2024) include some reasons for extending this range eastward to Cape Otway. 

The intuitive argument in favour of a greater zone of nutrient-rich upwelling follows on from recorded whale sightings:
     "Gill et al, 2011, discusses the monthly encounter rates of blue whales in the study region based on six upwelling seasons from November 2002 to May 2007. During this time, between 8 and 78 animals were sighted per up[welling season (November to May) in the main upwelling plume between Cape Nelson and Cape Jaffa, while between 6 and 30 animals were sighted along the continental shelf between Cape Otway and Cape Nelson."


     "On the other hand, the 'windforcing' creates a north westward flow along the entire continental shelf from Cape Otway to Cape Jaffa and beyond ... It should be noted that the continental shelf becomes narrower from Cape Otway to Cape Nelson. This feature supports the funnelling of krill swarms into a narrow, confined stream which makes it relatively easy for whales to obtain food."



The second of the two papers focuses on examining a series of five main submarine canyons situated off the eastern coastline toward Cape Otway. Here the continental shelf shrinks from approximately 250 kms in width to just 80 kms. 























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