Abstract
The region where the East Australian Current (EAC) separates from the coast is dynamic and the shelf circulation is impacted by the interplay of the western boundary current and its eddy field with the coastal ocean. This interaction can drive upwelling, retention or export. Hence understanding the connection between offshore waters and the inner shelf is needed as it influences the productivity potential of valuable coastal rocky reefs. Near urban centres, artificial reefs enhance fishing opportunities in coastal waters, however these reefs are located without consideration of the productivity potential of adjacent waters. Here we identify three dominant modes of mesoscale circulation in the EAC separation region (~31.5−34.5°S); the ‘EAC mode’ which dominates the flow in the poleward direction, and two eddy modes, the ‘EAC eddy mode’ and the ‘Eddy dipole mode’, which are determined by the configuration of a cyclonic and anticyclonic eddy and the relationship with the separated EAC jet. We use a Lagrangian approach to reveal the transport pathways across the shelf to understand the impact of the mesoscale circulation modes and to explore the productivity potential of the coastal waters. We investigate the origin (position and depth) of the water that arrives at the inner-mid shelf over a 21-day period (the plankton productivity timescale). We show that the proportion of water that is upwelled from below the euphotic zone varies spatially, and with each mesoscale circulation mode. Additionally, shelf transport timescales and pathways are also impacted by the mesoscale circulation. The highest proportion of upwelling (70%) occurs upstream of 32.5°S, associated with the EAC jet separation, with vertical displacements of 70–120 m. From 33 to 33.5°S, water comes from offshore above the euphotic layer, and shelf transport timescales are longest. The region of highest retention over the inner shelf is immediately downstream of the EAC separation region. The position of the EAC jet and the location of the cyclonic eddy determines the variability in shelf-ocean interactions and the productivity of shelf waters. These results are useful for understanding productivity of temperate rocky reefs in general and specifically for fisheries enhancements along an increasingly urbanised coast.
Highlights
Production on the continental shelf—and the proximity to ports and markets—is why more than 90% of the global fisheries catch is harvested from continental shelves (Pauly et al, 2002)
The second is the ‘East Australian Current (EAC) eddy mode’ where the EAC separates in the north of our domain, and a cyclonic eddy is over the shelf
The origin and transport pathways of water flowing across the inner-mid shelf of the Hawkesbury Bioregion off SE Australia was examined from Lagrangian backtracking simulations to reveal the connection between mesoscale circulation and shelf transport
Summary
Production on the continental shelf—and the proximity to ports and markets—is why more than 90% of the global fisheries catch is harvested from continental shelves (Pauly et al, 2002). The valuable coastal rocky reefs support a productive habitat which relies on the delivery of plankton to the reef (Holland et al, 2020). Phytoplankton and zooplankton growth responds when deep nutrient rich water is uplifted (Brink, 2016) over timescales of 7–21 days (Mongin et al, 2011; Rocha et al, 2019). Zooplankton production advected from upstream of a coastal reef provides a food source for vast schools of small forage fish (Truong et al, 2017; Morais and Bellwood, 2019). Understanding the physical oceanographic setting, including the source of water reaching the inner shelf and coastal reefs, the shelf transport pathways, residence times and their temporal variability is necessary for understanding the productivity and sustainability of temperate rocky reefs
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