Gliding through marine heatwaves: subsurface biogeochemical characteristics on the Australian continental shelf

Abstract

Marine heatwaves (MHWs) disrupt ecosystems across multiple trophic levels by altering oxygen and biological productivity through the water column and yet, most studies focus on the surface, overlooking subsurface processes that shape ecosystem responses. To address this gap, we analysed 16 years of routine and event-based glider observations on the continental shelf around Australia to present the first comprehensive assessment of the subsurface biogeochemical response during surface MHWs across four contrasting coastal regions. Across all regions and seasons, the distribution of chlorophyll concentrations shifted towards a decline in the mixed layer and an increase below the mixed layer during MHWs, modulated by the event categories. Dissolved oxygen shows a more complex distribution, which also varies during moderate and strong MHW events, arguably with more variation in the mixed layer than below. When regional and seasonal specificities are taken into account, the subsurface characteristics of MHWs vary in accordance with the environmental setting, including the continental shelf structure, tropical or sub-tropical regime, and boundary current influence, especially through the changes in stratification. Summer surface MHWs were characterised by a shallower mixed layer depth than normal conditions and enhanced stratification, confining warming to the upper ocean, while other seasons allow deeper penetration under weakly stratified conditions. The depth of maximum stratification therefore emerged as a useful proxy for the vertical extent of MHWs. The interaction between physical processes, such as seasonal circulation and stratification, and biological feedback, including the presence of deep chlorophyll maxima and potential oxygen production, highlights the complex biogeochemical responses to MHWs, and underscores the importance of region-specific dynamics and the need for more consistent observation strategy, including biogeochemical processes.