2021 State of the Environment Report Marine Chapter – Expert Assessment – Pressure – Ocean nutrients and dissolved oxygen

Researchers: Chamberlain, Matthew (Author) ,  Emma Flukes (Point of contact) ,  Lenton, Andrew (Author) ,  Matear, Richard (Author)

Brief description The Marine chapter of the 2021 State of the Environment (SoE) report incorporates multiple expert templates developed from streams of marine data. This metadata record describes the Expert Assessment "Pressure of Ocean nutrients and dissolved oxygen". ***A PDF of the full Expert Assessment, including figures and tables (where provided) is downloadable in the "On-line Resources" section of this record as "EXPERT ASSESSMENT 2021 - Pressure – Ocean nutrients and dissolved oxygen"*** ---------------------------------------- DESCRIPTION OF PRESSURE The vital physical processes supplying nutrients to the upper ocean (Figure 1) are the seasonal deepening of the surface ocean mixed layer, the upwelling and vertical mixing associated with ocean eddies, and wind-driven upwelling. In the Australian region, the first two processes dominate because wind-induced upwelling is confined to a few small areas (e.g. Bonnie Upwelling off South Australia). As the climate warms, the upper ocean will develop a stronger vertical gradient in temperature, becoming more stratified, reducing the vertical supply of nutrients to the surface ocean (Kwiatkowski et al., 2020). Climate change is also projected to modify winds, and strengthen boundary currents and increased eddy activity their vertical nutrient supply (e.g. the East Australia Current, Matear et al., 2013). Oxygen is essential to living aerobic organisms in the ocean. In the surface ocean, oxygen levels are high because air-sea exchange keeps oxygen near saturation levels. Due to aerobic respiration, the dissolved oxygen concentrations in the ocean interior decline and are lowest in the intermediate water (300 m to 1000 m) away from regions these waters exchange with the atmosphere (i.e. the Southern Ocean, Figure 2). High oxygen consumption in coastal regions can lower oxygen to a level where it can no longer support aerobic organisms leading to fish kills (Hobbs and McDonald, 2010). Solubility of oxygen is reduced in a warming ocean, and climate change is projected to lead to the expansion of areas with low oxygen (Kwiatkowski et al., 2020). With climate change warming of the ocean, marine heatwaves in Australian waters are projected to increase in frequency and intensity (Hayashida et al., 2020b). Additional pressures exist in coastal ocean regions, such as increased nutrient input (rives and estuary); which reduces oxygen with the potential for local anoxic events and increases nutrients with the increase phytoplankton blooms. DATA STREAM(S) USED IN EXPERT ASSESSMENT Data come from the CARS climatological atlas of nitrate, phosphate and oxygen (1996). Details can be found at http://www.globcolour.info/products_description.html. ---------------------------------------- 2021 SOE ASSESSMENT SUMMARY [see attached Expert Assessment for full details] • 2021 • Assessment grade: High impact Assessment trend: Unclear Confidence grade: Low Confidence trend: Low Comparability: Somewhat comparable (oxygen and nutrients assessed separately in 2016 ---------------------------------------- CHANGES SINCE 2016 SOE ASSESSMENT The seasonal climatologies are based on the latest version of CARS, which brings in additional observations collected since 2016. Additionally, dissolved oxygen and nutrient supply and cycling were previously assessed separately, but have been combined into a single ‘biogeochemistry’ assessment for 2021.

Lineage Statement: QUALITY OF DATA USED IN THE ASSESSMENT Spatial and temporal coverage are poor and the best we can do in the Australia region is to provide a seasonal climatology of these fields.

Lineage

Notes Credit
Peer reviews of this assessment were provided by: Mark Holzer (UNSW) Tom Trull (IMAS, UTAS)

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