2021 State of the Environment Report Marine Chapter – Expert Assessment – State and Trend – Water clarity (turbidity, transparency and colour)

Researchers: Davies, Claire (Author) ,  Doblin, Martina (Author) ,  Doblin, Martina (Author) ,  Dr Jason Everett (Author) ,  Emma Flukes (Point of contact)
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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 "State and Trend of Water clarity (turbidity, transparency and colour)". ***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 - Water Clarity"*** ---------------------------------------- DESCRIPTION OF PROCESS FOR EXPERT ASSESSMENT Marine environments (continental shelf and open ocean) are influenced by regional variation in climate, geomorphology and oceanography, all of which regulate the concentration and nature of the dissolved and particulate materials in seawater. These constituents collectively determine water quality. Australian marine waters are generally low in suspended sediments and colour resulting in relatively deep light penetration that allows pelagic primary producers (phytoplankton) to persist at depths greater than 100 m. Marine ecosystems are adapted to these conditions and therefore any deterioration in water quality (i.e., increased turbidity or decreased optical transparency) threatens key habitat-forming benthic primary producers such as kelps and seagrasses. In oceanic and outer continental shelf waters the major determinant of turbidity, transparency, and colour is the biomass of phytoplankton (Yentsch 1960), with phytoplankton growth largely being driven by the availability of dissolved nutrients. However, water transparency declines strongly toward shore due to increased sources of sediment, nutrients and greater phytoplankton biomass. The northern waters of Australia (Timor and Arafura Seas) have highest suspended sediment (lowest transparency) relative to southern waters, and those in the Coral Sea have greatest transparency (Secchi disk depth; Fig. 1). Water transparency is strongly seasonal, reaching a minimum in spring in the Temperate East and Southeast and in winter in the North, Northwest, Southwest and Coral Sea, due to growth of phytoplankton (as indicated by peaks in chlorophyll-a (Chl-a) concentration; Fig. 1; Thompson et al 2020). Tidal flows and waves contribute to turbidity, but extreme events such as tropical cyclones and storms can increase the level of suspended sediment by up to 3 orders of magnitude (x1000) due to both runoff and bottom disturbance by waves. In addition, tropical rivers in areas of high rainfall can deliver large amounts of sediment to the coastal zone, with plumes sometimes being quite extensive. DATA STREAM(S) USED IN EXPERT ASSESSMENT Water quality data was derived from two sources: i) IMOS National Reference Stations, sampled monthly ii) Globcolour, a time-series of ocean colour data merged from four satellite data sources: MERIS, MODIS, SeaWIFS. Globcolour data was accessed in a bounding box of 110-150 oE and -10:-45 oS over a time period that spanned the last 2 SOE periods, 2011 – 2020. The TSS product was only available from early 2016 onwards. For more information: https://www.globcolour.info/products_description.html Satellite sensors have the appropriate spatial (m to km) and temporal coverage (min to days), to assess water quality but require algorithms to convert satellite remote sensing reflectance into biogeochemical properties. While global algorithms are biased towards validation data collected predominantly in the northern hemisphere, satellite data remain the longest time-series with which to assess the status of marine water quality. These data are complemented by in-situ time series at IMOS National Reference Stations in a limited number of locations around Australia. ---------------------------------------- 2021 SOE ASSESSMENT SUMMARY [see attached Expert Assessment for full details] • 2021 • Assessment grade: Good Assessment trend: Stable Confidence grade: Somewhat adequate Confidence trend: Adequate Comparability: Grade and trend comparable to 2016 assessment • 2016 • Assessment grade: Good Assessment trend: Stable Confidence grade: Adequate high quality evidence and high level of consensus Confidence trend: Adequate high quality evidence and high level of consensus Comparability: Grade and trend are comparable to the 2011 assessment • 2011 • Assessment grade: Very good Assessment trend: Stable Confidence grade: Limited evidence or limited consensus Confidence trend: Limited evidence or limited consensus ---------------------------------------- CHANGES SINCE 2016 SOE ASSESSMENT There was a significant increase in Secchi disk depth from 2009-2015 in the Australian region (10°S to 45°S and 105°E to 160°E), indicating the water was becoming more transparent. The assessment from 2016-2021 indicates that this can mostly be attributed to a decrease in non-pigmented particles (total suspended solids), as Chl-a concentrations are increasing in some regions.

Lineage Statement: QUALITY OF DATA USED IN THE ASSESSMENT i) In situ data is collected using nationally consistent protocols and details can be found at https://s3-ap-southeast-2.amazonaws.com/content.aodn.org.au/Documents/IMOS/Facilities/national_mooring/IMOS_NRS_BGCManual_LATEST.pdf. ii) Satellite data has not been extensively validated for the Australian region, but in eastern Australia, the uncertainty in Chl-a has been estimated at ± 37 % (Laiolo et al. 2021).

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Notes Credit
Peer reviews of this assessment were provided by: Bozena Wojtasiewicz (CSIRO) Peter Strutton (IMAS, UTAS)

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