GBR1 - Freshwater exposure map (North Queensland Flooding 2019)

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    This map shows the freshwater exposure (based on the eReefs GBR1 Hydrodynamic model v2.0) as a result of the major flooding that occurred in North Queensland in late January and February 2019. This product shows estimates of the total freshwater exposure over the entire month of February 2019. Predicting the level of coral bleaching from freshwater exposure is difficult to determine due to uncertainties in the hydrodynamic modelling and the relative poor understanding of freshwater exposure. Based on the few observations available areas in these maps with an exposure of greater than 50 PSU days are likely to have experienced coral morality.

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    Background

    The salinity of normal ocean water is approximately 35 PSU. As freshwater is mixed with the sea water the salinity drops, with pure water having a PSU of zero. Coral reefs are sensitive to freshwater as exposure can lead to bleaching and death. The relationship between freshwater exposure and coral bleaching and mortality is poorly understood. However flooding from Fitzroy river in 2011 cause significant shallow water coral mortality on the nearby Keppel Islands (Berkelmans et al. 2012). 

    Average salinity

    The third panel in the map shows the average salinity for the month. This is quite similar to the freshwater exposure except that is includes contributions from areas where there was very little freshwater. 

    Freshwater exposure calculations

    The freshwater exposure maps show the level of exposure to salinity levels below a specified threshold over a month. In the maps above the freshwater exposure was calculated for two threshold levels: 26 and 28 PSU. These were chosen as different species have different tolerances to freshwater exposure, and the most appropriate level is not currently known.

    The freshwater exposure is calculated from the sum of the daily average salinity below the exposure threshold. Salinity levels above the threshold don't contribute at all to the exposure. Salinity levels below the threshold contribute in proportion to the amount below the threshold. As a result with a threshold of 28 PSU and an exposure to 22 PSU for 3 days the freshwater exposure would be (28-22)*3 = 18 PSU days. The same exposure at 25 PSU would take 6 days (28-25)*6 = 18 PSU days. This method of calculating exposure is conceptually very similar to temperature exposure measured in Degree Heating Weeks, where the higher the temperature is above the typical maximum summer temperature the faster the heat stress occurs. For freshwater exposure the stress occurs when the salinity drops below the threshold value. The lower the salinity the faster the exposure accumulates. More research is needed to determine if this is the best method for estimating stress from freshwater. 

    The maps show the accumulated exposure below the threshold over the month of February 2019, which covered most of the flooding event.

    For comparison the freshwater exposure calculations were applied to the observations taken during the Fitzroy flooding event in 2011 that caused significant coral mortality on reef flats corals in the Keppel Islands (Berkelmans et al. 2012). This shows that exposure to greater than 50 PSU days can result in significant mortality, particularly in sensitive coral species.

    Island

    Freshwater exposure

    (PSU days, 28 PSU threshold)

    Reef flat mortality (%)
    Great Keppel 130 100
    Halfway 110 100
    North Keppel 50 40
    Miall 40 0

    Freshwater exposure calculated from measurements taken on the Keppel Islands during the 2011 flooding. (Berkelmans et al. 2012)

    How accurate is the modelled freshwater exposure?

    The results on this page are generated from a model and not based on direct measurements of the salinity. It is difficult to reliably determine the level of error without direct measures of salinity during the event, however there are a few clues that give us some estimate of the error and confidence in the results. Figure 1 shows a comparison between the freshwater exposure calculated from the 1 km and 4 km eReefs hydrodynamic models (v2.0). This shows there is a significant difference between these models. For a threshold of 28 PSU at Magnetic Island the 1 km model shows freshwater exposure of 35 PSU days, where as the 4 km model shows 0 PSU days.  

    Freshwater exposure eReefs model comparison

    Observations taken along the eastern side of Magnetic Island taken on the 20th February 2019 showed no evidence of freshwater bleaching, although some corals were pale due to low light conditions. Corals on the rocks near AIMS (near the Haughton river) showed freshwater mortality. These two points of reference can be used to gauge whether the 1 km or 4 km results are more accurate.

    Location 1 km model (28 PSU threshold) 4 km model (28 PSU threshold) Observed freshwater mortality
    Freshwater exposure (PSU days) Predicted morality (Berkelmans,  2012) 4 km model freshwater exposure (28 PSU threshold) Predicted morality (Berkelmans,  2012)
    Magnetic Island 35 None 0 None None
    Cape Ferguson 75 High 15 None High

    Applying the previously estimated freshwater exposure 50 PSU days the 1 km model predicts there would be coral mortality at Cape Ferguson, which was the case, where as the 4 km model predicts none. At Magnetic Island both models predict no freshwater mortality and none was observed.

    There is currently little evidence to assess the accuracy of freshwater exposure maps, however the available evidence indicates that the 1 km model are a better match to observations.

    References

    Berkelmans, R., Jones, A.M. & Schaffelke, B. (2012) Salinity thresholds of Acropora spp. on the Great Barrier Reef. 31: 1103. Coral Reefs. https://doi.org/10.1007/s00338-012-0930-z