Port of Abbot Point: initial light thresholds for modelling impacts to seagrass from the Abbot Point Growth Gateway Project

McKenna, S.A., Chartrand, K.M., Jarvis, J.C., Carter, A.B., Davies, J.C., and Rasheed, M.A. (2015) Port of Abbot Point: initial light thresholds for modelling impacts to seagrass from the Abbot Point Growth Gateway Project. Report. TropWATER, James Cook University, Cairns, QLD, Australia.

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This technical report provides the support and basis for initial light threshold values for seagrasses to model zones of impact from dredging activities including return water discharge associated with the proposed Abbot Point Growth Gateway project.

Seagrass light thresholds for the model input were derived by:

1. A review of the range of species specific light requirements from existing literature as well as data from ongoing studies to establish likely ranges of light required for the species of concern.

2. Analysis of in situ data on light and seagrass change collected at Abbot Point between August 2013 and May 2015.

3. Examination of how the literature and ongoing study derived values fit with the recorded light history and occurrence of seagrass at the Abbot Point monitoring sites.

Based on these assessments we recommend the following initial modelling thresholds:

1. For the offshore areas of deep water Halophila species the modelling threshold is 1.5 mol m⁻² day⁻¹ over a rolling 7 day average.

2. For the shallow inshore areas potentially effected by outfall discharges (dominated by Halodule uninervis) the modelling threshold we recommend is 3.5 mol m⁻² day⁻¹ over a rolling 14 day average.

The upper limit of threshold values for the model was heavily informed by the in situ light monitoring at Abbot Point. The paired light and seagrass monitoring program provided critical information on the light environment associated with the maintenance or increase in seagrass at the sites. This emphasizes the value of keeping these loggers and seagrass monitoring in place leading up to dredging to further refine the relationships into an operational tool for management during dredging. Future plans for incorporating light based triggers for seagrass management at Abbot Point should remain flexible to allow additional information from the ongoing data collection on site, or any additional new studies to be incorporated in refining the initial threshold. As a priority to improve certainty around the threshold levels we recommend:

1. Continuing the quarterly seagrass monitoring and ongoing light assessment to provide additional data around the relationship. The primary concern is for coastal meadows where seagrass has only recently returned and only limited local information was available to inform the initial threshold; and

2. Increasing to monthly seagrass sampling between July and December 2015 (growing season) to develop better relationships between fluctuating light levels and seagrass change. There are a range of other additional research activities that would also be valuable to better understand the light requirements of local seagrasses including: field shading studies to manipulate light levels; laboratory trials to test observed relationships; and investigations of dredging related spectral shifts (colour) of light and its impacts on seagrass light requirements.

In the absence of additional information being available, we recommend adopting more conservative initial values for management especially for the coastal Halodule uninervis where the least local information was available at the time of this report. This could include using a value of 5 mol m⁻² day⁻¹ over a rolling 7 day average until further data is available.

Item ID: 39685
Item Type: Report (Report)
Keywords: recovery, resilience, management, environmental monitoring, marine monitoring, marine water quality, seagrass, reproduction, seagrass habitats, Great Barrier Reef, port development, dredging
Additional Information:

This report is confidential and cannot be made public.

Date Deposited: 20 Aug 2015 03:33
FoR Codes: 05 ENVIRONMENTAL SCIENCES > 0502 Environmental Science and Management > 050206 Environmental Monitoring @ 50%
05 ENVIRONMENTAL SCIENCES > 0502 Environmental Science and Management > 050205 Environmental Management @ 50%
SEO Codes: 96 ENVIRONMENT > 9605 Ecosystem Assessment and Management > 960507 Ecosystem Assessment and Management of Marine Environments @ 50%
96 ENVIRONMENT > 9605 Ecosystem Assessment and Management > 960503 Ecosystem Assessment and Management of Coastal and Estuarine Environments @ 50%
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