McCormack, C.V., Rasheed, M.A., and Carter, A.B. (2012) Effects of Shading on Zostera capricorni in the Gladstone Western Basin During the Senescent Season: April - July 2012. Report. Department of Agriculture, Fisheries and Forestry, Cairns, QLD, Australia.

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Abstract

This report summarises the findings of an in situ seagrass shading study conducted in the Port of Gladstone from the beginning of April to the beginning of July 2012 during the seagrass senescent season. The study replicated senescent season shading studies originally conducted in 2010 in the Gladstone area focus sing on the impacts of prolonged shading on Zostera capricorni. The main objective of the study was to test conclusions from previous shading studies that seagrasses were not affected by a reduction in light during the senescent season.

The key findings of this study are:

• Seagrass meadows experiencing prolonged (>9 weeks) shading in the 2012 senescent season had a small but significantly lower above- ground biomass of Z. capricorni compared with control plots.

• Biomass was reduced relative to control plots for seagrasses under all shade intensities applied, most likely due to all shade treatments resulting in light levels below the minimum light requirement for Z. capricorni during the study.

• The effect of prolonged shading during the senescent season was minimal compared with effects detected during in situ shading studies conducted over the seagrass growing season.

• All levels of shading resulted in seagrasses receiving less than 6 mol quanta m⁻² d⁻¹ over a two week rolling average for the majority of the study , whereas controls received slightly more than 6 mol quanta m⁻² d⁻¹ over a two week rolling average for the duration of the study.

• Despite receiving more than 6 mol quanta m⁻² d⁻¹ the control (unshaded) treatments declined significantly over the duration of the study, as expected during the senescent season.

These findings reinforce the established minimum light requirements for Gladstone seagrasses, with negative impacts to seagrass biomass detected with light below 6 mol quanta m⁻² d⁻¹ over a two week rolling average. However unlike the growing season where impacts were noted between 3 and 4 weeks of light below the threshold, reductions were not detected until 9 weeks of constant shading. So while the results indicate some merit in using the established growing season light trigger during the senescent season, implementing the trigger as a management tool during this time of year may be problematic as:

• The size of the effect was relatively small

• Seagrasses are naturally declining during this period even when receiving light above the threshold

• It required up to 9 weeks of shading below the light requirement to induce a measurable impact

Item ID:	45877
Item Type:	Report (Report)
Keywords:	seagrass, light thresholds, shading, Zostera, dredging, Gladstone,
Related URLs:	Publisher
Funders:	Gladstone Ports Corporation (GPC), Fisheries Queensland (FQ), Department of Agriculture, Fisheries and Forestry (DAFF)
Date Deposited:	28 Sep 2016 23:11
FoR Codes:	05 ENVIRONMENTAL SCIENCES > 0501 Ecological Applications > 050102 Ecosystem Function @ 70% 06 BIOLOGICAL SCIENCES > 0607 Plant Biology > 060705 Plant Physiology @ 30%
SEO Codes:	96 ENVIRONMENT > 9605 Ecosystem Assessment and Management > 960503 Ecosystem Assessment and Management of Coastal and Estuarine Environments @ 80% 96 ENVIRONMENT > 9611 Physical and Chemical Conditions of Water > 961102 Physical and Chemical Conditions of Water in Coastal and Estuarine Environments @ 20%
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