Light Thresholds to Prevent Dredging Impacts on the Great Barrier Reef Seagrass, Zostera muelleri ssp. capricorni

Abstract

Coastal seagrass habitats are at risk from a range of anthropogenic activities that modify the natural light environment, including dredging activities associated with coastal and port developments. On Australia's east coast, the tropical seagrass Zostera muelleri ssp. capricorni dominates intertidal mudbanks in sheltered embayments which are also preferred locations for harbors and port facilities. Dredging to establish and maintain shipping channels in these areas can degrade water quality and diminish light conditions that are required for seagrass growth. Based on this potential conflict, we simulated in-situ light attenuation events to measure effects on Z. muelleri ssp. capricorni condition. Semi-annual in situ shading studies conducted over 3 years were used to quantify the impact of prolonged light reduction on seagrass morphometrics (biomass, percent cover, and shoot density). Experimental manipulations were complimented with an assessment of 46 months of light history and concurrent natural seagrass change at the study site in Gladstone Harbour. There was a clear light-dependent effect on seagrass morphometrics during seagrass growing seasons, but no effect during senescent periods. Significant seagrass declines occurred between 4 and 8 weeks after shading during the growing seasons with light maintained in the range of 4-5 mol photons m −2 d −1 . Sensitivity to shading declined when applied in 2-week intervals (fortnightly) rather than continuous over the same period. Field observations were correlated to manipulative experiments to derive an applied threshold of 6 mol photons m −2 d −1 which formed the basis of a reactive light-based management strategy which has been successfully implemented to ensure positive ecological outcomes for seagrass during a large-scale dredging program.

Elevated Turbidity and the Nutrient Removal Capacity of Seagrass

“…of turbidity on seagrass nutrient removal are indirect. Water column turbidity impacts seagrass by reducing the quantity and quality of light available for photosynthesis (Petrou et al, 2013;Chartrand et al, 2016). During our experiment, the average benthic light availability at Mai C, and also at KKF, was less than the amount required for photosynthetic saturation in Zostera muelleri [≥195 µmol m −2 s −1 ; (Schwarz, 2004)], whereas light availability at the site with the clearest water, TPB, exceeded saturation requirements.…”

“…Consistent with the literature (van der Heide et al, 2008), our results demonstrate a positive relationship between nutrient uptake rates and seagrass biomass. The site with the lowest nutrient uptake capacity, Mai C, likely had a reduced capacity for nutrient uptake due to a combination of relatively low seagrass biomass and a low potential for photosynthetic activity as a result of high water column turbidity [which in turn is associated with reduced seagrass biomass (Goodman et al, 1995;Petrou et al, 2013;Chartrand et al, 2016)].…”

“…In addition, because nutrient uptake rates are generally related to primary production rates, stressors that affect seagrass primary production, such as increased turbidity, may therefore affect the nutrient removal capacity of seagrass. Turbidity in the water column is inversely correlated with seabed light levels, and high turbidity is generally considered a stressor of seagrass (Petrou et al, 2013;Chartrand et al, 2016). By moderating seagrass primary production rates, high turbidity has the potential to affect the growth and vigor of individual plants and the extent and lushness of whole meadows (Goodman et al, 1995;Petrou et al, 2013;Chartrand et al, 2016), which in turn may result in reduced nutrient removal or retention (van der Heide et al, 2008).…”

Effects of combined dredging-related stressors on sponges: a laboratory approach using realistic scenarios

“…These stressors can act either alone or, more frequently, in combination and can change rapidly according to sea-state, cloud cover, and diel and tidal cycles 4, 32 . This ‘protean’ characteristic of turbidity makes it very difficult to predict impacts for management purposes.…”

Proteome Analysis Reveals Extensive Light Stress-Response Reprogramming in the Seagrass Zostera muelleri (Alismatales, Zosteraceae) Metabolism

“…Seagrasses have unusually high light requirements for growth (10–37% of surface irradiance compared with 0.11% for most other marine macrophytes), which make them highly vulnerable to deterioration in water clarity (Petrou et al, 2013; Chartrand et al, 2016). In coastal habitats, increased light scattering, and/or light attenuation due to suspended particles or by the overgrowth of epiphytes or algal blooms in the water column affects light quality.…”