Chartrand, K.M., Ralph, P.J., and Rasheed, M.A. (2015) Under pressure – How to succeed as a deepwater seagrass. In: [Presented at CERF 2015]. From: CERF 2015: Grand Challenges in Coastal & Estuarine Science: Securing Our Future, 8-12 November 2015, Portland, Oregon USA.

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Abstract

What are the major drivers of deepwater (>10m) seagrasses and how best can this resource be managed to curb coastal development impacts such as dredge plumes? To address this, we constructed a multidimensional research program on seasonal dynamics and growth strategies of Halophila spp. in deep water at three locations along the Great Barrier Reef lagoon. We mapped seasonal abundance, reproductive effort, seed bank status, productivity and environmental parameters over three years. While all Halophila spp. thrived under relatively low light, the extent of the growing period and whether plants were present or absent depended on the species and latitudinal position. The light climate and growing phases at field sites guided customised light and temperature treatments for H. decipiens and H. spinulosa during laboratory experiments into tolerances and thresholds to maintain seagrass condition. Overall, H. decipiens and H. spinulosa maintained their condition in aquaria at light levels as low as 3.2 mol photons m-2 d-1, in line with observed seasonal germination and production at field sites. A 66% reduction in light from ambient led to decreased shoot density for H. decipiens and H. spinulosa after two and four weeks respectively. Differences in sexual reproductive effort between the two species indicate disparate life history strategies to cope with impacts. H. decipiens relies heavily on seed dispersal into local sediments, whereas H. spinulosa likely preserves a standing crop from which it can proliferate when conditions improve. Overall, small reductions in light over short timeframes can quickly exhaust a deepwater seagrass population. Safeguarding an annual seed bank should be a management priority under dredging scenarios to ensure subsequent germination and recruitment. Designing a light threshold for managing deepwater seagrasses must account for a monocarpic life history strategy to be an effective management tool in mitigating light-driven loss.

Item ID:	80218
Item Type:	Conference Item (Abstract / Summary)
Keywords:	deep-water seagrass, light, growth dynamics, Great Barrier Reef, Halophila, seed banks
Related URLs:	Organisation
Date Deposited:	30 Aug 2023 02:11
FoR Codes:	31 BIOLOGICAL SCIENCES > 3108 Plant biology > 310801 Phycology (incl. marine grasses) @ 30% 31 BIOLOGICAL SCIENCES > 3108 Plant biology > 310806 Plant physiology @ 40% 41 ENVIRONMENTAL SCIENCES > 4104 Environmental management > 410404 Environmental management @ 30%
SEO Codes:	18 ENVIRONMENTAL MANAGEMENT > 1805 Marine systems and management > 180501 Assessment and management of benthic marine ecosystems @ 100%
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