Effective strategies to manage dredge related threats to tropical seagrass systems based on seagrass ecological requirements

Major dredging projects have the potential to impact on tropical seagrass communities through direct removal and burial and indirectly through turbid dredge plumes reducing the amount of light available to seagrasses. This is a major concern in Australia and elsewhere in the Indo Pacific region where substantial expansion of tropical ports associated with the resources boom is occurring. In the majority of cases managing the impacts from turbid plumes has focussed on a turbidity threshold that has not been related to the true light requirements of the various seagrass species potentially impacted. Here we report on the value of an approach based on determining the minimum light requirements of species, their resilience to impacts and ability to recover and designing a dredge mitigation approach that is focussed on maintaining critical windows of light to support seagrass growth and longer term survival. Results show the value of experimentally determining locally relevant ecological requirements and the importance of understanding the relationships between light requirements, tidal exposure, shifts in spectral quality of light, seasonality and capacity for species to recover from light stress in determining ecologically relevant triggers. This information combined with a robust toolkit for assessing sub-lethal light stress provides an effective dredge mitigation strategy to protect seagrasses

Implementing a light-based seagrass management approach for a large scale dredging program

Seagrass has been highlighted as a key ecosystem component at risk from port developments and associated dredging activities on the central east coast of Queensland Australia. Fisheries Queensland has developed a light-based approach to manage impacts on seagrasses from turbidity plumes associated with a large-scale port dredging program. In situ and laboratory seagrass shading tolerance studies were conducted to simulate the effects of a dredge plume-related reduction in light from increased turbidity. A review of historical seagrass trends at permanent transect monitoring sites throughout the region and associated Photosynthetically Active Radiation (PAR) data refined species-specific light requirements to develop a working light trigger value for a management intervention that would protect seagrasses from dredge plume impacts. This light-based approach is being trialed in parallel with turbidity management plans. The development, implementation and outcomes of this Light-Based Seagrass Management Approach will be presented

Under pressure...how to succeed as a deepwater seagrass

As a functional group seagrasses are ubiquitous throughout the Great Barrier Reef World Heritage Area (GBRWHA), from offshore coral reefs to inshore estuaries and embayments. Mounting concern for acute and chronic anthropogenic pressures on seagrasses have steered efforts to develop indicators of light attenuation stress. Developing a “one-size-fits-all” model for the 15 species found in the GBRWHA is ineffective without accounting for functional differences in their life history traits. We investigated the growth attributes and adaptation of deepwater (>10m) seagrass communities with a goal to improve coastal management strategies impacting this functional group. We explored what role density and stratification of a seed bank plays in meadow resilience, and how the optical challenges of growing at depth may affect seed germination in these ephemeral meadows. Sediment cores were collected over three years from three Halophila spp. meadows within the GBRWHA. Seed densities varied over time and with depth (≤ 10cm) but followed similar trends among meadows. Collections of mature fruits afforded fresh seeds to establish the first successful seed viability test known for this genus. Fresh seeds were also used to examine the role spectrally-refined light at depth has on germination success. While strategically Halophila spp. are known to rely on seeds, having a more intimate understanding of the functionality of their seed banks and how they respond to environmental cues, bridges a science gap for effectively mitigating light-driven loss of tropical deepwater seagrasses

Implementing a light-based seagrass management approach for a large scale dredging program

Seagrass has been highlighted as a key ecosystem component at risk from port developments and associated dredging activities on the central east coast of Queensland Australia. Fisheries Queensland has developed a light-based approach to manage impacts on seagrasses from turbidity plumes associated with a large-scale port dredging program. In situ and laboratory seagrass shading tolerance studies were conducted to simulate the effects of a dredge plume-related reduction in light from increased turbidity. A review of historical seagrass trends at permanent transect monitoring sites throughout the region and associated Photosynthetically Active Radiation (PAR) data refined species-specific light requirements to develop a working light trigger value for a management intervention that would protect seagrasses from dredge plume impacts. This light-based approach is being trialed in parallel with turbidity management plans. The development, implementation and outcomes of this Light-Based Seagrass Management Approach will be presented

Impacts of a large-scale flood event on sub-tropical intertidal seagrass meadows

Long term monitoring of coastal seagrasses in the Port of Gladstone has provided the opportunity to better understand the impacts of extreme weather events on these marine plant communities. We studied the effects of a large-scale flood event in the summer of 2010-2011on seven intertidal seagrass meadows at Gladstone Harbour on the central east coast of Queensland. Significant declines in percent cover and above-ground biomass were recorded at several meadows following the weather event with some meadows yet to recover to pre-flood levels by early 2012. Seagrass condition, assessed quarterly and more recently monthly from November 2009 (and biannually from 2005 for some locations), was tested against potential drivers (rainfall, river flow, PAR, temperature and tidal exposure) using multiple linear regression analysis. We present the results from this analysis. We also present results of an investigation of the capacity for meadow recovery from a below ground seed bank

Understanding tropical seagrass light requirements and resilience to effectively manage seagrass health during major dredging projects

Major dredging projects have the potential to impact on tropical seagrass communities through direct removal and burial and indirectly through turbid dredge plumes reducing the amount of light available to seagrasses. This is a major concern in Australia and elsewhere in the Asia Pacific region where substantial expansion of tropical ports associated with the resources boom is occurring. In the majority of cases managing the impacts from turbid plumes has focussed on a turbidity threshold that has not been related to the true light requirements of the various seagrass species potentially impacted. Here we report on the value of an approach based on determining the minimum light requirements of species, their resilience to impacts and ability to recover and designing a dredge mitigation approach that is focussed on maintaining critical windows of light to support seagrass growth and longer term survival. Results show the value of experimentally determining locally relevant light requirements and the importance of understanding the relationships between light requirements, tidal exposure, shifts in spectral quality of light, seasonality and capacity for species to recover from light stress in determining ecologically relevant triggers. This information combined with a robust toolkit for assessing sub-lethal light stress provides an effective dredge mitigation strategy to protect seagrasses

Light thresholds for seagrasses of the GBRWHA: a synthesis and guiding document

[Extract]. Key Findings. This synthesis contains light thresholds for seagrass species in the Great Barrier Reef World Heritage Area (GBRWHA). The thresholds can be applied to ensure protection of seagrasses from activities that impact water quality and the light environment over the short-term, such as coastal and port developments. Thresholds for long-term maintenance of seagrasses are also proposed. •The synthesis provides clear and consistent guidance on light thresholds to apply in managing potential water quality impacts to seagrass. •All available information on biological light thresholds was tabulated and conservative management thresholds were identified to ensure seagrass protection. •Acute management thresholds are suited to compliance guidelines for managing short-term impacts and these and are the focus of this synthesis. Long-term thresholds are suited to the setting of water quality guidelines for catchment management. •The synthesis identified key areas where further information is required, including: ◦species for which almost no information on light thresholds exists; ◦location and population-specific thresholds particularly for the most at-risk species; ◦definitions of desired state to underpin the development of long-term light guidelines to meet them; ◦the effect of spectral quality on light thresholds; and, consideration of cumulative impacts (temperature, nutrients, sedimentary conditions) on acute and long-term light thresholds. •Light management thresholds for acute impacts are presented for twelve species. Colonising species are the most sensitive to light reduction (i.e. lowest thresholds) and have the shortest time to impact while larger, persistent species have higher light thresholds and a longer time to impact. •The recommended acute management thresholds are ready for application, as the conservative approach (higher light threshold, shortest time to impact) for species with low confidence should ensure protection to seagrass meadows at risk from acute light stress

Deep-water seagrasses in the tropics: resilience, recovery and establishing thresholds and drivers of change

Global seagrass research and assessment efforts have focused on shallow coastal and estuarine seagrass populations. Comparatively little is known about the dynamics of deep-water (>10m) seagrasses despite evidence they form extensive meadows in some parts of the world and may be highly productive compared with their shallow counterparts. Deep-water seagrasses are subject to a similar range of anthropogenic threats as shallow meadows particularly along the Great Barrier Reef (GBR) in Queensland, where they occur close to major population centres and adjacent to the coast. We examine the dynamics of deep-water seagrass populations in the GBR through a range of research studies including long term (>8 years) assessments of change; impacts of major dredging programs; resilience and recovery from severe tropical storms and; targeted research investigating the drivers, thresholds and tolerances behind seasonal and inter-annual change. Collectively these re- search programs have provided new insight into deep-water seagrass dynamics. Despite considerable inter-annual variability deep-water seagrasses had a regular annual pattern of occurrence at some locations, a low level of resilience to reduced water quality, but a high capacity for recolonisation on the cessation of impacts. While susceptible to large scale loss from severe storms these meadows were quick to re-establish compared with nearby shallow coastal seagrasses. The results of the work are establishing a series of key management thresholds and stress indictors that can be applied to ensure greater protection of these seagrasses

Measuring the dynamics and thresholds of tropical deepwater seagrasses

Extensive areas of deepwater (>10m) seagrass meadows are known to occur in many tropical regions. More than 31,000 km2 are found in the Great Barrier Reef Region of Queensland alone and one of the world's largest continuous seagrass meadows has been mapped recently in deeper waters of the Torres Strait. Despite their extensive distribution, little is known about the ecological roles, tolerances and dynamics of these deepwater communities compared with the much more commonly studied shallow seagrass meadows from the same region. Existing information suggests deepwater meadows may be highly productive but also highly dynamic between and within years. Increasingly these meadows are coming under threat from anthropogenic disturbances and it is critical to develop our understanding of the drivers of change and tolerances of these meadows to effectively manage them. We present initial findings from a major research program to establish a better understanding of the drivers of seasonal and interannual dynamics in these seagrass communities, the role of seed banks and seagrass recruitment, and the environmental cues that drive the seasonal patterns of decline and recovery. The research program includes a detailed study of the light requirements of these deepwater species as well as developing a range of tools for monitoring and managing anthropogenic impacts such as dredging

The influence of light and tidal exposure on primary production in the tropical seagrass Zostera capricorni and Halophila ovalis

The growth, survival and depth penetration of seagrass is directly related to light availability, which drives photosynthesis. The amount of light reaching seagrass beds is highly variable and can be easily disrupted by human activities, such as dredging. Dredging results in increased turbidity and decreased light penetration to the seagrass beds, invariably influencing overall productivity and seagrass health. To better understand seagrass light requirements and resilience to environmental stressors such as dredging requires knowledge on seagrass photophysiology and the impact air exposure during a tidal cycle has on photosynthesis. Oxygen, fluorescence and bio-optical properties were measured over a tidal cycle in seagrass beds of Zostera capricorni and Halophila ovalis in Gladstone Harbour to provide insight into the variability in carbon production in intertidal seagrass meadows. Both species showed an increase in photosynthetic activity with increased irradiance as the tide receded. However, sensitivity to desiccation was observed during air-exposure with a significant decline in photosynthesis irrespective of increased light availability. Understanding the complex dynamics of seagrass photosynthesis over a tidal cycle will help in the mitigation of dredging-related light loss to Gladstone seagrass meadows