Keep off the grass: Using herbivore exclusion cages to understand herbivory in seagrass meadows

Seagrasses provide important habitat that delivers ecosystem services and provides food to a wide diversity of herbivores globally. In the Great Barrier Reef (GBR) we find the full size spectrum of herbivores; from small mesograzers such as amphipods, to macrograzers such as fish and large megagrazers such as turtles and dugongs. These herbivores can structurally alter seagrass beds in either positive or negative ways depending on their size, feeding preferences and methods and grazing intensity. These structural changes can subsequently interact with the delivery of other ecosystem services, or the benefits to humans, provided by the seagrass meadow. In the tropics, we know little about the impact of herbivores and how different groups interact to structure seagrass meadows, despite the number and variety of herbivores present in tropical seagrass habitats. We carried out exclusion experiments that targeted each herbivore group individually and in combination in subtidal and intertidal seagrass meadows in Queensland, Australia to understand the role of herbivores in structuring meadows and the interaction between herbivore groups. Our results show different feeding strategies of herbivores in each habitat, especially megaherbivores, and these impact the meadow in different ways. The effects on biomass, shoot density and shoot height depended on the type of grazing observed. All herbivore groups acted to structure the seagrass and interacted to influence overall meadow properties. Grazer mediated changes in meadow structure will have important implications for the ecosystem services delivered by tropical seagrass ecosystems

Exclusion studies reveal the interactions between herbivores in structuring seagrass meadows their ecosystem services and the implications for effective management

Seagrasses provide important habitat that delivers ecosystem services and provides food to a wide diversity of herbivores globally. In the Great Barrier Reef we find large seagrass meadows that are grazed on by a diverse herbivore community. This presents a challenge for managers trying to conserve herbivores, the habitats they rely on and maintain ecosystem service delivery in coastal ecosystems. Herbivore communities can structurally alter seagrass meadows in positive or negative ways depending on their size, feeding methods and grazing intensity. These structural changes can alter the ecosystem services provided by the seagrass meadow. We carried out exclusion experiments targeting each herbivore group individually and in combination in subtidal and intertidal seagrass meadows in Queensland, to understand how herbivores can structure meadows and the interactions between herbivore groups. Our results show different feeding strategies of herbivores in each habitat, especially megaherbivores, which impact the meadow in different ways. The effects on biomass, shoot density and shoot height depended on the type of grazing observed. Grazer mediated changes in meadow structure will have important implications for the ecosystem services delivered by tropical seagrass ecosystems and the management of these ecosystems, including incorporating grazing dynamics into monitoring projects

Spatial and temporal patterns in macroherbivore grazing in a multi-species tropical seagrass meadow of the Great Barrier Reef

Macroherbivory is an important process in seagrass meadows worldwide; however, the impact of macroherbivores on seagrasses in the Great Barrier Reef (GBR) has received little attention. We used exclusion cages and seagrass tethering assays to understand how the intensity of macroherbivory varies over space and time in the seagrass meadows around Green Island (Queensland), and what impact this has on overall meadow structure. Rates of macroherbivory were comparatively low, between 0.25–44% of daily seagrass productivity; however, rates were highly variable over a one-year period, and among sites. Loss of seagrass material to macroherbivory was predominantly due to fish; however, urchin herbivory was also taking place. Macroherbivory rates were of insufficient intensity to impact overall meadow structure. No macroherbivory events were identified on video cameras that filmed in the day, indicating that feeding may be occurring infrequently in large shoals, or at night. While relatively low compared to some meadows, seagrass macroherbivory was still an important process at this site. We suggest that in this highly protected area of the GBR, where the ecosystem and food webs remain largely intact, macroherbivory was maintained at a low level and was unlikely to cause the large-scale meadow structuring influence that can be seen in more modified seagrass systems

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

Exclusion studies reveal the interactions between herbivores in structuring seagrass meadows

Seagrasses provide important habitat that delivers ecosystem services and provides food to a wide diversity of herbivores globally. In the Great Barrier Reef we find large seagrass meadows that are grazed on by a diverse herbivore community. This presents a challenge for managers trying to conserve herbivores, the habitats they rely on and maintain ecosystem service delivery in coastal ecosystems. Herbivore communities can structurally alter seagrass meadows in positive or negative ways depending on their size, feeding methods and grazing intensity. These structural changes can alter the ecosystem services provided by the seagrass meadow. We carried out exclusion experiments targeting each herbivore group individually and in combination in subtidal and intertidal seagrass meadows in Queensland, Australia to understand how herbivores can structure meadows and the interactions between herbivore groups. Our results show different feeding strategies of herbivores in each habitat, especially megaherbivores, which impact the meadow in different ways. The effects on biomass, shoot density and shoot height depended on the type of grazing observed. Grazer mediated changes in meadow structure will have important implications for the ecosystem services delivered by tropical seagrass ecosystems and the management of these ecosystems, including incorporating grazing dynamics into monitoring projects

Richness of primary producers and consumer abundance mediate epiphyte loads in a tropical seagrass system

Consumer communities play an important role in maintaining ecosystem structure and function. In seagrass systems, algal regulation by mesograzers provides a critical maintenance function which promotes seagrass productivity. Consumer communities also represent a key link in trophic energy transfer and buffer negative effects to seagrasses associated with eutrophication. Such interactions are well documented in the literature regarding temperate systems, however, it is not clear if the same relationships exist in tropical systems. This study aimed to identify if the invertebrate communities within a tropical, multispecies seagrass meadow moderated epiphyte abundance under natural conditions by comparing algal abundance across two sites at Green Island, Australia. At each site, paired plots were established where invertebrate assemblages were perturbed via insecticide manipulation and compared to unmanipulated plots. An 89% increase in epiphyte abundance was seen after six weeks of experimental invertebrate reductions within the system. Using generalised linear mixed-effect models and path analysis, we found that the abundance of invertebrates was negatively correlated with epiphyte load on seagrass leaves. Habitat species richness was seen to be positively correlated with invertebrate abundance. These findings mirrored those of temperate systems, suggesting this mechanism operates similarly across latitudinal gradients

Community-specific "desired" states for seagrasses through cycles of loss and recovery

Seagrass habitats provide critical ecosystem services, yet there is ongoing concern over mounting pressures and continuing degradation. Defining a desired state for these habitats is a key step in implementing appropriate management but is often difficult given the challenges of available data and an evaluation of where to set benchmarks. We use more than 20 years of historical seagrass biomass data (1995–2018) for the diverse seagrass communities of Australia's Great Barrier Reef World Heritage Area (GBRWHA) to develop desired state benchmarks. Desired state for seagrass biomass was estimated for 25 of 36 previously defined seagrass communities with the remainder having insufficient data. Desired state varied by more than one order of magnitude between community types and was influenced by the mix of species in the communities and the range of environmental conditions. We identify a historical, decadal-scale cycle of decline with recovery to desired state in coastal intertidal communities. In contrast a number of the estuary and coastal subtidal communities have not recovered to desired state biomass. Understanding a historical context is critically important for setting benchmarks and making informed management decisions on the present state of seagrass in the GBRWHA. The approach we have developed is scalable for monitoring, management and assessment of pressures for other management areas and for other jurisdictions. Our results guide conservation planning through prioritization of the at-risk seagrass communities that are continuing to fall below their desired state

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

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

Seagrass mapping synthesis: a resource for coastal management in the Great Barrier Reef World Heritage Area

This project provides an up to date synthesis of the available information on seagrass in the Great Barrier Reef World Heritage Area (GBRWHA). It brings together more than 30 years of spatial information and data collection into easy to use spatial GIS layers that provide key information on species, meadow type and age and reliability of the data. The project provides: Seagrass site and meadow-specific data in Geographic Information System (GIS) layers to provide seagrass data to inform research analysis and management advice. A site layer that includes >66,000 individual survey sites with information including latitude/longitude, Natural Resource Management region, site depth, seagrass presence/absence, dominant seagrass species, presence/absence of individual species, survey date, survey method, and data custodian. A meadow layer that includes 1169 individual and/or composite seagrass meadows with information including individual meadow persistence, meadow location (intertidal/subtidal), meadow density based on mean biomass and/or mean percent cover, meadow area, dominant seagrass species, seagrass species present, range of survey dates, survey method, and data custodian. Metadata to enable interpretation of the information and to identify the original data custodians for assistance with interpretation. Outcomes: This study consolidates all available seagrass spatial data for the GBRWHA collected from 1984 to December 2014 by the TropWATER Seagrass Group and CSIRO in a GIS database. It assembles and documents the state of spatial knowledge of seagrass in the GBRWHA. The spatial data is based on methods developed by TropWATER and CSIRO for seagrass habitat surveys of subtidal meadows, and TropWATER methods for intertidal surveys. Methods include sampling by boat (free divers, underwater video camera, grabs, sled with net sampling), helicopter and walking. 447,530 hectares of seagrasses were mapped (modelled deep water seagrass areas are not included in area figures in this report) within the GBRWHA; much of which provides habitat for commercial and traditional fishery species, and an important food resource for dugong and green turtle populations. Data is included for twelve seagrass species from three families. Seagrass was present at 39% of all sites visited. The study identifies areas where much of the data available for management is more than 20 years old or where there are specific habitats unsurveyed. Large areas of central and northern Queensland require updating. Several key habitat types such as reef platform seagrass meadows are poorly represented in the data