Spatial patterns of seagrass dispersal and settlement

Vegetative fragment production as a means of propagule dispersal for tropical seagrass meadows

BACKGROUND AND AIMS

Long distance dispersal (LDD) contributes to the replenishment and recovery of tropical seagrass habitats exposed to disturbance, such as cyclones and infrastructure development. However, our current knowledge regarding the physical attributes of seagrass fragments that influence LDD predominantly stems from temperate species and regions. The goal of this paper is to measure seagrass fragment density and viability in two tropical species, assessing various factors influencing their distribution.

METHODS

We measured the density and viability of floating seagrass fragments for two tropical seagrass species (Zostera muelleri and Halodule uninervis) in two coastal seagrass meadows in the central Great Barrier Reef World Heritage Area, Australia. We assessed the effect of wind speed, wind direction, seagrass growing/senescent season, seagrass meadow density, meadow location and dugong foraging intensity on fragment density. We also measured seagrass fragment structure and fragment viability; i.e., potential to establish into a new plant.

KEY RESULTS

We found that seagrass meadow density, season, wind direction and wind speed influenced total fragment density, while season and wind speed influenced the density of viable fragments. Dugong foraging intensity did not influence fragment density. Our results indicate that wave action from winds combined with high seagrass meadow density increases seagrass fragment creation, and that more fragments are produced during the growing than the senescent season. Seagrass fragments classified as viable for Z. muelleri and H. uninervis had significantly more shoots and leaves than non-viable fragments. We collected 0.63 (±0.08 SE) floating viable fragments 100 m-2 in the growing season, and 0.13 (±0.03 SE) viable fragments 100 m-2 in the senescent season. Over a third (38%) of all fragments collected were viable.

CONCLUSION

There is likely to be a large number of viable seagrass fragments available for long distance dispersal. This study's outputs can inform dispersal and connectivity models that are used to direct seagrass ecosystem management and conservation strategies.

Biophysical models resolution affects coral connectivity estimates

Estimating connectivity between coral reefs is essential to inform reef conservation and restoration. Given the vastness of coral reef ecosystems, connectivity can only be simulated with biophysical models whose spatial resolution is often coarser than the reef scale. Here, we assess the impact of biophysical models resolution on connectivity estimates by comparing the outputs of five different setups of the same model with resolutions ranging from 250 m to 4 km. We show that increasing the model resolution around reefs yields more complex and less directional dispersal patterns. With a fine-resolution model, connectivity graphs have more connections but of weaker strength. The resulting community structure therefore shows larger clusters of well-connected reefs. Virtual larvae also tend to stay longer close to their source reef with a fine-resolution model, leading to an increased local retention and self-recruitment for species with a short pre-competency period. Overall, only about half of the reefs with the largest connectivity indicator values are similar for the finest and coarsest resolution models. Our results suggest that reef management recommendations should only be made at scales coarser than the model resolution. Reef-scale recommendations can hence only be made with models not exceeding about 500 m resolution.

Quantifying the environmental impact of a major coal mine project on the adjacent Great Barrier Reef ecosystems

A major coal mine project in Queensland, Australia, is currently under review. It is planned to be located about 10 km away from the Great Barrier Reef World Heritage Area (GBRWHA). Sediment dispersal patterns and their impact on marine ecosystems have not been properly assessed yet. Here, we simulate the dispersal of different sediment types with a high-resolution ocean model, and derive their environmental footprint. We show that sediments finer than 32 μm could reach dense seagrass meadows and a dugong sanctuary within a few weeks. The intense tidal circulation leads to non-isotropic and long-distance sediment dispersal patterns along the coast. Our results suggest that the sediments released by this project will not be quickly mixed but rather be concentrated where the most valuable ecosystems are located. If accepted, this coal mine could therefore have a far-reaching impact on the GBRWHA and its iconic marine species.

A spatial analysis of seagrass habitat and community diversity in the Great Barrier Reef World Heritage Area

The Great Barrier Reef World Heritage Area (GBRWHA) in north eastern Australia spans 2500 km of coastline and covers an area of ~ 350,000 km². It includes one of the world’s largest seagrass resources. To provide a foundation to monitor, establish trends and manage the protection of seagrass meadows in the GBRWHA we quantified potential seagrass community extent using six random forest models that include environmental data and seagrass sampling history. We identified 88,331 km² of potential seagrass habitat in intertidal and subtidal areas: 1111 km² in estuaries, 16,276 km² in coastal areas, and 70,934 km² in reef areas. Thirty-six seagrass community types were defined by species assemblages within these habitat types using multivariate regression tree models. We show that the structure, location and distribution of the seagrass communities is the result of complex environmental interactions. These environmental conditions include depth, tidal exposure, latitude, current speed, benthic light, proportion of mud in the sediment, water type, water temperature, salinity, and wind speed. Our analysis will underpin spatial planning, can be used in the design of monitoring programs to represent the diversity of seagrass communities and will facilitate our understanding of environmental risk to these habitats.

Fate and transport of coastal driftwood: A critical review

Driftwood originating from natural and anthropogenic sources is abundant in coastal regions and plays an important role in ecosystems, providing habitat, structure, nutrients, and carbon storage. Conversely, large accumulations of driftwood can litter coastal zones, negatively impact coastal ecosystems and pose hazards to navigation, infrastructure and communities. Knowledge of the processes underlying the fate and transport of coastal driftwood is therefore needed to inform sustainable management practices. The present state of understanding is limited, and predominantly founded on studies of rivers and tsunamis, where the spatio-temporal scales and driving processes are significantly different from typical climatic or storm conditions in coastal waters. The authors critically review research on fate and transport of driftwood in coastal waters, and identify research needs and opportunities. Key knowledge gaps relate to: interactions between driftwood, littoral zone hydrodynamics and geomorphology; mechanisms of driftwood rafting and accumulation; and influence of weathering and degradation on mobility.

Seagrass ecosystems of the Pacific Island Countries and Territories: A global bright spot

Seagrass ecosystems exist throughout Pacific Island Countries and Territories (PICTs). Despite this area covering nearly 8% of the global ocean, information on seagrass distribution, biogeography, and status remains largely absent from the scientific literature. We confirm 16 seagrass species occur across 17 of the 22 PICTs with the highest number in Melanesia, followed by Micronesia and Polynesia respectively. The greatest diversity of seagrass occurs in Papua New Guinea (13 species), and attenuates eastward across the Pacific to two species in French Polynesia. We conservatively estimate seagrass extent to be 1446.2 km², with the greatest extent (84%) in Melanesia. We find seagrass condition in 65% of PICTs increasing or displaying no discernible trend since records began. Marine conservation across the region overwhelmingly focuses on coral reefs, with seagrass ecosystems marginalised in conservation legislation and policy. Traditional knowledge is playing a greater role in managing local seagrass resources and these approaches are having greater success than contemporary conservation approaches. In a world where the future of seagrass ecosystems is looking progressively dire, the Pacific Islands appears as a global bright spot, where pressures remain relatively low and seagrass more resilient.

Over a decade monitoring Fiji's seagrass condition demonstrates resilience to anthropogenic pressures and extreme climate events

Seagrass are an important marine ecosystem of the Fiji Islands. We confirm six seagrass species from the archipelago and defined five broad categories of seagrass habitat. We report, with high confidence, seagrass meadows covering 59.19 km² of Fiji's shallow water habitats from literature and this study. Long-term monitoring of seagrass abundance, species composition, and seed banks at eight sentinel sites, found no long-term trends. Examination of key attributes that affect seagrass resilience identified meadows as predominately enduring and dominated by opportunistic species which had moderate physiological resistance, and high recovery capacity. We examined threats to Fiji's seagrass meadows from extreme climatic events and anthropogenic activities using a suite of indicators, identifying water quality as a major pressure. Based on these findings, we assessed existing protections in Fiji afforded to seagrass and their services. This understanding will help to better manage for seagrass resilience and focus future seagrass research in Fiji.

A general framework for propagule dispersal in mangroves

Dispersal allows species to shift their distributions in response to changing climate conditions. As a result, dispersal is considered a key process contributing to a species' long-term persistence. For many passive dispersers, fluid dynamics of wind and water fuel these movements and different species have developed remarkable adaptations for utilizing this energy to reach and colonize suitable habitats. The seafaring propagules (fruits and seeds) of mangroves represent an excellent example of such passive dispersal. Mangroves are halophytic woody plants that grow in the intertidal zones along tropical and subtropical shorelines and produce hydrochorous propagules with high dispersal potential. This results in exceptionally large coastal ranges across vast expanses of ocean and allows species to shift geographically and track the conditions to which they are adapted. This is particularly relevant given the challenges presented by rapid sea-level rise, higher frequency and intensity of storms, and changes in regional precipitation and temperature regimes. However, despite its importance, the underlying drivers of mangrove dispersal have typically been studied in isolation, and a conceptual synthesis of mangrove oceanic dispersal across spatial scales is lacking. Here, we review current knowledge on mangrove propagule dispersal across the various stages of the dispersal process. Using a general framework, we outline the mechanisms and ecological processes that are known to modulate the spatial patterns of mangrove dispersal. We show that important dispersal factors remain understudied and that adequate empirical data on the determinants of dispersal are missing for most mangrove species. This review particularly aims to provide a baseline for developing future research agendas and field campaigns, filling current knowledge gaps and increasing our understanding of the processes that shape global mangrove distributions.

Global-scale dispersal and connectivity in mangroves

Mangroves are of considerable ecological and socioeconomical importance; however, substantial areal losses have been recorded in many regions, driven primarily by anthropogenic disturbances and sea level rise. Oceanic dispersal of mangrove propagules provides a key mechanism for shifting distributions in response to environmental change. Here we provide a model framework for describing global dispersal and connectivity in mangroves. We identify important dispersal routes, barriers, and stepping-stones and quantify the influence of minimum and maximum floating periods on simulated connectivity patterns. Our study provides a baseline to improve our understanding of observed mangrove species distributions and, in combination with climate data, the expected range shifts under climate change.

Dispersal provides a key mechanism for geographical range shifts in response to changing environmental conditions. For mangroves, which are highly susceptible to climate change, the spatial scale of dispersal remains largely unknown. Here we use a high-resolution, eddy- and tide-resolving numerical ocean model to simulate mangrove propagule dispersal across the global ocean and generate connectivity matrices between mangrove habitats using a range of floating periods. We find high rates of along-coast transport and transoceanic dispersal across the Atlantic, Pacific, and Indian Oceans. No connectivity is observed between populations on either side of the American and African continents. Archipelagos, such as the Galapagos and those found in Polynesia, Micronesia, and Melanesia, act as critical stepping-stones for dispersal across the Pacific Ocean. Direct and reciprocal dispersal routes across the Indian Ocean via the South Equatorial Current and seasonally reversing monsoon currents, respectively, allow connectivity between western Indian Ocean and Indo-West Pacific sites. We demonstrate the isolation of the Hawaii Islands and help explain the presence of mangroves on the latitudinal outlier Bermuda. Finally, we find that dispersal distance and connectivity are highly sensitive to the minimum and maximum floating periods. We anticipate that our findings will guide future research agendas to quantify biophysical factors that determine mangrove dispersal and connectivity, including the influence of ocean surface water properties on metabolic processes and buoyancy behavior, which may determine the potential of viably reaching a suitable habitat. Ultimately, this will lead to a better understanding of global mangrove species distributions and their response to changing climate conditions.

Seascape genetics and biophysical connectivity modelling support conservation of the seagrass Zostera marina in the Skagerrak-Kattegat region of the eastern North Sea

Maintaining and enabling evolutionary processes within meta-populations are critical to resistance, resilience and adaptive potential. Knowledge about which populations act as sources or sinks, and the direction of gene flow, can help to focus conservation efforts more effectively and forecast how populations might respond to future anthropogenic and environmental pressures. As a foundation species and habitat provider, Zostera marina (eelgrass) is of critical importance to ecosystem functions including fisheries. Here, we estimate connectivity of Z. marina in the Skagerrak-Kattegat region of the North Sea based on genetic and biophysical modelling. Genetic diversity, population structure and migration were analysed at 23 locations using 20 microsatellite loci and a suite of analytical approaches. Oceanographic connectivity was analysed using Lagrangian dispersal simulations based on contemporary and historical distribution data dating back to the late 19th century. Population clusters, barriers and networks of connectivity were found to be very similar based on either genetic or oceanographic analyses. A single-generation model of dispersal was not realistic, whereas multigeneration models that integrate stepping-stone dispersal and extant and historic distribution data were able to capture and model genetic connectivity patterns well. Passive rafting of flowering shoots along oceanographic currents is the main driver of gene flow at this spatial-temporal scale, and extant genetic connectivity strongly reflects the "ghost of dispersal past" sensu Benzie, 1999. The identification of distinct clusters, connectivity hotspots and areas where connectivity has become limited over the last century is critical information for spatial management, conservation and restoration of eelgrass.

Timing anthropogenic stressors to mitigate their impact on marine ecosystem resilience

Better mitigation of anthropogenic stressors on marine ecosystems is urgently needed to address increasing biodiversity losses worldwide. We explore opportunities for stressor mitigation using whole-of-systems modelling of ecological resilience, accounting for complex interactions between stressors, their timing and duration, background environmental conditions and biological processes. We then search for ecological windows, times when stressors minimally impact ecological resilience, defined here as risk, recovery and resistance. We show for 28 globally distributed seagrass meadows that stressor scheduling that exploits ecological windows for dredging campaigns can achieve up to a fourfold reduction in recovery time and 35% reduction in extinction risk. Although the timing and length of windows vary among sites to some degree, global trends indicate favourable windows in autumn and winter. Our results demonstrate that resilience is dynamic with respect to space, time and stressors, varying most strongly with: (i) the life history of the seagrass genus and (ii) the duration and timing of the impacting stress.

Long distance biotic dispersal of tropical seagrass seeds by marine mega-herbivores

Terrestrial plants use an array of animals as vectors for dispersal, however little is known of biotic dispersal of marine angiosperms such as seagrasses. Our study in the Great Barrier Reef confirms for the first time that dugongs (Dugong dugon) and green sea turtles (Chelonia mydas) assist seagrass dispersal. We demonstrate that these marine mega-herbivores consume and pass in faecal matter viable seeds for at least three seagrass species (Zostera muelleri, Halodule uninervis and Halophila decipiens). One to two seagrass seeds per g DW of faecal matter were found during the peak of the seagrass reproductive season (September to December), with viability on excretion of 9.13% ± 4.61% (SE). Using population estimates for these mega-herbivores, and data on digestion time (hrs), average daily movement (km h) and numbers of viable seagrass seeds excreted (per g DW), we calculated potential seagrass seed dispersal distances. Dugongs and green sea turtle populations within this region can disperse >500,000 viable seagrass seeds daily, with a maximum dispersal distance of approximately 650 km. Biotic dispersal of tropical seagrass seeds by dugongs and green sea turtles provides a large-scale mechanism that enhances connectivity among seagrass meadows, and aids in resilience and recovery of these coastal habitats.