An integrated study of the temporal and spatial variation in the supply of propagules, recruitment and assemblages of intertidal macroalgae on a wave-exposed rocky coast, Victoria, Australia

Recruitment is known to influence distributions and abundances of benthic marine organisms. It is therefore important to document patterns of variability in recruitment and how these relate to patterns in established assemblages. This study provides an integrated assessment of the temporal and spatial variation in supply and recruitment of propagules and established populations of several macroalgae. Propagules in water samples from two stages of the incoming tide, recruitment to artificial substrata and percentage cover of species established on the shore were recorded every 2 months from December 1994 to October 1995, in two zones of an intertidal, wave-exposed rocky shore. Variability in recruitment was measured at three spatial scales: 10s cm, 100s cm and 100s m. Availability and recruitment of most taxa were greatest between April and August, although many species had available propagules and recruited throughout the year. Temporal variation in the established assemblages was, however, more species-specific. Differences in established assemblages between zones were reflected in differences in availability and recruitment of propagules between zones. Recruitment could not be predicted directly from supply of propagules, but the two processes were linked. For most species, the greatest variation in recruitment occurred at the smallest spatial scale of 10s cm, although there was also considerable large-scale (between site) variation in recruitment of several species. Results indicate that while pre-and post-settlement mortality are likely to influence macroalgal distribution and abundance, the temporal and spatial variability in supply and recruitment of propagules can explain much of the patchiness in macroalgal assemblages.

Can macroalgae contribute to blue carbon? An Australian perspective

Macroalgal communities in Australia and around the world store vast quantities of carbon in their living biomass, but their prevalence of growing on hard substrata means that they have limited capacity to act as long-term carbon sinks. Unlike other coastal blue carbon habitats such as seagrasses, saltmarshes and mangroves, they do not develop their own organic-rich sediments, but may instead act as a rich carbon source and make significant contributions in the form of detritus to sedimentary habitats by acting as a “carbon donor” to “receiver sites” where organic material accumulates. The potential for storage of this donated carbon however, is dependent on the decay rate during transport and the burial efficiency at receiver sites. To better understand the potential contribution of macroalgal communities to coastal blue carbon budgets, a comprehensive literature search was conducted using key words, including carbon sequestration, macroalgal distribution, abundance and productivity to provide an estimation of the total amount of carbon stored in temperate Australian macroalgae. Our most conservative calculations estimate 109.9 Tg C is stored in living macroalgal biomass of temperate Australia, using a coastal area covering 249,697 km2. Estimates derived for tropical and subtropical regions contributed an additional 23.2 Tg C. By extending the search to include global studies we provide a broader context and rationale for the study, contributing to the global aspects of the review. In addition, we discuss the potential role of calcium carbonate-containing macroalgae, consider the dynamic nature of macroalgal populations in the context of climate change, and identify the knowledge gaps that once addressed will enable robust quantification of macroalgae in marine biogeochemical cycling of carbon. We conclude that macroalgal communities have the potential to make ecologically meaningful contributions toward global blue carbon sequestration, as donors, but given that the fate of detached macroalgal biomass remains unclear, further research is needed to quantify this contribution.

Recovery of omega-3 profiles of cultivated abalone by dietary macroalgae supplementation

Formulated feeds for cultivated abalone have received considerable attention to improve the feeding costs, efficiency and productivity of the growing cultivated abalone industry in Australia. Less attention has been given to the resulting quality of the seafood product from formulated feeds and the effect of the reduction of marine ingredients in these feeds. Marine n-3 fatty acids are particularly important nutritional factors of seafood that are lacking from feed ingredients at the base of the aquaculture production chain. Considering that this important category of nutrients is already deficient in the western diet, further erosion through cultivated seafood products is of concern. This study tested the effect of three diet categories on the fatty acid profiles of commercially farmed abalone. Two commercial formulated feeds, Ulva spp. macroalgae, and combinations of the formulated feeds and macroalgae were fed to six replicate baskets of 20 abalone (~62 mm) for 12 months in Victoria, Australia. The macroalgae diets represented overall lower fatty acid content, including eicosapentaenoic (EPA) and docosahexaenoic (DHA), but higher relative amounts of alpha linolenic acid (ALA) and docosapentaenoic acid (DPA), than the formulated feeds. The total lipid content of abalone tissue did not vary substantially across diet categories; however, the ratio of n-6:n-3 fatty acids decreased incrementally with the increasing macroalgae content in the diet and by a factor of two with a 100 % macroalgae diet. Also, equal or higher content of all important long chain n-3 fatty acids was achieved with a macroalgae diet despite the lower dietary content of some of these fatty acids. Marine fatty acid-deplete aquaculture feeds can result in decreased n-3 fatty acid content in the abalone tissue. The inclusion of macroalgae in the diet of abalone can offset this reduction.

Variation in gametophyte dominance in populations of Chondrus verrucosus (Gigartinaceae, Rhodophyta)

We describe the abundance, including spatial and temporal variability, of phases of the isomorphic Chondrus verrucosus Mikami from Japan. Chondrus verrucosus occurred in a dense (∼90% cover) and temporally stable bed on a small, isolated rocky outcrop (Oyakoiwa) in Shizuoka Prefecture. Small vegetative fronds were always much more abundant than large vegetative and fertile fronds over the spring to late summer periods in 1999 and 2000. Over the same period, fertile carposporophytic fronds were generally more abundant than fertile tetrasporophytic fronds, and fertile male fronds appeared infrequently at low densities. Using the resorcinol-acetal test, we determined the proportion of gametophytes and tetrasporophytes in three populations of C. verrucosus: Oyakoiwa and Noroshi (Shizuoka) in the summers of 1999 and 2000 and Kamehana Point (Miyagi) in autumn 2000. All populations had a significantly higher proportion of gametophytes than tetrasporophytes in both years, although gametophytic proportions were lower at Noroshi (∼70%) than at Oyakoiwa (∼80%) and Kamehana Point (∼97%). However, examination of all isolated individuals sampled on Noroshi showed equal proportions of each phase in 1999, but gametophyte dominance (74%) in 2000. Differences in dispersal and spore production between phases are discussed as mechanisms potentially contributing to variation in gametophyte dominance.

Dispersal of Hormosira banksii (phaeophyceae) via detached fragments : reproductive viability and longevity

Drifting, fertile thalli are well documented to be the primary long-distance dispersal vector for many marine macroalgae, but little information about reproductive viability of drift is known. This study examined the reproductive viability and longevity of floating fragments of the intertidal Australasian fucoid Hormosira banksii (Turner) Decne. Beach wrack surveys and field experiments were conducted to test the model that long-distance dispersal is achieved in H. banksii via floating, fertile fronds. High densities of beach wrack fragments were evident during summer compared to autumn. The majority of beach wrack occurred on sandy beaches rather than rocky shores. Both male and female fragments were present in the beach wrack. Detached fronds were capable of releasing gametes up to 8 weeks after detachment. Beach wrack produced high fertilization rates and recruited successfully onto artificial panels. Results suggest that detached fragments are reproductively viable and that floating, fertile fronds may be an important mechanism for facilitating long-distance dispersal in this species. Nevertheless, the frequency of fronds reaching a suitable habitat and contributing to gene flow between populations, or colonizing new populations, may not be proportional to the total density of beach wrack.

Dislodgment and attachment strength of the intertidal macroalga Hormosira banksii (Fucales, Phaeophyceae)

Hormosira banksii is a dominant intertidal alga characterised by limited dispersal of propagules, yet it has a broad distribution throughout temperate Australasia. However, the high abundance of beach wrack of H. banksii on sandy beaches along southwest Victoria, the frequency of frond dislodgment and the force required to break it from the substratum remain unknown. Dislodgment of macroalgae has been shown to influence dispersal processes; therefore, we tested the model that long-distance dispersal of H. banksii is facilitated by weak attachment to the substratum and frequent dislodgment of fertile fronds. We monitored the dislodgment of H. banksii individuals and conducted in situ pull tests to determine the attachment strength of fronds of H. banksii. We further tested whether thallus size or pull direction influenced attachment strength of H. banksii. We found that breakage of vesicles and fronds was a regular event during the survey period and that the mean attachment strength of fronds of H. banksii was weak compared to other intertidal algae. Furthermore, we found that thallus size did not influence the force required to break fronds of H.banksii from the substratum but that the direction that fronds were pulled from the substratum did. We suggest that fronds of H. banksii do drift frequently and that long-distance dispersal is likely to be an important mechanism for the distributional success of this species.

Comparing towed and baited underwater video techniques for assessing temperate marine fishes

Accurate estimates of fish species occurrence are important to any species’ assessments and distribution model. With increasing emphasis on nondestructive sampling, underwater video techniques are commonly used without a thorough understanding of their advantages and disadvantages. This study compared data collected from baited remote underwater stereo-video systems (stereo BRUVS) and towed-video systems to determine; (1) the differences between these video techniques in terms of fish assemblages, functional groups (i.e. pelagic carnivore, epibenthic carnivore/omnivore or herbivore) and observability (i.e. conspicuous or cryptic), and (2) what impact do these two techniques have on the interpretation of spatially-explicit, predictive models. We found stereo BRUVS and towedvideo techniques recorded very different assemblages, functional groups and observability categories across structurally complex benthic biological habitats (i.e. macroalgae dominated habitats). However, as the habitat complexity became less (e.g. seagrass and areas with no visible macro-biota) both techniques appeared to provide similar fish assemblage information. We also found considerable differences in the predicted extents of habitat suitability between the two video techniques.

Trophic cul-de-sac, Pyrazus ebeninus, limits trophic transfer through an estuarine detritus-based food web

The importance to food-webs of trophic cul-de-sacs, species that channel energy flow away from higher trophic levels, is seldom considered outside of the pelagic systems in which they were first identified. On intertidal mudflats, inputs of detritus from saltmarshes, macroalgae or microphytobenthos are generally regarded as a major structuring force underpinning food-webs and there has been no consideration of trophic cul-de-sacs to date. A fully orthogonal three-factor experiment manipulating the density of the abundant gastropod, Pyrazus ebeninus, detritus and macrobenthic predators on a Sydney mudflat revealed large deleterious effects of the gastropod, irrespective of detrital loading or the presence of predators. Two months after experimental manipulation, the standing-stock of microphytobenthos in plots with high (44 per m²) densities of P. ebeninus was 20% less than in plots with low (4 per m²) densities. Increasing densities of P. ebeninus from low to high halved the abundance of macroinvertebrates and the average number of species. In contrast, the addition of detritus had differing effects on microphytobenthos (positively affected) and macroinvertebrates (negatively affected). Over the two-months of our experiment, no predatory mortality of P. ebeninus was observed and high densities of P. ebeninus decreased impacts of predators on macroinvertebrate abundances. Given that the dynamics of southeast Australian mudflats are driven more by disturbance than seasonality in predators and their interactions with prey, it is likely that Pyrazus would be similarly resistant to predation and have negative effects on benthic assemblages at other times of the year, outside of our study period. Thus, in reducing microphytobenthos and the abundance and species richness of macrofauna, high abundances of the detritivore P. ebeninus may severely limit the flow of energy up the food chain to commercially-important species. This study therefore suggests that trophic cul-de-sacs are not limited to the eutrophied pelagic systems in which they were first identified, but may exist in other systems as well.

Movement patterns of green turtles in Brazilian coastal waters described by satellite tracking and flipper tagging

The movements of 8 green turtles Chelonia mydas in Brazilian coastal waters were tracked using transmitters linked to the Argos system for periods of between 1 and 197 d. These were the first tracking data gathered on juveniles of this species in this important foraging ground. Information was integrated with that collected over a decade using traditional flipper-tagging methods at the same site. Both satellite telemetry and flipper tagging suggested that turtles undertook 1 of 3 general patterns of behaviour: pronounced long range movements (>100 km), moderate range movements (<100 km) or extended residence very close to the capture/release site. There seemed to be a general tendency for the turtles recaptured/tracked further afield to have been among the larger turtles captured. Satellite tracking of 5 turtles which moved from the release site showed that they moved through coastal waters; a factor which is likely to predispose migrating turtles to incidental capture as a result of the prevailing fishing methods in the region. The movements of the 3 turtles who travelled less than 100 km from the release site challenge previous ideas relating to home range in green turtles feeding in sea grass pastures. We hypothesise that there may be a fundamental difference in the pattern of habitat utilisation by larger green turtles depending on whether they are feeding on seagrass or macroalgae. Extended tracking of 2 small turtles which stayed near the release point showed that small juvenile turtles, whilst in residence in a particular feeding ground, can also exhibit high levels of site-fidelity with home ranges of the order of several square kilometers.

Implications of climate change for macrophytic rafts and their hitchhikers

Most models predicting changes to species distributions under future climate scenarios ignore dispersal processes, despite their importance in determining community structure in both terrestrial and aquatic systems ('supply-side ecology'). In the marine environment, facilitation of long-distance dispersal of coastal organisms by macrophytic rafts may be severely modified by climate impacts on raft supply, quality, and persistence, and on transport processes. Increasing storminess in the coastal zone, higher water temperatures, and changes in water circulation represent some of the key mechanisms that will directly affect rafts, while increases in herbivore metabolism due to higher water temperatures are likely to indirectly reduce raft longevity through raft consumption. Accurate predictions of climate impacts on coastal biodiversity will be con - tingent on resolution of factors influencing rafting so that this and other dispersal mechanisms can be incorporated into species distribution models. © 2011 Inter-Research.

Habitat classification of temperate marine macroalgal communities using bathymetric LiDAR

Here, we evaluated the potential of using bathymetric Light Detection and Ranging (LiDAR) to characterise shallow water (<30 m) benthic habitats of high energy subtidal coastal environments. Habitat classification, quantifying benthic substrata and macroalgal communities, was achieved in this study with the application of LiDAR and underwater video groundtruth data using automated classification techniques. Bathymetry and reflectance datasets were used to produce secondary terrain derivative surfaces (e.g., rugosity, aspect) that were assumed to influence benthic patterns observed. An automated decision tree classification approach using the Quick Unbiased Efficient Statistical Tree (QUEST) was applied to produce substrata, biological and canopy structure habitat maps of the study area. Error assessment indicated that habitat maps produced were primarily accurate (>70%), with varying results for the classification of individual habitat classes; for instance, producer accuracy for mixed brown algae and sediment substrata, was 74% and 93%, respectively. LiDAR was also successful for differentiating canopy structure of macroalgae communities (i.e., canopy structure classification), such as canopy forming kelp versus erect fine branching algae. In conclusion, habitat characterisation using bathymetric LiDAR provides a unique potential to collect baseline information about biological assemblages and, hence, potential reef connectivity over large areas beyond the range of direct observation. This research contributes a new perspective for assessing the structure of subtidal coastal ecosystems, providing a novel tool for the research and management of such highly dynamic marine environments. © 2014 by the authors; licensee MDPI, Basel, Switzerland.

Forests of the sea: predictive habitat modelling to assess the abundance of canopy forming kelp forests on temperate reefs

 Large brown seaweeds (kelps) form forests in temperate and boreal marine systems that serve as foundations to the structure and dynamics of communities. Mapping the distributions of these species is important to understanding the ecology of coastal environments, managing marine ecosystems (e.g., spatial planning), predicting consequences of climate change and the potential for carbon production. We demonstrate how combining seafloor mapping technologies (LiDAR and multibeam bathymetry) and models of wave energy to map the distribution and relative abundance of seaweed forests of Ecklonia radiata can provide complete coverage over hundreds of square kilometers. Using generalized linear mixed models (GLMMs), we associated observations of E. radiata abundance from video transects with environmental variables. These relationships were then used to predict the distribution of E. radiata across our 756.1km2 study area off the coast of Victoria, Australia. A reserved dataset was used to test the accuracy of these predictions. We found that the abundance distribution of E. radiata is strongly associated with depth, presence of rocky reef, curvature of the reef topography, and wave exposure. In addition, the GLMM methodology allowed us to adequately account for spatial autocorrelation in our sampling methods. The predictive distribution map created from the best GLMM predicted the abundance of E. radiata with an accuracy of 72%. The combination of LiDAR and multibeam bathymetry allowed us to model and predict E. radiata abundance distribution across its entire depth range for this study area. Using methods like those presented in this study, we can map the distribution of macroalgae species, which will give insight into ecological communities, biodiversity distribution, carbon uptake, and potential sequestration.