Mike van Keulen

Fluid dynamics in seagrass ecology - from molecules to ecosystems

Fluid dynamics is the study of the movement of fluids. Among other things, it addresses velocity, acceleration, and the forces exerted by or upon fluids in motion (Daugherty et al.. 1985; White. 1999: Kundu and Cohen, 2002). Fluid dynamics affects every aspect of the existence of seagrasses from the smallest to the largest scale: from the nutrients they obtain to the sediment they colonize; from the pollination of their flowers to the import/export of organic matter to adjacent systems; from the light that reaches their leaves to the organisms that live in the seagrass habitats. Therefore, fluid dynamics is of major importance in seagrass biology, ecology, and ecophysiology. Unfortunately, fluid dynamics is often overlooked in seagrass systems (Koch, 2001). This chapter provides a general background in fluid dynamics and then addresses increasingly larger scales of fluid dynamic processes relevant to seagrass ecology and physiology: molecules (μm), leaves and shoots (mm to cm), seagrass canopies (m), sea- grass landscapes (100—1.000 m), and seagrasses as part of the biosphere (>1.000 m). Although gases are also fluids, this chapter is restricted to water (i.e. compressed fluids), how it flows through seagrasses, the forces it exerts on the plants, and the implications that this has for seagrass systems. Seagrasses are not only affected by water in motion, they also affect the currents, waves and turbulence of the water masses surrounding them. This capacity to alter their own environment is referred to as “ecosystem engineering” (Jones et al.. 1994, 1997; Thomas et al., 2000). Readers are also encouraged to consult a recent review by Okubo et al. (2002) for a discussion on flow in terrestrial and aquatic vegetation including freshwater plants, seagrasses, and kelp.

Epiphytes of seagrasses

In all aquatic environments, available surfaces are rapidly colonized by a variety of organisms. If these organisms grow on plants they are called epiphytes. Seagrasses provide an excellent substratum for epiphytic organisms and these organisms are an integral component of seagrass ecosystems. The ecology and physiology of seagrass epiphytes have been reviewed previously (Harlin, 1980; Borowitzka and Lethbridge, 1989) and this chapter focuses primarily on new developments in our understanding of seagrass epiphyte biology and ecology that have occurred since then.

Coral‐associated bacterial communities on Ningaloo Reef, Western Australia

Coral-associated microbial communities from three coral species (Pocillopora damicornis, Acropora tenuis and Favites abdita) were examined every 3 months (January, March, June, October) over a period of 1 year on Ningaloo Reef, Western Australia. Tissue from corals was collected throughout the year and additional sampling of coral mucus and seawater samples was performed in January. Tissue samples were also obtained in October from P. damicornis coral colonies on Rottnest Island off Perth, 1200 km south of Ningaloo Reef, to provide comparisons between coral-microbial associates in different locations. The community structures of the coral-associated microorganisms were analysed using phylogenetic analysis of 16S rRNA gene clone libraries, which demonstrated highly diverse microbial profiles among all the coral species sampled. Principal component analysis revealed that samples grouped according to time and not species, indicating that coral-microbial associations may be a result of environmental drivers such as oceanographic characteristics, benthic community structure and temperature. Tissue samples from P. damicornis at Rottnest Island revealed similarities in bacteria to the samples at Ningaloo Reef. This study highlights that coral-associated microbial communities are highly diverse; however, the complex interactions that determine the stability of these associations are not necessarily dependent on coral host specificity.

Coral-bacterial communities before and after a coral mass spawning event on Ningaloo Reef

Bacteria associated with three coral species, Acropora tenuis, Pocillopora damicornis and Tubastrea faulkneri, were assessed before and after coral mass spawning on Ningaloo Reef in Western Australia. Two colonies of each species were sampled before and after the mass spawning event and two additional samples were collected for P. damicornis after planulation. A variable 470 bp region of the 16 S rRNA gene was selected for pyrosequencing to provide an understanding of potential variations in coral-associated bacterial diversity and community structure. Bacterial diversity increased for all coral species after spawning as assessed by Chao1 diversity indicators. Minimal changes in community structure were observed at the class level and data at the taxonomical level of genus incorporated into a PCA analysis indicated that despite bacterial diversity increasing after spawning, coral-associated community structure did not shift greatly with samples grouped according to species. However, interesting changes could be detected from the dataset; for example, α-Proteobacteria increased in relative abundance after coral spawning and particularly the Roseobacter clade was found to be prominent in all coral species, indicating that this group may be important in coral reproduction.

Seagrass meadows in a globally changing environment

Seagrass meadows are valuable ecosystem service providers that are now being lost globally at an unprecedented rate, with water quality and other localised stressors putting their future viability in doubt. It is therefore critical that we learn more about the interactions between seagrass meadows and future environmental change in the anthropocene. This needs to be with particular reference to the consequences of poor water quality on ecosystem resilience and the effects of change on trophic interactions within the food web. Understanding and predicting the response of seagrass meadows to future environmental change requires an understanding of the natural long-term drivers of change and how these are currently influenced by anthropogenic stress. Conservation management of coastal and marine ecosystems now and in the future requires increased knowledge of how seagrass meadows respond to environmental change, and how they can be managed to be resilient to these changes. Finding solutions to such issues also requires recognising people as part of the social-ecological system. This special issue aims to further enhance this knowledge by bringing together global expertise across this field. The special issues considers issues such as ecosystem service delivery of seagrass meadows, the drivers of long-term seagrass change and the socio-economic consequences of environmental change to seagrass.

Improving mechanical seagrass transplantation

Until recently seagrass transplantation efforts have met with limited success in areas with high wave energies. Survival in Western Australia has been markedly improved by the deployment of large, mechanically transplanted units which provide sufficient anchorage to overcome water motion. ECOSUB1 was an underwater seagrass harvesting and planting machine designed to extract and plant large seagrass units with minimal disturbance. Over 2000 sods have been planted, with an average survival of approximately 70% over 3 years. New machines (ECOSUB2) have now been constructed to improve efficiency; these are located semi-permanently on the seafloor and allow for concurrent seagrass harvesting and planting.

Seasonal variability in sediment distribution along an exposure gradient in a seagrass meadow in Shoalwater Bay, Western Australia

Seagrasses, by blocking water flow as it passes through the leafy canopy, are believed to have a significant impact on sediment dynamics, and the formation of sand and mud banks in some areas. The role of seagrasses in sedimentation processes is poorly understood in high-energy environments, such as those found in south-Western Australia. Studies of sediment size fraction distribution were conducted over a 14-month period within a Posidonia sinuosa meadow, at an exposed and a sheltered site, to investigate the role of seagrass canopies on sediment dynamics. Sediment size analyses, obtained by sieving sediment samples, showed a difference between exposed and sheltered sites, as well as seasonal influences on the data. A three-way ANOVA run on the data indicates a summer and winter pattern superimposed over the sheltered and exposed pattern. This suggests that during the calmer conditions experienced in summer there was an increased proportion of finer grain sizes at the sheltered site, while in winter the grain sizes tended to become coarser, more closely matching the pattern observed at the exposed site. These results suggest that reduced water motion at the sheltered site during summer permitted finer sediment grain sizes to settle out, while increased water motion during winter increased the proportion of coarse grain sizes. At the exposed site this seasonal difference was not observed. It therefore appears that the P. sinuosa canopy reduces flow through a dense meadow, but this effect appears to be modified by overall wave energy, observed to operate seasonally.

Bioturbation by stingrays at Ningaloo Reef, Western Australia

Stingrays are an important part of the biomass of the fishes in shallow, coastal ecosystems, particularly in inter-reefal areas. In these habitats they are thought to be keystone species, responsible for modifying physical and biological habitats through their foraging and predation. However, there have been few attempts to quantify the effects of these animals on benthic environments. Here, we examine the effects of bioturbation by rays on sand flats of the lagoon of Ningaloo Reef WA. At Mangrove Bay we surveyed 15, 10x10 m quadrats during August 2009, September 2010 and February 2011. We recorded all pits that could be identified as due to ray feeding. Of these, 98 were selected randomly to be measured (length, breadth, depth) on a daily basis for a week. Over the 21 day period, a total of 2.01 cubic m of sediment was excavated by ray pits equating to a wet weight of 1,411.3 kg and 2.42% of the total area sampled. Based on these figures, up to 42% of the soft sediments in our study area would be reworked by stingray feeding to an average depth of 5.16 cm over a year. Within the 15 quadrats, new pits were formed at a rate of 0.41/day and then maintained a clear shape for an average of 1.57 days, after which time they could no longer be measured. All evidence of the pit was lost after an average of 3.3 d. In addition to the turnover of sediment and removal of prey, pits created sheltered habitats for a range of organisms, including larval fish, crabs and gastropods. The role of stingrays is compared with that of other organisms that are seen as important ecosystem engineers in soft-sediment environments such as dugongs, crabs and callianasiid shrimps.

Macroalgae Inhibits Larval Settlement and Increases Recruit Mortality at Ningaloo Reef, Western Australia

Globally, many coral reefs are degraded and demonstrate reduced coral cover and increased macroalgal abundance. While negative correlations between macroalgae and coral recruitment have commonly been documented, the mechanisms by which macroalgae affects recruitment have received little attention. Here we examined the effect of macroalgae on larval settlement and the growth and survival of coral recruits, in a field experiment over nine months. Exclusion treatments were used to manipulate herbivory and macroalgal biomass, while settlement tiles measured coral settlement and survival. After nine months the volume of macroalgae was up to 40 times greater in the caged treatments than in controls and the settlement of coral larvae on the undersides of tiles within caged plots was 93% lower than in the uncaged treatments. The growth and survival of coral recruits was also severely reduced in the presence of macroalgae: survival was 79% lower in caged treatments and corals were up to 58% smaller with 75% fewer polyps. These data indicate that macroalgae has an additive effect on coral recruitment by reducing larval settlement and increasing recruit mortality. This research demonstrates that macroalgae can not only inhibit coral recruitment, but also potentially maintain dominance through a positive feedback system.

Linking Seed Photosynthesis and Evolution of the Australian and Mediterranean Seagrass Genus Posidonia

Recent findings have shown that photosynthesis in the skin of the seed of Posidonia oceanica enhances seedling growth. The seagrass genus Posidonia is found only in two distant parts of the world, the Mediterranean Sea and southern Australia. This fact led us to question whether the acquisition of this novel mechanism in the evolution of this seagrass was a pre-adaptation prior to geological isolation of the Mediterranean from Tethys Sea in the Eocene. Photosynthetic activity in seeds of Australian species of Posidonia is still unknown. This study shows oxygen production and respiration rates, and maximum PSII photochemical efficiency (Fv : Fm) in seeds of two Australian Posidonia species (P. australis and P. sinuosa), and compares these with previous results for P. oceanica. Results showed relatively high oxygen production and respiratory rates in all three species but with significant differences among them, suggesting the existence of an adaptive mechanism to compensate for the relatively high oxygen demands of the seeds. In all cases maximal photochemical efficiency of photosystem II rates reached similar values. The existence of photosynthetic activity in the seeds of all three species implicates that it was an ability probably acquired from a common ancestor during the Late Eocene, when this adaptive strategy could have helped Posidonia species to survive in nutrient-poor temperate seas. This study sheds new light on some aspects of the evolution of marine plants and represents an important contribution to global knowledge of the paleogeographic patterns of seagrass distribution.