No abstract

As coastal cities around the world expand, and sea levels and the frequency of storms rise, natural shorelines are steadily being replaced by artificial defences such as seawalls. A growing number of studies have documented the assemblages that inhabit these novel environments, and some have contrasted them against those found in their natural analogues: rocky shores. Most of this work has, however, been conducted in temperate regions, and there is limited research on seawalls in the tropics. To address this, we conducted monthly surveys of adjacent seawall and rocky shores at multiple sites around Singapore for 1 yr. Our results concur with previous temperate studies -artificial seawalls support a lower diversity but share a substantial number of species with rocky shores. Multivariate analyses reveal that assemblage differences were largely driven by species that were found in both habitats (e.g. detritivore Ligia exotica, grazer Monodonta labio and carnivorous whelk Drupella margariticola) but occurred in different abundances. We also conducted (for the first time on seawalls) stable isotope analyses to elucidate the diets of the common species found in both habitats. Turf algae, which were found to be present in significantly lower abundances on seawalls, could possibly contribute substantially to the diets of many dominant herbivores. Future seawall enhancement efforts in the tropics could therefore look into whether enhancing turf algae will improve biodiversity.

Microplastic accumulation on marine macrophytes, such as macroalgae and seagrasses, is a potentially critical but overlooked pathway by which microplastics enter the marine food web. Despite the possible significance of this pathway, few studies have examined the presence of microplastics on macrophytes found in situ. We quantified the density of microplastics found on the surfaces of three species of intertidal seagrasses (Cymodocea rotundata, Cymodocea serrulata and Thalassia hemprichii) and two species of subtidal macroalgae (Padina sp. and Sargassum ilicifolium), and found significantly higher microplastic densities on seagrasses than on macroalgae. However, we found no relationships between microplastic density and epibiont cover in either seagrass or macroalgae. Our study has provided early evidence of microplastics on macrophyte surfaces in situ, being the first such evidence for macroalgae, and the second for seagrasses.

No abstract

The dispersal of seagrasses is important to promoting the resilience and long-term survival of populations. Most of the research on long-distance dispersal to date has focused on sexual propagules while the dispersal of vegetative fragments has been largely overlooked, despite the important role this mechanism might play. In this study, we proposed a conceptual model that categorizes vegetative fragment dispersal into seven fundamental steps: i.e., (i) fragment formation, (ii) transport, (iii) decay, (iv) substrate contact, (v) settlement, (vi) establishment, and (vii) dislodgement. We present two experiments focusing on the final steps of the model from substrate contact to dislodgement in four tropical seagrass species (Cymodocea rotundata, Halophila ovalis, Halodule uninervis, and Thalassia hemprichii), which are critical for dispersed vegetative fragments to colonize new areas. We first conducted a mesocosm experiment to investigate the effect of fragment age and species on settlement (i.e., remains on the substrate in a rising tide) and subsequently establishment (i.e., rooting in substrate) rates. To determine dislodgement resistance of settled fragments, we also subjected fragments under different burial treatments to wave and currents in a flume. We found that both initial settlement and subsequent establishment rates increased with fragment age. H. ovalis was the only species that successfully established within the study period. After settlement, dislodgement resistance depended primarily on burial conditions. Smaller species H. ovalis and H. uninervis were also able to settle more successfully, and withstand higher bed shear stress before being dislodged, compared to the larger species T. hemprichii and C. rotundata. However, the ordinal logistic regressions did not reveal relationships between the tested plant morphometrics and the energy needed for dislodgement (with the exception of C. rotundata), indicating that there are potentially some untested species-specific traits that enable certain species to withstand dislodgement better. We discuss the implication our findings have on the dispersal potential for different species and the conservation of seagrasses. This study represents the first effort toward generating parameters for a bio-physical model to predict vegetative fragment dispersal.