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Seagrasses need dissolved nutrients to maintain their productivity through uptake processes, from substrate pore-water via their roots and/or from the water column via their leaves. Here, we present the first study of exchanges of dissolved nutrients between pore water and the water column in the vicinity of seagrass canopies. We address the following research questions, using a laboratory flume experiment: (1) How does solute exchange between the water column and substrate pore water vary spatially within seagrass patches? (2) How does seagrass leaf length affect this solute exchange? (3) How does the measured rate of solute exchange compare with seagrasses' rates of uptake of dissolved nutrients? Our results indicate that solute intrusion from the water column into the substrate pore water is highest in the area around seagrass patches' leading edges, where flow deceleration is strongest, and decreases approximately linearly with downstream distance into the patch. The decrease in measured flow speed in the canopy fits well the predictions of previously reported models of arrays of rigid obstacles. The length of the region in which the concentration of solute that has infiltrated into the substrate at the upstream end of the seagrass patches is similar to the length scale predicted from model estimates of infiltration rate (based on the substrate permeability) and the length of time over 24-h runs. We conclude that the mechanism we identify only pertains near canopy edges, and therefore that other mechanisms must govern nutrient supply in the interior of seagrass meadows. [ABSTRACT FROM AUTHOR]

Abstract: In remote, tropical areas human influences increase, potentially threatening pristine seagrass systems. We aim (i) to provide a bench-mark for a near-pristine seagrass system in an archipelago in East Kalimantan, by quantifying a large spectrum of abiotic and biotic properties in seagrass meadows and (ii) to identify early warning indicators for river sediment and nutrient loading, by comparing the seagrass meadow properties over a gradient with varying river influence. Abiotic properties of water column, pore water and sediment were less suitable indicators for increased sediment and nutrient loading than seagrass properties. Seagrass meadows strongly responded to higher sediment and nutrient loads and proximity to the coast by decreasing seagrass cover, standing stock, number of seagrass species, changing species composition and shifts in tissue contents. Our study confirms that nutrient loads are more important than water nutrient concentrations. We identify seagrass system variables that are suitable indicators for sediment and nutrient loading, also in rapid survey scenarios with once-only measurements. [Copyright &y& Elsevier]

Seagrasses comprise 78 species and are rarely invasive. But the seagrass Halophila stipulacea , firstly recorded in the Caribbean in the year 2002, has spread quickly throughout the region. Previous works have described this species as invasive in the Caribbean, forming dense mats that exclude native seagrass species. During a reconnaissance field survey of Caribbean seagrass meadows at the islands of Bonaire and Sint Maarten in 2013, we observed that this species was only extremely dense at 5 out of 10 studied meadows. Compared to areas with sparse growth of H. stipulacea , these dense meadows showed consistently higher nutrient concentrations, as indicated by higher leaf tissue N contents of the seagrass Thalassia testudinum (dense when C:N < 22.5) and sediments (dense when %N > 11.3). Thus, the potential invasiveness of this non-native seagrass most likely depends on the environmental conditions, especially the nutrient concentrations. [ABSTRACT FROM AUTHOR]

Abstract: Seagrasses have declined at a global scale due to light reduction and toxicity events, caused by eutrophication and increased sediment loading. Although several studies have tested effects of light reduction and toxicants on seagrasses, there is at present no information available on their interacting effects. In a full-factorial 5-day laboratory experiment, we studied short-term interactive effects of light conditions, pH and reduced nitrogen (NH x ) in the water layer, mimicking pulses of river discharge, on the tropical early successional species Halodule uninervis and the late successional species Thalassia hemprichii. In contrast to recent results reported for the temperate species Zostera marina, increased NH x supply did not affect leaf mortality or photochemical efficiency in H. uninervis and in 7 out of 8 treatments for T. hemprichii. However, both tropical species demonstrated striking differences in nitrogen accumulation, free amino acid composition and free NH3 accumulation. The increase in tissue nitrogen content was two times higher for H. uninervis than for T. hemprichii. Nitrogen stored as free amino acids (especially asparagine) only increased in H. uninervis. High pH only affected T. hemprichii, but only when not shaded, by doubling its free NH3 concentrations, concomitantly decreasing its photosynthetic efficiency. Our results indicate that the early successional H. uninervis has higher tolerance to high NH x loads as compared to the late successional T. hemprichii. H. uninervis was better able to avoid toxic internal NH x levels by further assimilating glutamine into asparagine in contrast to T. hemprichii. Moreover, both tropical species seem to cope much better with high NH x than the temperate Z. marina. The implications for the distribution and succession of seagrass species under high nutrient loads are discussed. [Copyright &y& Elsevier]

An important ecosystem service of tropical coastal vegetation including seagrass beds and mangrove forests is their ability to accumulate carbon. Here we attempt to establish the driving forces for the accumulation of surface organic carbon in southern Thailand coastal systems. Across 12 sites we found that in line with expectations, seagrass beds (0.6 ± 0.09%) and mangrove forests (0.9 ± 0.3%) had higher organic carbon in the surface (top 5 cm) sediment than un-vegetated mudflats (0.4 ± 0.04%). Unexpectedly, however, mangrove forests in this region retained organic carbon, rather than outwell it, under normal tidal conditions. No relationship was found between organic carbon and substrate grain size. The most interesting finding of our study was that climax and pioneer seagrass species retained more carbon than mixed-species meadows, suggesting that plant morphology and meadow characteristics can be important factors in organic carbon accumulation. Insights such as these are important in developing carbon management strategies involving coastal ecosystems such as offsetting of carbon emissions. The ability of tropical coastal vegetation to sequester carbon is an important aspect for valuing the ecosystems. Our results provide some initial insight into the factors affecting carbon sequestration in these ecosystems, but also highlight the need for further research on a global scale. [ABSTRACT FROM AUTHOR]

While attention in coastal ecosystem restoration has increased over the last two decades, the success rate of efforts remains relatively low. To increase success rates, physical restoration techniques often utilize supporting or protective materials to provide a stable surface for transplantation, and in some cases reduce herbivory and hydrodynamic disturbances. In this study, we evaluated the effectiveness of traditional (staples, burlap) and novel (BESE- elements, a biodegradable potato starch lattice) physical restoration techniques on the growth of transplanted Halodule wrightii seagrass. A first experiment revealed that seagrass planted in both two-stacked BESE structure without planting holes and four-stacked BESE with holes had significantly higher shoot count and blade length than four-stacked BESE without holes, with the latter design losing all seagrass shortly after deployment as shoots could not float through the structure. In a second experiment, the BESE lattice treatment (four-stacked with holes) had three times the shoot count and equal to greater blade length compared to traditional methods of physical restoration (staples and burlap), likely due to BESE providing some protection from hydrodynamic activity. However, disturbances, possibly including herbivory and hydrodynamic activity (culminating with Hurricane Irma), prevented long term study, illustrating the importance of stochastic abiotic factors in seagrass planting success. Overall our study demonstrates the effectiveness of using BESE lattice designs and similar physical techniques in the restoration of seagrass beds. • Seagrass restoration is expensive and often unsuccessful. • Traditional and novel physical restoration techniques were compared. • Biodegradable starch (BESE) lattices yielded better growth. • BESE may improve restoration in hydrodynamically active environments. [ABSTRACT FROM AUTHOR]

Abstract: Although several recent studies point at the importance of seagrass roots for nutrient acquisition in oligotrophic tropical ecosystems, remarkably little is known about the root architecture of tropical seagrasses. The present study provides a detailed description of the root architecture of six seagrass species that were extracted from three sites differing in sediment type and nutrient availability (i.e., except for one species that was not present at one of the sites). Number of roots per node, order of root branching, length and diameter per root order, root hair density and length were determined and used to calculate a topological index for the different species and habitats. Root architecture differed strongly between species. The relatively long-lived and slowly-growing species Thalassia hemprichii and Enhalus acoroides were characterised by short internodes with relatively few unbranched roots per node and a high root hair density. More fast growing species such as Cymodocea rotundata, Cymodocea serrulata and Syringodium isoetifolium had many roots per node, the majority of which were branched. Surprisingly, differences in sediment type and nutrient availability, as present between locations, had little effect on root architecture. We expect this to be due to a relatively homogeneous nutrient distribution at the small scale within water saturated sediments. Overall, all seagrass species had relatively simple branching (topological index > 0.7), comparable to angiosperms of the low temperate salt marsh. We speculate that relatively simple root architecture of plants in flooded systems reflects the need for a minimal path length for oxygen transport from shoots to roots. [Copyright &y& Elsevier]