Restoration depth mediates seasonal recalcitrant carbon retention capacity in tropical seagrass meadows

Summary There is an ongoing world‐wide decline of seagrass ecosystems, one of the world's most efficient carbon sink habitats. In spite of this, there is a clear lack of studies experimentally testing the effects of anthropogenic disturbances on carbon sequestration of seagrass systems. We assessed the effects of two disturbances of global concern on the carbon sink function in a five‐month in situ experiment within a tropical seagrass (Thalassia hemprichii) meadow by testing the impacts of shading and simulated grazing at two levels of intensity using shading cloths and clipping of shoot tissue. We measured the effects of these disturbances on the carbon sequestration process by assessing the net community production (NCP), carbon and nitrogen content in tissue biomass, and organic matter and THAA (total hydrolysable amino acids) in the sediment down to 40 cm depth. Treatments of high‐intensity shading and high‐intensity clipping were similarly impacted and showed a significantly lower NCP and carbon content in the below‐ground biomass compared to the seagrass control. No significant effects were seen in organic carbon, total nitrogen, C:N ratio and THAA in the sediment for the seagrass treatments. However, both clipping treatments showed different depth profiles of carbon and THAA compared to the seagrass control, with lower organic carbon and THAA content in the surface sediment. This can be explained by the clipping of shoot tissue causing a less efficient trapping of allochthonous carbon and reduced input of shredded seagrass leaves to the detritus sediment layer. In the clipping plots, erosion of the surface sediment occurred, which was also most likely caused by the removal of above‐ground plant biomass. Synthesis. Our findings show that during the course of this experiment, there were no impacts on the sedimentary carbon while the high‐intensity disturbances caused a clear depletion of carbon biomass and reduced the seagrass meadow's capacity to sequester carbon. From a long‐term perspective, the observed effect on the carbon biomass pool in the high‐intensity treatments and the sediment erosion in the clipping plots may lead to loss in sedimentary carbon.

Coastal vegetative habitats are known to be highly productive environments with a high ability to capture and store carbon. During disturbance this important function could be compromised as plant photosynthetic capacity, biomass, and/or growth are reduced. To evaluate effects of disturbance on CO2 capture in plants we performed a five-month manipulative experiment in a tropical seagrass (Thalassia hemprichii) meadow exposed to two intensity levels of shading and simulated grazing. We assessed CO2 capture potential (as net CO2 fixation) using areal productivity calculated from continuous measurements of diel photosynthetic rates, and estimates of plant morphology, biomass and productivity/respiration (P/R) ratios (from the literature). To better understand the plant capacity to coping with level of disturbance we also measured plant growth and resource allocation. We observed substantial reductions in seagrass areal productivity, biomass, and leaf area that together resulted in a negative daily carbon balance in the two shading treatments as well as in the high-intensity simulated grazing treatment. Additionally, based on the concentrations of soluble carbohydrates and starch in the rhizomes, we found that the main reserve sources for plant growth were reduced in all treatments except for the low-intensity simulated grazing treatment. If permanent, these combined adverse effects will reduce the plants’ resilience and capacity to recover after disturbance. This might in turn have long-lasting and devastating effects on important ecosystem functions, including the carbon sequestration capacity of the seagrass system.

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 382:41-47 (2009) - DOI: https://doi.org/10.3354/meps07973 Seagrass photosynthesis controls rates of calcification and photosynthesis of calcareous macroalgae in a tropical seagrass meadow I. Sware Semesi1,2,3,*, Sven Beer1,4, Mats Björk1,3 1Institute of Marine Sciences, University of Dar es Salaam, PO Box 668, Zanzibar, Tanzania 2Department of Aquatic Resources and Conservation, University of Dar es Salaam, Box 35064, Dar es Salaam, Tanzania 3Botany Department, Stockholm University, 10691 Stockholm, Sweden 4Department of Plant Sciences, Tel Aviv University, Tel Aviv 69978, Israel *Email: sware@udsm.ac.tz ABSTRACT: Diel fluctuations in seawater pH can be >1 pH unit (7.9 to >8.9) in the seagrass meadows of Chwaka Bay (Zanzibar, Tanzania). The high daily pH values are generated by the photosynthetic activity of the bay’s submerged seagrasses and macroalgae, and maintained by the relatively low, tide-dominated, water exchange rate. Since pH in principle can affect rates of both calcification and photosynthesis, we investigated whether diel variations in pH caused by photosynthesis could affect rates of calcification and photosynthesis of the calcareous red (Hydrolithon sp. and Mesophyllum sp.) and green (Halimeda renschii) algae growing within these meadows. This was done by measuring rates of calcification and relative photosynthetic electron transport (rETR) of the algae in situ in open-bottom incubation cylinders either in the natural presence of the rooted seagrasses or after the leaves had been removed. The results showed that seagrass photosynthesis increased the seawater pH within the cylinders from 8.3–8.4 to 8.6–8.9 after 2.5 h (largely in conformity with that of the surrounding seawater), which, in turn, enhanced the rates of calcification 5.8-fold for Hydrolithon sp. and 1.6-fold for the other 2 species. The rETRs of all algae largely followed the irradiance throughout the day and were (in Mesophyllum sp.) significantly higher in the presence of seagrasses despite the higher pH values generated by the latter. We conclude that algal calcification within seagrass meadows such as those of Chwaka Bay is considerably enhanced by the photosynthetic activity of the seagrasses, which in turn increases the seawater pH. KEY WORDS: Calcareous algae · Calcification · Halimeda sp. · Hydrolithon sp. · Mesophyllum sp. · Photosynthesis · Halimeda renschii Full text in pdf format PreviousNextCite this article as: Semesi IS, Beer S, Björk M, (2009) Seagrass photosynthesis controls rates of calcification and photosynthesis of calcareous macroalgae in a tropical seagrass meadow. Mar Ecol Prog Ser 382:41-47. https://doi.org/10.3354/meps07973Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 382. Online publication date: April 30, 2009 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2009 Inter-Research.

Methane (CH4) is a potent greenhouse gas that reduces the carbon sequestration capacity of seagrass meadows. However, our understanding of CH4 production and emission from these important carbon sinks is limited. Here we conducted biogeochemical experiments to identify methylotrophic methanogenesis as the primary methanogenesis pathway in a tropical seagrass meadow. The production rate constant was 2.2–3.9 d−1, significantly higher than those in temperate meadows (<0.02 d−1). The CH4 emission rate from the meadow was 835 ± 124 μmol m−2 d−1. A global meta‐analysis further revealed the CH4 emission rate in tropical seagrass meadows is approximately 8 times higher than temperate meadows. Global seagrass meadows emit 0.3 Tg CH4 yr−1, with over 90% from tropical meadows and resulting in an 8.6% reduction of the seagrass carbon burial on a 20‐year time horizon. These findings highlight the role of tropical seagrass meadows as CH4 sources, which can offset the benefits of seagrass carbon sequestration.

In the tropical ecosystem, sea cucumbers are associated with seagrass meadows in various ways, often forming a network of ecological interactions. From this myriad of interactions, the trophic relationship between the seagrasses and sea cucumbers has received recent attention with the advent of analytical techniques. However, little is understood about the exact mechanism by which seagrasses are sustaining the sea cucumber populations in the food chain, considering the high number of refractory components in seagrasses and the lack of digestive enzymes among sea cucumbers. This manuscript aims to review existing concepts in ecology concerning the association between tropical seagrasses and sea cucumbers to provide directions for research and management of this vital resource. We searched literature from electronic databases and identified key concepts concerning sea cucumber and seagrass communities based on geographic distribution, nutrient compositions, seagrass decomposition process, and trophic enrichments in the food chain. A conceptual model was then developed detailing the factors influencing the association between the seagrass meadows and sea cucumbers. Despite the limited published information on the seagrass–sea cucumber association, a synthesis of the current understanding of this topic is provided to address the declining sea cucumber populations in the tropical seagrass meadows. We suggest that the successful restoration of sea cucumber fisheries requires a thorough understanding of the seagrass decomposition process, which is vital to the diet of sea cucumbers.

The influence of quantitative and qualitative aspects of habitat complexity in tropical sea-grass meadows

Shrimp burrow in tropical seagrass meadows: An important sink for litter

Decade changes of the food web structure in tropical seagrass meadow: Implication of eutrophication effects

Shading and simulated grazing increase the sulphide pool and methane emission in a tropical seagrass meadow

Seagrass meadows support key ecosystem services, via provision of food directly for herbivores, and indirectly to their predators. The importance of herbivores in seagrass meadows has been well-documented, but the links between food webs and ecosystem services in seagrass meadows have not previously been made explicit. Herbivores interact with ecosystem services – including carbon sequestration, cultural values, and coastal protection. Interactions can be positive or negative and depend on a range of factors including the herbivore identity and the grazing type and intensity. There can be unintended consequences from management actions based on a poor understanding of trade-offs that occur with complex seagrass-herbivore interactions. Tropical seagrass meadows support a diversity of grazers spanning the meso-, macro-, and megaherbivore scales. We present a conceptual model to describe how multiple ecosystem services are influenced by herbivore pressure in tropical seagrass meadows. Our model suggests that a balanced ecosystem, incorporating both seagrass and herbivore diversity, is likely to sustain the broadest range of ecosystem services. Our framework suggests the pathway to achieve desired ecosystem services outcomes requires knowledge on four key areas: (1) how size classes of herbivores interact to structure seagrass; (2) desired community and management values; (3) seagrass responses to top–down and bottom–up controls; (4) the pathway from intermediate to final ecosystem services and human benefits. We suggest research should be directed to these areas. Herbivory is a major structuring influence in tropical seagrass systems and needs to be considered for effective management of these critical habitats and their services.

What lies beneath: Why knowledge of belowground biomass dynamics is crucial to effective seagrass management

Seagrass meadows deliver a range of ecosystem services, where one of the more important is the capacity to store carbon and serve as sinks for atmospheric carbon dioxide. The capacity of seagrass meadows for carbon storage might, however, be modified and complicated by several factors; one important factor is the possible effects of calcification within the meadows. In tropical areas, seagrass meadows can contain high proportions of calcareous organisms, which through their calcification may cause release of CO2. To study this aspect of the CO2 balance within tropical seagrass systems, we investigated the air-water CO2 flux in seagrass mesocosms with different plant community compositions, i.e. mixtures of seagrass and calcifying macroalgae, having similar overall photosynthetic oxygen evolution rates. The measured CO2 fluxes changed both in rate and direction over the day and were significantly related to plant community composition. Downward fluxes of CO2 were found only over vegetation with high proportion of seagrass and in the afternoon, whereas occurrence of calcifying algae appeared to reverse the flow. A partial least squares (PLS) regression model indicated that pH, pCO2 and dissolved inorganic carbon (DIC) were the primary environmental variables predicting the CO2 fluxes. Our findings show that algal calcification might partly counteract the carbon sequestration in seagrass meadows.

Benthic bacteria, in particular those existing in seagrass rhizosphere, play pivotal roles in supporting the growth and health of their hosts and also in nutrient cycling. Abundant (AT, relative abundance ≥ 0.05%) and rare (RT, relative abundance ≤ 0.001%) taxa reflect two distinct species pools in bacterial communities that differ in their structure and function and are assembled by different ecological processes. However, the mechanisms and factors controlling their spatial β-diversity patterns and ecological assembly are least understood in tropical seagrasses compared to their temperate counterparts. As rhizospheric effect vary between single and mixed plant communities, we examined AT and RT in both mono- and mixed species seagrass meadows and compared them with bulk (un-vegetated) sediments in a tropical coastal lagoon, Chilika (India). Results showed that the β-diversity (Bray-Curtis dissimilarity) of the AT and RT differed across seagrass meadows. RT exhibited a much stronger decay in community similarity with increasing spatial distance between samples than the AT. Spatial variation in RT was driven almost entirely by species turnover, whereas in AT both nestedness and turnover components played an important role. All AT were habitat generalists with broader niche breadth and environmental tolerances, while the majority of RT (66%) were specialists possessing narrower niche breadth and lower environmental tolerances. Stochastic processes (mostly dispersal limitation, 70.65-89.71%) contributed to the assembly of AT in both seagrass and bulk sediments, while deterministic factors (primarily variable selection, 45.78-60.78%) controlled the assembly of RT. Overall, this study highlighted the importance of examining AT and RT in bacterial communities for a broader understanding of the spatial patterns and underlying assembly mechanisms in tropical seagrass meadows.

Seagrass provides habitat and resources for various organisms in coastal areas. However, the productivity of a seagrass meadow might vary by its size, which can influence the contribution of primary sources to fauna and the food web structure. This study uses stable isotopes to investigate the contribution of sources to faunas and the associated food web structure in two tropical seagrass meadows. The carbon (δ13C) and nitrogen (δ15N) stable isotopes of primary sources such as mangrove leaf, coastal and riverine particulate organic matter, seagrass material and associated fauna (fish, bivalves, gastropods, crab, shrimp) were investigated from samples collected seagrass meadows of different sizes, at Libong Island and Tharai Island in Thailand. The contribution of the primary sources to the fauna diet showed that seagrass material was the main food source for fauna in both sites. Moreover, the trophic niche did not overlap among the groups of each fauna guild, reflecting the support seagrass provides to resident fauna. The study emphasizes the important role of seagrass as a habitat and feeding ground in tropical coastal ecosystems.

Two potential measures of habitat complexity, plant species number and aboveground plant biomass, are considered in relation to the species richness and abundance of motile macroinvertebrates inhabiting tropical sea- grass (Thalassia testudinum (Konig)) meadows. Plant species number was not significantly related to either invertebrate species number or abundance and therefore does not appear to adequately represent habitat complexity for the invertebrate species being considered. However, aboveground plant biomass is significantly correlated with both invertebrate species number and abundance and appears to be a reasonable measure of habitat complexity for these species. A consideration of several alternative explanations for these results suggests that the significant correlation between aboveground plant biomass and inverte- brate species number probably results from the addition of cryptic species which inhabit protected habitats that are formed among the foliose branches of certain plant species when they are very dense. The significant aboveground plant biomass-invertebrate abundance correlation is most likely due to the protection from predators that thick vegetation provides, but may also be partly a result of the greater plant surface area that is available for habitation in heavily vegetated areas. The results of this first step toward quantifying relationships among plants and animals in seagrass meadows clearly indicate that experimental studies are needed to differentiate conclusively among the competing explanations which seek to explain community organization in seagrass meadows.