Núria Marbà

Estimating Global “Blue Carbon” Emissions from Conversion and Degradation of Vegetated Coastal Ecosystems

Recent attention has focused on the high rates of annual carbon sequestration in vegetated coastal ecosystems—marshes, mangroves, and seagrasses—that may be lost with habitat destruction (‘conversion’). Relatively unappreciated, however, is that conversion of these coastal ecosystems also impacts very large pools of previously-sequestered carbon. Residing mostly in sediments, this ‘blue carbon’ can be released to the atmosphere when these ecosystems are converted or degraded. Here we provide the first global estimates of this impact and evaluate its economic implications. Combining the best available data on global area, land-use conversion rates, and near-surface carbon stocks in each of the three ecosystems, using an uncertainty-propagation approach, we estimate that 0.15–1.02 Pg (billion tons) of carbon dioxide are being released annually, several times higher than previous estimates that account only for lost sequestration. These emissions are equivalent to 3–19% of those from deforestation globally, and result in economic damages of

Seagrass community metabolism: Assessing the carbon sink capacity of seagrass meadows

US 6–42 billion annually. The largest sources of uncertainty in these estimates stems from limited certitude in global area and rates of land-use conversion, but research is also needed on the fates of ecosystem carbon upon conversion. Currently, carbon emissions from the conversion of vegetated coastal ecosystems are not included in emissions accounting or carbon market protocols, but this analysis suggests they may be disproportionally important to both. Although the relevant science supporting these initial estimates will need to be refined in coming years, it is clear that policies encouraging the sustainable management of coastal ecosystems could significantly reduce carbon emissions from the land-use sector, in addition to sustaining the well-recognized ecosystem services of coastal habitats.

Seagrass sediments as a global carbon sink: Isotopic constraints

[1] The metabolic rates of seagrass communities were synthesized on the basis of a data set on seagrass community metabolism containing 403 individual estimates derived from a total of 155 different sites. Gross primary production (GPP) rates (mean ± SE = 224.9 ± 11.1 mmol O 2 m ―2 d ―1 ) tended to be significantly higher than the corresponding respiration (R) rates (mean ± SE = 187.6 ± 10.1 mmol O 2 m ―2 d ―1 ), indicating that seagrass meadows tend to be autotrophic ecosystems, reflected in a positive mean net community production (NCP 27.2 ± 5.8 mmol O 2 m ―2 d ―1 ) and a mean P/R ratio above 1 (1.55 ± 0.13). Tropical seagrass meadows tended to support higher metabolic rates and somewhat lower NCP than temperate ones. The P/R ratio tended to increase with increasing GPP, exceeding, on average, the value of 1 indicative of metabolic balance for communities supporting a GPP greater than 186 mmol O 2 m ―2 d ―1 , on average. The global NCP of seagrass meadows ranged (95% confidence limits of mean values) from 20.73 to 50.69 Tg C yr ―1 considering a low global seagrass area of 300,000 km and 41.47 to 101.39 Tg C yr ―1 when a high estimate of global seagrass area of 600,000 km 2 was considered. The global loss of 29% of the seagrass area represents, therefore, a major loss of intense natural carbon sinks in the biosphere.

Will the Oceans Help Feed Humanity

Seagrass meadows are highly productive habitats found along many of the world’scoastline, providing important services that support the overall functioning of the coastalzone. The organic carbon that accumulates in seagrass meadows is derived not only fromseagrass production but from the trapping of other particles, as the seagrass canopiesfacilitate sedimentation and reduce resuspension. Here we provide a comprehensivesynthesis of the available data to obtain a better understanding of the relative contributionof seagrass and other possible sources of organic matter that accumulate in the sedimentsof seagrass meadows. The data set includes 219 paired analyses of the carbon isotopiccomposition of seagrass leaves and sediments from 207 seagrass sites at 88 locationsworldwide. Using a three source mixing model and literature values for putative sources,we calculate that the average proportional contribution of seagrass to the surfacesediment organic carbon pool is ∼50%. When using the best available estimates ofcarbon burial rates in seagrass meadows, our data indicate that between 41 and66 gC m

Effects of fish farm waste on Posidonia oceanica meadows: Synthesis and provision of monitoring and management tools

Constraints on the availability of freshwater and land plants and animals to feed the 9.2 billion humans projected to inhabit Earth by 2050 can be overcome by enhancing the contribution the ocean makes to food production. Catches from ocean fisheries are unlikely to recover without adequate conservation measures, so the greater contribution of the oceans to feeding humanity must be derived largely from mariculture. For the effort to be successful, mariculture must close the production cycle to abandon its current dependence on fisheries catches; enhance the production of edible macroalgae and filter-feeder organisms; minimize environmental impacts; and increase integration with food production on land, transferring water-intensive components of the human diet (i.e., production of animal protein) to the ocean. Accommodating these changes will enable the oceans to become a major source of food, which we believe will constitute the next food revolution in human history.

Leaf nutrient resorption, leaf lifespan and the retention of nutrients in seagrass systems

This paper provides a synthesis of the EU project MedVeg addressing the fate of nutrients released from fish farming in the Mediterranean with particular focus on the endemic seagrass Posidonia oceanica habitat. The objectives were to identify the main drivers of seagrass decline linked to fish farming and to provide sensitive indicators of environmental change, which can be used for monitoring purposes. The sedimentation of waste particles in the farm vicinities emerges as the main driver of benthic deterioration, such as accumulation of organic matter, sediment anoxia as well as seagrass decline. The effects of fish farming on P. oceanica meadows are diverse and complex and detected through various metrics and indicators. A safety distance of 400 m is suggested for management of P. oceanica near fish farms followed by establishment of permanent seagrass plots revisited annually for monitoring the health of the meadows.

Sulfur cycling and seagrass (Posidonia oceanica) status in carbonate sediments

Abstract Efficient nutrient resorption from senescing leaves, and extended leaf life spans are important strategies in order to conserve nutrients for plants in general. Despite the fact that seagrasses often grow in oligotrophic waters, these conservation strategies are not strongly developed in seagrasses. A compilation of literature data on nutrient resorption from seagrass leaves shows that the mean resorption of nitrogen is 20.4%, and that of phosphorus 21.9%, which is lower than comparable values for various groups of perennial terrestrial plants. The actual realised resorption in seagrasses may be even less as a result of premature losses of leaf fragments due to herbivory and hydrodynamic stresses, and due to leaching losses. The leaf lifespan in seagrasses on average is 88.4 days, but is highly variable, ranging from 345 days in Posidonia oceanica to only a few days in Halophila ovalis. Leaf lifespan increases with increasing leaf weight, and decreases with increasing leaf formation rate. Furthermore, leaf longevity increases going from tropical to temperate latitudes. We compared seagrass leaf lifespan with those of freshwater angiosperms, terrestrial herbaceous plants, shrubs and trees. Considerable variability in leaf lifespan was also found in these plant groups, but comparison among data sets shows that seagrass leaf lifespan is significantly lower than the leaf lifespan of terrestrial herbaceous plants, shrubs and trees. No significant difference was found between the leaf lifespan of seagrasses and freshwater angiosperms. Leaves are usually the major sink for nutrients in seagrasses. The combination of low nutrient resorption from the leaves and a short leaf lifespan is, therefore, expected to result in a low nutrient residence time in the plants. Indeed, field experiments with 15N labelled Thalassia hemprichii showed that less than 5% of the initial 15N amount was still within the living plant biomass 240 days after labelling. Limited nutrient retention in the plant biomass necessitates the capture of new nutrients for persistent growth. We speculate that effective nutrient uptake by seagrass leaves is an important strategy to maintain an adequate nutrient balance in seagrasses, particularly in thin vegetation or in small patches. The constraints imposed by the marine environment may have favoured the development of this strategy over the development of efficient nutrient conservation strategies.

Implications of Extreme Life Span in Clonal Organisms: Millenary Clones in Meadows of the Threatened Seagrass Posidonia oceanica

Sulfur cycling was investigated in carbonate-rich and iron-poor sediments vegetated with Posidonia oceanica in oligotrophic Mediterranean around Mallorca Island, Spain, to quantify sulfate reduction and pools of sulfide in seagrass sediments. The oxygen penetration depth was low (< 4.5 mm) and sulfate reduction rates were relatively high (0.7–12 mmol m−2d−1). The total pools of reduced sulfides were remarkably low (< 5 mol S m−2) indicating a fast turnover of reduced sulfides in these iron-poor sediments. The sulfate reduction rates were generally higher in vegetated compared to bare sediments possible due to enhanced sedimentation of sestonic material inside the seagrass meadows. The sulfate reduction rates were positively correlated with the seasonal variation in water temperature and negatively correlated with the shoot density indicating that the microbial activity was controlled by temperature and release of oxygen from the roots. The pools of reduced sulfides were low in these iron-poor sediments leading to high oxygen consumption for reoxidation. The sediments were highly anoxic as shown by relatively low oxygen penetration depths (< 4.5 mm) in these low organic sediments. The net shoot recruitment rate was negative in sediments enriched with organic matter, suggesting that organic matter enrichment may be an important factor for seagrass status in these iron-depleted carbonate sediments.

Seagrass Beds and Coastal Biogeochemistry

The maximum size and age that clonal organisms can reach remains poorly known, although we do know that the largest natural clones can extend over hundreds or thousands of metres and potentially live for centuries. We made a review of findings to date, which reveal that the maximum clone age and size estimates reported in the literature are typically limited by the scale of sampling, and may grossly underestimate the maximum age and size of clonal organisms. A case study presented here shows the occurrence of clones of slow-growing marine angiosperm Posidonia oceanica at spatial scales ranging from metres to hundreds of kilometres, using microsatellites on 1544 sampling units from a total of 40 locations across the Mediterranean Sea. This analysis revealed the presence, with a prevalence of 3.5 to 8.9%, of very large clones spreading over one to several (up to 15) kilometres at the different locations. Using estimates from field studies and models of the clonal growth of P. oceanica, we estimated these large clones to be hundreds to thousands of years old, suggesting the evolution of general purpose genotypes with large phenotypic plasticity in this species. These results, obtained combining genetics, demography and model-based calculations, question present knowledge and understanding of the spreading capacity and life span of plant clones. These findings call for further research on these life history traits associated with clonality, considering their possible ecological and evolutionary implications.

Direct Evidence of Imbalanced Seagrass (Posidonia oceanica) Shoot Population Dynamics in the Spanish Mediterranean

Capitulo en: LARKUM, Anthony W.D.; ORTH, Robert J.; DUARTE, Carlos M. (eds.). Seagrasses: Biology, Ecology and Conservation. Repr. with Corrections, 2007. [Dordrecht]: Springer, 2006, p.135-157