Seagrass sediments as a global carbon sink: Isotopic constraints

Seagrass meadows are highly productive habitats found along many of the world's coastline, providing important services that support the overall functioning of the coastal zone. The organic carbon that accumulates in seagrass meadows is derived not only from seagrass production but from the trapping of other particles, as the seagrass canopies facilitate sedimentation and reduce resuspension. Here we provide a comprehensive synthesis of the available data to obtain a better understanding of the relative contribution of seagrass and other possible sources of organic matter that accumulate in the sediments of seagrass meadows. The data set includes 219 paired analyses of the carbon isotopic composition of seagrass leaves and sediments from 207 seagrass sites at 88 locations worldwide. Using a three source mixing model and literature values for putative sources, we calculate that the average proportional contribution of seagrass to the surface sediment organic carbon pool is ∼50%. When using the best available estimates of carbon burial rates in seagrass meadows, our data indicate that between 41 and 66 gC m−2 yr−1 originates from seagrass production. Using our global average for allochthonous carbon trapped in seagrass sediments together with a recent estimate of global average net community production, we estimate that carbon burial in seagrass meadows is between 48 and 112 Tg yr−1, showing that seagrass meadows are natural hot spots for carbon sequestration.

Environmental Dynamics of Dissolved Black Carbon in Aquatic Ecosystems

Black carbon (BC), the incomplete combustion product from biomass and fossil fuel burning, is ubiquitously found in soils, sediments, ice, water and atmosphere. Because of its polyaromatic molecular characteristic, BC is believed to contribute significantly to the global carbon budget as a slow-cycling, refractory carbon pool. However, the mass balance between global BC generation and accumulation does not match, suggesting a removal mechanism of BC to the active carbon pool, most probable in a dissolved form. The presence of BC in waters as part of the dissolved organic matter (DOM) pool was recently confirmed via ultrahigh resolution mass spectrometry, and dissolved black carbon (DBC), a degradation product of charcoal, was found in marine and coastal environments. However, information on the loadings of DBC in freshwater environments and its global riverine flux from terrestrial systems to the oceans remained unclear. The main objectives of this study were to quantify DBC in diverse aquatic ecosystems and to determine its environmental dynamics. Surface water samples were collected from aquatic environments with a spatially significant global distribution, and DBC concentrations were determined by a chemical oxidation method coupled with HPLC detection. While it was clear that biomass burning was the main sources of BC, the translocation mechanism of BC to the dissolved phase was not well understood. Data from the regional studies and the developed global model revealed a strong positive correlation between DBC and dissolved organic carbon (DOC) dynamics, indicating a co-generation and co-translocation between soil OC and BC. In addition, a DOC-assistant DBC translocation mechanism was identified. Taking advantage of the DOC-DBC correlation model, a global riverine DBC flux to oceans on the order of 26.5 Mt C yr-1 (1 Mt = 1012 g) was determined, accounting for 10.6% of the global DOC flux. The results not only indicated that DOC was an important environmental intermediate for BC transfer and storage, but also provided an estimate of a major missing link in the global BC budget. The ever increasing DBC export caused by global warming will change the marine DOM quality and may have important consequences for carbon cycling in marine ecosystem.

The effects of climate on the growth and physiology of tropical rainforest trees

Tropical rainforests account for more than a third of global net primary production and contain more than half of the global forest carbon. Though these forests are a disproportionately important component of the global carbon cycle, the relationship between rainforest productivity and climate remains poorly understood. Understanding the link between current climate and rainforest tree stem diameter increment, a major constituent of forest productivity, will be crucial to efforts at modeling future climate and rainforest response to climate change. This work reports the physiological and stem growth responses to micrometeorological and phenological states of ten species of canopy trees in a Costa Rican wet tropical forest at sub-annual time intervals. I measured tree growth using band dendrometers and estimated leaf and reproductive phenological states monthly. Electronic data loggers recorded xylem sap flow (an indicator of photosynthetic rate) and weather at half-hour intervals. An analysis of xylem sap flow showed that physiological responses were independent of species, which allowed me to construct a general model of weather driven sap flow rates. This model predicted more than eighty percent of climate driven sap flow variation. Leaf phenology influenced growth in three of the ten species, with two of these species showing a link between leaf phenology and weather. A combination of rainfall, air temperature, and irradiance likely provided the cues that triggered leaf drop in Dipteryx panamensis and Lecythis ampla. Combining the results of the sap flow model, growth, and the climate measures showed tree growth was correlated to climate, though the majority of growth variation remained unexplained. Low variance in the environmental variables and growth rates likely contributed to the large amount of unexplained variation. A simple model that included previous growth increment and three meteorological variables explained from four to nearly fifty percent of the growth variation. Significant growth carryover existed in six of the ten species, and rainfall was positively correlated to growth in eight of the ten species. Minimum nighttime temperature was also correlated to higher growth rates in five of the species and irradiance in two species. These results indicate that tropical rainforest tree trunks could act as carbon sinks if future climate becomes wetter and slightly warmer. ^

Local biomass control on the composition and reactivity of particulate organic matter in aquatic environments

Freshwater ecosystems have been recognized as important components of the global carbon cycle, and the flux of organic matter (OM) from freshwater to marine environments can significantly affect estuarine and coastal productivity. The focus of this study was the assessment of carbon dynamics in two aquatic environments, namely the Florida Everglades and small prairie streams in Kansas, with the aim of characterizing the biogeochemistry of OM. In the Everglades, particulate OM (POM) is mostly found as a layer of flocculent material (floc). While floc is believed to be the main energy source driving trophic dynamics in this oligotrophic wetland, not much is known about its biogeochemistry. The objective of this study was to determine the origin/sources of OM in floc using biomarkers and pigment-based chemotaxonomy to assess specific biomass contributions to this material, on a spatial (freshwater marshes vs. mangrove fringe) and seasonal (wet vs. dry) scales. It was found that floc OM is derived from the local vegetation (mainly algal components and macrophyte litter) and its composition is controlled by seasonal drivers of hydrology and local biomass productivity. Photo-reactivity experiments showed that light exposure on floc resulted in photo-dissolution of POC with the generation of significant amounts of both dissolved OM (DOM) and nutrients (N & P), potentially influencing nutrient dynamics in this ecosystem. The bio-reactivity experiments determined as the amount and rate of CO2 evolution during incubation were found to vary on seasonal and spatial scales and were highly influenced by phosphorus limitation. Not much is known on OM dynamics in small headwater streams. The objective of this study was to determine carbon dynamics in sediments from intermittent prairie streams, characterized by different vegetation cover for their watershed (C4 grasses) vs. riparian zone (C3 plants). In this study sedimentary OM was characterized using a biomarker and compound specific carbon stable isotope approach. It was found that the biomarker composition of these sediments is dominated by higher plant inputs from the riparian zone, although inputs from adjacent prairie grasses were also apparent. Conflicting to some extent with the River Continuum Concept, sediments of the upper reaches contained more degraded OM, while the lower reaches were enriched in fresh material deriving from higher plants and plankton sources as a result of hydrological regimes and particle sorting.

Modeling Light Use Efficiency in a Subtropical Mangrove Forest Equipped with CO2 Eddy Covariance °C

Despite the importance of mangrove ecosystems in the global carbon budget, the relationships between environmental drivers and carbon dynamics in these forests remain poorly understood. This limited understanding is partly a result of the challenges associated with in situ flux studies. Tower-based CO2 eddy covariance (EC) systems are installed in only a few mangrove forests worldwide, and the longest EC record from the Florida Everglades contains less than 9 years of observations. A primary goal of the present study was to develop a methodology to estimate canopy-scale photosynthetic light use efficiency in this forest. These tower-based observations represent a basis for associating CO2 fluxes with canopy light use properties, and thus provide the means for utilizing satellite-based reflectance data for larger scale investigations. We present a model for mangrove canopy light use efficiency utilizing the enhanced green vegetation index (EVI) derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) that is capable of predicting changes in mangrove forest CO2 fluxes caused by a hurricane disturbance and changes in regional environmental conditions, including temperature and salinity. Model parameters are solved for in a Bayesian framework. The model structure requires estimates of ecosystem respiration (RE), and we present the first ever tower-based estimates of mangrove forest RE derived from nighttime CO2 fluxes. Our investigation is also the first to show the effects of salinity on mangrove forest CO2 uptake, which declines 5% per each 10 parts per thousand (ppt) increase in salinity. Light use efficiency in this forest declines with increasing daily photosynthetic active radiation, which is an important departure from the assumption of constant light use efficiency typically applied in satellite-driven models. The model developed here provides a framework for estimating CO2 uptake by these forests from reflectance data and information about environmental conditions.

Dissolved Black Nitrogen (DBN) in Freshwater Environments

Biomass burning results in the formation and accumulation of pyrogenic products such as black carbon (BC) and black nitrogen (BN) in soils. The ubiquitous presence of pyrogenic products in natural dissolved organic matter (DOM) and potential implications in global carbon cycling have recently been reported. However, little is known about the environmental dynamics or the importance in the global N cycle of dissolved BN (DBN; or heteroaromatic N). Here we report the coupling between DBN and dissolved BC (DBC) in ultrafiltered DOM from six headwater streams across a climatic region of North America, suggesting similar combustion sources, and that DOC may play an important role in the translocation of soil BN to the dissolved phase. The export of potentially recalcitrant riverine DBN to the ocean may affect the biogeochemical cycling of N and possibly the microbial community structure in aquatic environments.

Changing Bacterial Growth Efficiencies across a Natural Nutrient Gradient in an Oligotrophic Estuary

Recent studies have characterized coastal estuarine systems as important components of the global carbon cycle. This study investigated carbon cycling through the microbial loop of Florida Bay by use of bacterial growth efficiency calculations. Bacterial production, bacterial respiration, and other environmental parameters were measured at three sites located along a historic phosphorus-limitation gradient in Florida Bay and compared to a relatively nutrient enriched site in Biscayne Bay. A new method for measuring bacterial respiration in oligotrophic waters involving tracing respiration of 13C-glucose was developed. The results of the study indicate that 13C tracer assays may provide a better means of measuring bacterial respiration in low nutrient environments than traditional dissolved oxygen consumption-based methods due to strong correlations between incubation length and δ13C values. Results also suggest that overall bacterial growth efficiency may be lower at the most nutrient limited sites.

Mangrove production and carbon sinks: A revision of global budget estimates

Mangrove forests are highly productive but globally threatened coastal ecosystems, whose role in the carbon budget of the coastal zone has long been debated. Here we provide a comprehensive synthesis of the available data on carbon fluxes in mangrove ecosystems. A reassessment of global mangrove primary production from the literature results in a conservative estimate of ∼218 ± 72 Tg C a−1. When using the best available estimates of various carbon sinks (organic carbon export, sediment burial, and mineralization), it appears that >50% of the carbon fixed by mangrove vegetation is unaccounted for. This unaccounted carbon sink is conservatively estimated at ∼112 ± 85 Tg C a−1, equivalent in magnitude to ∼30–40% of the global riverine organic carbon input to the coastal zone. Our analysis suggests that mineralization is severely underestimated, and that the majority of carbon export from mangroves to adjacent waters occurs as dissolved inorganic carbon (DIC). CO2 efflux from sediments and creek waters and tidal export of DIC appear to be the major sinks. These processes are quantitatively comparable in magnitude to the unaccounted carbon sink in current budgets, but are not yet adequately constrained with the limited published data available so far.

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 $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.