Joseph M. Smoak

Avoiding timescale bias in assessments of coastal wetland vertical change

Abstract There is concern that accelerating sea‐level rise will exceed the vertical growth capacity of coastal‐wetland substrates in many regions by the end of this century. Vertical vulnerability estimates rely on measurements of accretion and/or surface‐elevation‐change derived from soil cores and/or surface elevation tables (SETs). To date there has not been a broad examination of whether the multiple timescales represented by the processes of accretion and elevation change are equally well‐suited for quantifying the trajectories of wetland vertical change in coming decades and centuries. To examine the potential for timescale bias in assessments of vertical change, we compared rates of accretion and surface elevation change using data derived from a review of the literature. In the first approach, average rates of elevation change were compared with timescale‐averaged accretion rates from six regions around the world where sub‐decadal, decadal, centennial, and millennial timescales were represented. Second, to isolate spatial variability, temporal comparisons were made for regionally unique environmental categories within each region. Last, comparisons were made of records from sites where SET‐MH stations and radiometric measurements were co‐located in close proximity. We find that rates vary significantly as a function of measurement timescale and that the pattern and magnitude of variation between timescales are location‐specific. Failure to identify and account for temporal variability in rates will produce biased assessments of the vertical change capacity of coastal wetlands. Robust vulnerability assessments should combine accretion rates from multiple timescales with the longest available SET record to provide long‐term context for ongoing monitoring observations and projections.

Accuracy and Precision of Tidal Wetland Soil Carbon Mapping in the Conterminous United States

Tidal wetlands produce long-term soil organic carbon (C) stocks. Thus for carbon accounting purposes, we need accurate and precise information on the magnitude and spatial distribution of those stocks. We assembled and analyzed an unprecedented soil core dataset, and tested three strategies for mapping carbon stocks: applying the average value from the synthesis to mapped tidal wetlands, applying models fit using empirical data and applied using soil, vegetation and salinity maps, and relying on independently generated soil carbon maps. Soil carbon stocks were far lower on average and varied less spatially and with depth than stocks calculated from available soils maps. Further, variation in carbon density was not well-predicted based on climate, salinity, vegetation, or soil classes. Instead, the assembled dataset showed that carbon density across the conterminous united states (CONUS) was normally distributed, with a predictable range of observations. We identified the simplest strategy, applying mean carbon density (27.0u2009kg Cxa0m−3), as the best performing strategy, and conservatively estimated that the top meter of CONUS tidal wetland soil contains 0.72 petagrams C. This strategy could provide standardization in CONUS tidal carbon accounting until such a time as modeling and mapping advancements can quantitatively improve accuracy and precision.

Coastal Blue Carbon Assessment of Mangroves, Salt Marshes, and Salt Barrens in Tampa Bay, Florida, USA

Compared to other terrestrial environments, coastal “blue carbon” habitats such as salt marshes and mangrove forests sequester disproportionately large amounts of carbon as standing plant biomass and peat deposits. This study quantified organic carbon stocks in 16 salt marshes, salt barrens, and mangrove forests in Tampa Bay, Florida, USA. The sites included natural, restored, and created wetlands of varying ages and degrees of anthropogenic impacts. Peat deposits were generally less than 30-cm deep and organic content rapidly decreased with depth in all habitats. The top 15xa0cm of mangrove soil contained an average of 11.0% organic carbon by weight, salt marshes contained 6.6%, and salt barrens contained 1.0%. Total organic carbon stock in mangroves was 133.6u2009±u200912.8xa0Mgxa0ha−1, with 69.5% of that carbon stored belowground. Salt marshes contained 66.4u2009±u200925.0xa0Mgxa0ha−1 (93.5% belowground carbon), and salt barrens contained 26.6u2009±u20098.3xa0Mgxa0ha−1 (96.1% belowground carbon). These values were much lower than global averages for carbon stocks in mangroves and salt marshes, likely due to Tampa Bay’s location near the northern limit of mangrove habitat, sandy soil, young age of the restored wetlands, presence of mosquito ditches, and recent habitat conversion from salt marshes to mangroves. In the late 1800s, Tampa Bay’s coastal wetlands were dominated by salt marshes, but today they are dominated by mangroves. Based on the blue carbon values from the natural sites in this study, this habitat switching has led to the additional sequestration of 141,000xa0Mg of carbon in remaining wetlands in the Tampa Bay watershed.

Land use change assessment in coastal mangrove forests of Iran utilizing satellite imagery and CA–Markov algorithms to monitor and predict future change

AbstractMangrove forest stores large organic carbon stocks in a setting that is highly vulnerable to climate change and direct anthropogenic influences.n As such there is a need to elucidate the causes and consequences of land use change on these ecosystems that have high value in terms of ecosystem services. We examine the areal pattern of land types in a coastal region located in southern Iran over a period of 14xa0years to predict future loss and gain in land types to the year 2025. We applied a CA–Markov model to simulate and predict mangrove forest change. Landsat satellite images from 2000 to 2014 were used to analyze the land cover changes between soil, open water and mangroves. Major changes during this period were observed in soil and water which could be attributed to rising sea level. Furthermore, the mangrove area in the more seaward position was converted to open water due to sea-level rise. A cellular automata model was then used to predict the land cover changes that would occur by the year 2025. Results demonstrated that approximately 21xa0ha of mangrove area will be converted to open water, while mangroves are projected to expand by approximately 28xa0ha in landward direction. These changes need to be delineated to better inform precise mitigation and adaptation measures.

Influence of land cover on riverine dissolved organic carbon concentrations and export in the Three Rivers Headwater Region of the Qinghai-Tibetan Plateau

The Qinghai-Tibetan plateau (QTP) stores a large amount of soil organic carbon and is the headwater region for several large rivers in Asia. Therefore, it is important to understand the influence of environmental factors on river water quality and the dissolved organic carbon (DOC) export in this region. We examined the water physico-chemical characteristics, DOC concentrations and export rates of 7 rivers under typical land cover types in the Three Rivers Headwater Region during August 2016. The results showed that the highest DOC concentrations were recorded in the rivers within the catchment of alpine wet meadow and meadow. These same rivers had the lowest total suspended solids (TSS) concentrations. The rivers within steppe and desert had the lowest DOC concentrations and highest TSS concentrations. The discharge rates and catchment areas were negatively correlated with DOC concentrations. The SUVA254 values were significantly negatively correlated with DOC concentrations. The results suggest that the vegetation degradation, which may represent permafrost degradation, can lead to a decrease in DOC concentration, but increasing DOC export and soil erosion. In addition, some of the exported DOC will rapidly decompose in the river, and therefore affect the regional carbon cycle, as well as the water quality in the source water of many large Asian rivers.

Radionuclide Enrichment Near Coal Processing in Southern Brazil

This work highlights environmental radionuclide enrichment as a result of coal mining activity, including disbursement related to the coal processing in southern Brazil. Soil cores collected near the coal mining area showed 238U, 226Ra and 210Pb activities approximately 2 times greater than the background values. The highest activities were found at the surface and decrease downcore, indicating recent enrichment. Furthermore, atmospheric analyses of particulate matter indicated radionuclide enrichment up to 352 and 463 μBq m–3 of 226Ra and 210Pb, respectively, at the sites nearest to the coal-fired power plant. The atmospheric particulate material showed 210Pb/226Ra ratios similar to the fly ash near the coal-fired power plant, suggesting an enhanced source of 222Rn to the atmosphere likely associated with the power plant. However, no risk to the population near the coal processing was found, as noted by the total air absorbed γ-ray dose rate in the topsoil samples, 74 ± 19 nGy h–1, statistically similar to the background radioactivity (60 nGy h–1).