Robert R. Twilley

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

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.

Restoration of the Mississippi Delta: Lessons from Hurricanes Katrina and Rita

Hurricanes Katrina and Rita showed the vulnerability of coastal communities and how human activities that caused deterioration of the Mississippi Deltaic Plain (MDP) exacerbated this vulnerability. The MDP formed by dynamic interactions between river and coast at various temporal and spatial scales, and human activity has reduced these interactions at all scales. Restoration efforts aim to re-establish this dynamic interaction, with emphasis on reconnecting the river to the deltaic plain. Science must guide MDP restoration, which will provide insights into delta restoration elsewhere and generally into coasts facing climate change in times of resource scarcity.

Mapping Height and Biomass of Mangrove Forests in Everglades National Park with SRTM Elevation Data

We produced a landscape scale map of mean tree height in mangrove forests in Everglades National Park (ENP) using the elevation data from the Shuttle Radar Topography Mission (SRTM). The SRTM data was calibrated using airborne lidar data and a high resolution USGS digital elevation model (DEM). The resulting mangrove height map has a mean tree height error of 2.0 m (RMSE) over a pixel of 30 m. In addition, we used field data to derive a relationship between mean forest stand height and biomass in order to map the spatial distribution of standing biomass of mangroves for the entire National Park. The estimation showed that most of the mangrove standing biomass in the ENP resides in intermediate-height mangrove stands around 8 m. We estimated the total mangrove standing biomass in ENP to be 5.6 � 10 9 kg.

Litter dynamics in riverine mangrove forests in the Guayas River estuary, Ecuador

Abstract The hypothesis that rates of litter turnover in mangroves are controlled by local geophysical processes such as tides has been studied at sites with mostly small tides (<1 m) and minor crab consumption of leaf litter. Our study describes litter dynamics of three riverine mangrove sites (M1, M2, M3), inhabited by the mangrove crab Ucides occidentalis, located in a macrotidal (>3 m) river-dominated tropical estuary in Ecuador (2.5°S latitude). There were statistical effects of site and depth on soil salinities, but all mean salinities were <17 g kg−1. Daily rates of leaf litter fall were higher in the rainy compared to the dry season, although no seasonal effects were observed for other components of litter fall. Annual total litter fall rates were significantly different among sites at 10.64, 6.47, and 7.87 Mg ha−1 year−1 for M1, M2, and M3, respectively. There were significant site (M3 > M2 > M1) and season (rainy > dry) effects on leaf degradation, and both effects were related to differences in the initial nitrogen content of senescent leaves. Mean leaf litter standing crop among the sites ranged from 1.53 to 9.18 g m−2, but amounts were strongly seasonal with peak values during September in both years of our study (no significant year effect) at all three sites. Leaf turnover rates based on leaf fall estimates and litter standing crop were 10- to 20-fold higher than estimated from rates of leaf degradation, indicating the significant effect of leaf transport by tides and crabs. Field experiments demonstrated that the mangrove crab can remove daily additions of leaf material within 1 h at all three sites, except during August–October, when the crab is inactive on the forest floor. Even though there is seasonally elevated leaf accumulation on the forest floor during this time, leaf turnover rates are much higher than expected based on leaf degradation, demonstrating the importance of tidal export. This is the first description of how crabs influence litter dynamics in the New World tropics, and results are similar to higher rates of crab transport of leaf litter in the Old World tropics. Even in riverine mangroves with high geophysical energies, patterns of litter dynamics can be influenced by ecological processes such as crab transport.

Patterns of mangrove forest structure and soil nutrient dynamics along the Shark River estuary, Florida

The basal area and productivity of managrove wetlands are described in relation to selected soil properties to understand the general pattern of optimum forest stature at the mouth of estuaries in the Everglades, such as the Shark River Slough, Florida (U.S.). The basal area of mangroves decreases from 40.4 m2 ha−1 and 39.7 m2 ha−1 at two stations 1.8 km and 4.1 km from the estuary mouth to 20.7 m2 ha−1 and 19.6 m2 ha−1 at two sites 9.9 km and 18.2 km from the mouth, respectively. The gradient in basal area at these four sites is mostly the result of approximately 34 yr of growth since Hurricane Donna. Wood productivity is higher in the lower estuary (10.7 Mg ha−1 yr−1 and 12.0 Mg ha−1 yr−1) than in the upper estuary (3.2 Mg ha−1 yr−1 and 4.2 Mg ha−1 yr−1). Porewater salinity among these four mangrove sites during seasonal sampling in 1994 and 1995 ranged from 1.6 g kg−1 to 33.5 g kg−1, while sulfide was generally<0.15 mM at all sites. These soil values indicate that abiotic stress cannot explain the decrease in forest structure along this estuarine gradient. Concentrations of nitrogen (N) and phosphorus (P) are more closely related to patterns of forest development, with higher soil fertility at the mouth of the estuary as indicated by higher concentrations of extractable ammonium, total soil P, and available P, along with higher ammonium production rates. The more fertile sites of the lower estuary are dominated by Laguncularia racemosa, whereas the less fertile sites in the intermediate and upper estuary are dominated by Rhizophora mangle. Relative N mineralization per unit of total N is higher in the lower estuary and is related positively to concentrations of available P, indicating the importance of turnover rates and nutrient interactions to soil fertility. Concentrations of Ca-bound P per volume soil in the lower estuary is 40-fold higher than in the upper estuary, and along with an increase in residual P in the upper estuary, indicate a shift from mineral to organic P along the estuarine gradient. Mineral inputs to the mouth of Shark River estuary from the Gulf of Mexico (rather than upland inputs) apparently control the patterns of mangrove structure and productivity.

Recent accretion in mangrove ecosystems based on137Cs and210Pb

Accretion rates were measured in fringe and basin mangrove forests in river and tidally dominated sites in Terminos Lagoon, Mexico, and a basin mangrove forest in Rookery Bay, Florida, USA. Accretion rates were determined using the radionuclides210Pb and137Cs. Consolidation-corrected accretion rates for the Rookery Bay cores, ranged from 1.4 to 1.7 mm yr−1, with an average rate of 1.6 mm yr−1. Rates at the Mexico sites ranged from 1.0 to 4.4 mm yr−1, with an average of 2.4 mm yr−1. Determination of rates in these mangrove forests was greatly affected by the consolidation corrections which decreased the apparent accretion rate by over 50% in one case. Accretion rates at basin sites compare favorably with a reported 1.4 to 1.6 mm yr−1 rate of sea-level rise, indicating little or no subsidence at inland locations. Accretion rates in fringe sites are generally greater than basin sites, indicating greater subsidence rates in these sediments over longer time intervals.

A simulation model of organic matter and nutrient accumulation in mangrove wetland soils

The distribution and accumulation of organic matter, nitrogen (N) and phosphorus (P) in mangrove soils at four sites along the Shark River estuary of south Florida were investigated with empirical measures and a process-based model. The mangrove nutrient model (NUMAN) was developed from the SEMIDEC marsh organic matter model and parameterized with data from mangrove wetlands. The soil characteristics in the four mangrove sites varied greatly in both concentrations and profiles of soil carbon, N and P. Organic matter decreased from 82% in the upstream locations to 30% in the marine sites. Comparisons of simulated and observed results demonstrated that landscape gradients of soil characteristics along the estuary can be adequately modeled by accounting for plant production, litter decomposition and export, and allochthonous input of mineral sediments. Model sensitivity analyses suggest that root production has a more significant effect on soil composition than litter fall. Model simulations showed that the greatest change in organic matter, N, and P occurred from the soil surface to 5 cm depth. The rapid decomposition of labile organic matter was responsible for this decrease in organic matter. Simulated N mineralization rates decreased quickly with depth, which corresponded with the decrease of labile organic matter. The increase in organic matter content and decrease in soil bulk density from mangrove sites at downstream locations compared to those at upstream locations was controlled mainly by variation in allochthonous inputs of mineral matter at the mouth of the estuary, along with gradients in mangrove root production. Research on allochthonouns sediment input and in situ root production of mangroves is limited compared to their significance to understanding nutrient biogeochemistry of these wetlands. More accurate simulations of temporal patterns of nutrient characteristics with depth will depend on including the effects of disturbance such as hurricanes on sediment redistribution and biomass production.

Is It Feasible to Build New Land in the Mississippi River Delta

What if the Mississippi River levees were cut below New Orleans? What if much of the water and sediment were allowed to flow out and build new deltas? Could deltaic land loss be reversed, and indeed restored? Using a conservative sediment supply rate and a range of rates of sea level rise and subsidence, a physically based model of deltaic river sedimentation [Kim et al., 2009] predicts that approximately 700–1200 square kilometers of new land (exposed surface and in-channel freshwater habitat) could be built over a century (Figure 1).

A 7 YEAR RECORD OF ABOVE-GROUND NET PRIMARY PRODUCTION IN A SOUTHEASTERN MEXICAN MANGROVE FOREST

Spatial and temporal variations in net above-ground primary production (NPP) and litter turnover rate were studied, from 1987 to 1993, in a mangrove forest bordering Laguna de Terminos, Mexico. NPP, the sum of total litter fall and wood production, was measured over the entire study period in three zones in a basin forest: zone I, where Rhizophora mangle (red mangrove) occurs but Avicennia germinans (black mangrove) is the dominant species; zone II, a scrub forest of A. germinans; zone III, where larger A. germinans trees occur. In 1991, a fringe zone dominated by A. germinans and R. mangle was added to the study. Three distinctive climatic seasons occur in the region: rainy, dry, and cold front (locally named ‘nortes’). Average total litter fall in the fringe zone (793 g m−2 year−1) was significantly higher than in the basin forest (496, 307, and 410 g m−2 year−1 for basin zones I, II, and III, respectively). All zones showed significant differences among seasons with the norte season having significantly lower litter fall. Litter turnover rates were about 7 months in zones I and II and 10 months in zone III, reflecting the low tidal range that occurs in the basin forest. Low litter turnover rates in the basin forest were reflected in a high organic matter standing crop. Annual average stem growth was significantly higher in zones I and III (1.27 and 1.36 kg per tree year−1, respectively) than in zone II (0.62 kg per tree year−1). Above-ground NPP rates in the basin forest (399–695 g m−2 year−1) were lower than in fringe and riverine forests, reflecting patterns of litter fall and wood production. There was no seasonal variation in soil salinity but the basin forest had significantly higher soil salinity than the fringe forest. Spatially, mean soil salinity was inversely related to litter fall. Long-term patterns in soil salinity, precipitation and air temperature explained 74% of the inter-annual litter fall variability. Over the 7 year study, productivity in zone II was more variable than in zones I and III, and productivity (litter fall and wood growth) were less variable than litter standing crop and turnover.

Flux of nitrogen and sediment in a fringe mangrove forest in Terminos Lagoon, Mexico

Fluxes of dissolved inorganic and organic nitrogen, particulate nitrogen, and total suspended sediments were measured in a fringe mangrove forest using the flume technique during a 15-month period in Terminos Lagoon, Mexico. The 12-m flume extended through a fringe forest from a tidal creek to a basin forest. There was a net import of dissolved inorganic nitrogen (NH+4 and NO−2+NO−3) from the creek and basin forest, while particulate (PN) and dissolved organic nitrogen (DON) were exported to the creek and basin forest. The tidal creek was the principal source of NH+4 (0·53 g m−2 year−1) and NO−2+NO−3 (0·08 g m−2 year−1) to the fringe forest, while the basin forest was the main source of total suspended sediments (TSS; 210 g m−2 year−1). Net export of PN occurred from the fringe forest to the tidal creek (0·52 g m−2 year−1) while less PN was exported to the basin forest (0·06 g m−2 year−1). The decrease in salinity during the rainy season indicated that nutrient concentrations in the tidal creek may have been influenced by inputs from rainfall and river discharge to the lagoon. There was a net import of TSS to the fringe forest from both the creek and basin forests, but the net input was 3·5 times higher at the fringe/basin interface. Particulate material exported from the forest during ebb tides generally had a higher C/N ratio than particulate matter imported into the forest on the flooding tide. This suggested that there was a greater nitrogen demand during ebb tide caused by the export of nitrogen-deficient detritus from fringe and basin mangroves. The exchange of nutrients among the tidal creek, the fringe, and basin forests in Estero Pargo is strongly influenced by seasonal weather forcing, such as winter storms, that can influence the magnitude and direction of water flow. The net annual import of inorganic nitrogen and the export of DON and PN suggest, in contrast to other mangrove systems, that the fringe mangrove forest in Estero Pargo acts as a sink of inorganic nitrogen and as a source of dissolved and particulate nitrogen.