Gail L. Chmura

A blueprint for blue carbon: toward an improved understanding of the role of vegetated coastal habitats in sequestering CO2

Recent research has highlighted the valuable role that coastal and marine ecosystems play in sequestering carbon dioxide (CO(2)). The carbon (C) sequestered in vegetated coastal ecosystems, specifically mangrove forests, seagrass beds, and salt marshes, has been termed blue carbon. Although their global area is one to two orders of magnitude smaller than that of terrestrial forests, the contribution of vegetated coastal habitats per unit area to long-term C sequestration is much greater, in part because of their efficiency in trapping suspended matter and associated organic C during tidal inundation. Despite the value of mangrove forests, seagrass beds, and salt marshes in sequestering C, and the other goods and services they provide, these systems are being lost at critical rates and action is urgently needed to prevent further degradation and loss. Recognition of the C sequestration value of vegetated coastal ecosystems provides a strong argument for their protection and restoration; however, it is necessary to improve scientific understanding of the underlying mechanisms that control C sequestration in these ecosystems. Here, we identify key areas of uncertainty and specific actions needed to address them.

Methane and carbon dioxide flux from a macrotidal salt marsh, Bay of Fundy, New Brunswick

Fluxes of methane (CH4) and carbon dioxide (CO2) to the atmosphere at 52 sites within a salt marsh were measured by a dark static chamber technique from mid July to mid September. Mean CH4 fluxes ranged from 0.2 mg m−2 d−1 to 11.0 mg m−2 d−1, with an overall average of 1.6 mg m−2 d−1. Flux of CH4 was inversely correlated (r2=0.23, p = 0.001) with salinity of the upper porewater at the site, suggesting the dominant role of SO42− in inhibiting methanogenesis in salt-marsh sediments. The combination of salinity and water table position was able to explain only 29% of the variance in CH4 emission. Mean soil flux of CO2 ranged from 0.3 g m−2 d−1 to 3.7 g m−2 d−1, with an overall average of 2.5 g m−2 d−1; it was correlated with aboveground biomass (positive, r2=0.38, p = 0.001) and position of the water table (negative, r2 = 0.55, p = 0.001). The combination of biomass and water table position accounted for 63% of the variance in CO2 flux. There were high variations in gas flux within the six plant communities. The sequences were CH4: upland edge > panne > pool > middle marsh > low marsh > high marsh, and CO2: middle marsh > low marsh > upland edge > high marsh > panne > pool. Compared to other salt-marsh systems, this Bay of Fundy marsh emits small amounts of CH4 and CO2.

Dinoflagellate cyst records and human disturbance in two neighboring estuaries, New Bedford Harbor and Apponagansett Bay, Massachusetts (USA)

The dinoflagellate cyst records in sediments from New Bedford Harbor and Apponagansett Bay demonstrate sensitivity to environmental change caused by human activity in the watersheds over the last 500 years. Changes in the species richness, as well as absolute and relative abundance of dinoflagellate cyst taxa reflect recent periods of development around the estuaries. Cyst taxa sensitive to these changes include Dubridinium spp., Polykrikos schwartzii, Lingulodinium machaerophorum, Operculodinium israelianum and Selenopemphix quanta. The greatest changes in the dinoflagellate cyst record occur during the 20th century, when New Bedford Harbor was exposed to both toxic pollution and heavy nutrient loading from point and non-point sources. Apponagansett Bay was not subject to industrial pollution and nutrient enrichment has been lower (from non-point sources). In Apponagansett Bay there is an increase in the dinoflagellate cyst species richness while species richness first increased, then declined in New Bedford Harbor. During the same period, the total dinoflagellate cyst concentration in New Bedford Harbor fluctuated over a wide range. The decline of species richness and the large fluctuations in the total cyst abundances signal the intensified anthropogenic disturbance in the watershed, notably a high degree of eutrophication and toxic pollution.

Environmental factors influencing the spatial distribution of dinoflagellate cyst assemblages in shallow lagoons of southern New England (USA)

Surface sediment samples from 24 sites within eleven back-barrier lagoons of Rhode Island and Massachusetts (USA) contain abundant (200–6000 cysts cm−3) and diverse (up to 40 taxa) dinoflagellate cyst assemblages. The lowest cyst concentrations and diversity are observed in lagoons with low salinity (<10). The pattern of spatial distribution of dinoflagellate cysts in these shallow estuarine environments is described. We assessed the relationship between the available multi-year water quality data and the composition of the dinoflagellate cyst assemblages using canonical correspondence analysis. Temperature and salinity are found to be the primary abiotic factors influencing cyst distribution in the coastal lagoons.

Controls on Salt Marsh Accretion: A Test in Salt Marshes of Eastern Canada

We used137Cs-dating to determine vertical accretion rates of 15 salt marshes on the Bay of Fundy, the Gulf of St. Lawrence, and the Atlantic coast of Nova Scotia. Accretion rates are compared to a number of factors assumed to influence vertical marsh accretion: rates of relative sea-level rise, climatic parameters (average daily temperatures and degree days) and latitude (related to insolation and day length), sediment characteristics (organic matter inventory, bulk, mineral, and organic matter density), distance of the core site from the nearest source of tidal waters, and the tidal range. Uniques to our study is a consideration of climatic parameters and latitude, which should influence organic matter production, and thus vertical accretion rates. Significant predictors of accretion rates (in order of importance) were found to be organic matter inventory, distance from a creek, and range of mean tides. Contrary to conclusions from previous studies, we found that accretion rates decreased with increasing tidal range, probably because we considered a wider span of tidal ranges, from micro- to macrotidal. Although four marshes with low organic matter inventories also show a deficit in accretion with respect to relative sea-level rise, organic matter is not limiting in two-thirds of the marshes studied, despite shorter growing seasons.

Sedimentary and botanical factors influencing peat accumulation in the Mississippi Delta

The Holocene Mississippi Delta System consists of six cyclic deltaic packages. Associated peat deposits are planar and eutrophic, and accumulate in areas abandoned by deltaic sedimentation. Peats that formed in large-scale interlobe basins are the subject of this paper. True peats (>75% organic matter) average 81.7% organic matter on a dry-weight basis. Controls on quality and lateral distribution of peat deposits are: botanical parent material, the balance between subsidence and accretion rates, detrital clastic influx and marine inundation. Botanical parent material, inferred from 13C signatures and microtome sections, is always freshwater, originating either in forested swamps, or under herbaceous (floating) mats. Sphagnum spp. is essentially absent. In order for peat to accumulate, rates of accretion and subsidence must balance. Long-term subsidence rates in the study area increase from about 0.10 to 0.30 cm a-1 over a distance of 40 km from upper to lower delta plain. Subsurface accretion rates for true peats at a depth of 2-3 m, however, average about 0.05 cm a-1. Renewed coarse detrital clastic influx terminates peat accumulation. The largest quantities of true peat are found together with relatively large amounts of organic-poor clastic sediments (mostly clays). Delta lobe abandonment and consequent coastal erosion causes salt water intrusion into freshwater environments. Resulting saline marsh sediments have maximum organic matter contents of 35%. In a transgressive setting, these may overlie older high-quality (low-ash) freshwater peats. Leaching experiments reveal the presence of water-soluble salts in both transgressed and fresh peats. Leaching reduces ash values by 1/5 to 1/3, suggesting that such losses may occur during early diagenesis. It is possible that some commercial coal seams originated in analogous interlobe environments, where leaching and possibly silica mobilization reduced the originally high ash contents to acceptable levels.

Pollen transport through distributaries and depositional patterns in coastal waters

Abstract In this study we compare seasonal pollen and spore assemblages in river waters from the Mississippi River at Vicksburg, Mississippi to those downstream at Belle Chasse, Louisiana and in a distributary, the Atchafalaya River at Morgan City, Louisiana. Assemblages of pollen in waters at Vicksburg and Belle Chasse are similar to each other, but distinctively different from those in the Atchafalaya River. Local pollen inputs are more important in the anastomosing Atchafalaya River channel, as compared to the meandering main stem Mississippi channel which discharges through the bird-foot delta. Riverine pollen and spore assemblages are also compared to those of surface sediments from the Louisiana continental shelf in the vicinity of the Mississippi River plume. In this area the primary source of marine pollen and spore assemblages appears to be the discharge from the main stem Mississippi, with limited input from the Atchafalaya. Thus, pollen deposited in these coastal waters provides a record of vegetation in the Mississippi drainage basin, rather than local coastal areas. Pollen assemblages on the continental shelf vary with distance from the river source, but not in a systematic manner. The pattern is explained by biological controls on fine-particle deposition in marine waters, i.e., deposition of pollen in copepod fecal pellets. As copepod grazing is dependent upon phytoplankton production, pollen deposition from the river plume will be limited by high turbidities proximal to the river mouth. We hypothesize that in neritic environments, pollen is preferentially deposited in areas corresponding to high phytoplankton production, which are characterized by reduced turbidity and high nutrient availability.

Assessing Coastal Squeeze of Tidal Wetlands

ABSTRACT Torio, D.D. and Chmura, G.L., 2013. Assessing coastal squeeze of tidal wetlands. As sea level rise accelerates and land development intensifies along coastlines, tidal wetlands will become increasingly threatened by coastal squeeze. Barriers that protect inland areas from rising sea level prevent or reduce tidal flows, and impermeable surfaces prevent wetland migration to the adjacent uplands. As vegetation succumbs to submergence by rising sea levels on the seaward edge of a wetland, those wetlands prevented from inland migration will decrease in area, if not disappear completely. Tools to identify locations where coastal squeeze is likely to occur are needed for coastal management. We have developed a “Coastal Squeeze Index” that can be used to assess the potential of coastal squeeze along the borders of a single wetland and to rank the threats faced by multiple wetlands. The index is based on surrounding topography and impervious surfaces derived from light detection and ranging and advanced spaceborne thermal emission and reflection radiometry imagery, respectively, and uses a fuzzy logic approach. We assume that coastal squeeze varies continuously over the coastal landscape and tested several fuzzy logic functions before assigning a continuous weight, from 0 to 1, corresponding to the influence of slope and impervious surfaces on coastal squeeze. We then combined the ranked variables to produce a map of coastal squeeze as a continuous index. Using this index, we compare the present and future threat of coastal squeeze to marshes in Wells and Portland, Maine, in the United States and Kouchibouguac National Park in New Brunswick, Canada.

An inventory of historical mercury emissions in Maritime Canada: implications for present and future contamination.

Mercury is a longstanding concern in Maritime Canada due to high levels of contamination in a number of fish and bird species. The recycled component of past releases of anthropogenic mercury may be a significant source of ongoing pollution in many areas. Historical information on mercury releases can be used to quantify past and present anthropogenic contamination. We present an inventory of historical mercury emissions from anthropogenic sources in Maritime Canada for the years 1800-1995. Long-term trends in mercury emissions and the significance of the cumulative burden of mercury released from local sources are discussed. Emissions are calculated using both historical monitoring data and the application of emission factors. The nature of current anthropogenic sources of mercury is quite different than it was several decades ago when many of the existing policies governing mercury pollution were created. Our inventory illustrates that many of the most significant sources in the past such as the chlor-alkali industry, paint containing mercury additives, and pharmaceuticals, have been largely phased out with fossil fuel combustion and waste disposal remaining as the most significant modern sources. Atmospheric emissions in Maritime Canada peaked in 1945 (> 1,750 kg year-1), and again between 1965 and 1970 (> 2,600 kg year-1). Cumulative releases of mercury from anthropogenic sources for the years 1800-1995 were between 115 and 259 t to the atmosphere alone, and 327-448 t when discharges to wastewater and effluents were included. Assuming that only 0.2% (Nriagu, 1994.) of these releases become part of the recycled fraction of current fluxes, we estimate that between 570 and 900 kg Hg year-1 is deposited in Maritime Canada from past anthropogenic sources. Modern sources within Maritime Canada contribute at least 405 kg year-1 to the total annual deposition of 1.71 t over the provinces of New Brunswick, Nova Scotia and Prince Edward Island, leaving approximately 735 kg year-1 from natural sources and long-range contamination. Further study is needed to verify these estimates and clarify the significance of natural and long-range sources of mercury in Maritime Canada.

Modelling coastal marsh stability in response to sea level rise: a case study in coastal Louisiana, USA

Chmura, G.L., Costanza, R. and Kosters, E., 1992. Modelling coastal marsh stability in response to sea level rise: a case study in coastal Louisiana, USA. Ecol. Modelling, 64: 47-64. In some regions coastal marsh stability is threatened by high rates of sea level rise. The deltaic plain of the Mississippi River is a natural laboratory for the study of marsh stability under conditions of rising sea level because it has been experiencing high rates of local submergence which cause relatively high rates of apparent sea level rise. We constructed a dynamic simulation model to study the relationship of accretion to three components of relative sea level rise: compaction, eustatic rise and submergence. The model is then used to project marsh stability under various future scenarios of sea level rise as well as enhancement of sediment supplies and marsh accretion. The model was calibrated to a 14C-dated sediment deposit which provides a long-term record of sediment accretion. Results indicate that an equilibrium between relative sea level and accretion rates can be attained, but that in this region of coastal Louisiana only the most optimistic assumptions yield coastal marshes that are stable in the long term.