Anthropogenic modification of New England salt marsh landscapes

Salt marshes play a critical role in the ecology and geology of wave-protected shorelines in the Western Atlantic, but as many as 80% of the marshes that once occurred in New England have already been lost to human development. Here we present data that suggest that the remaining salt marshes in southern New England are being rapidly degraded by shoreline development and eutrophication. On the seaward border of these marshes, nitrogen eutrophication stimulated by local shoreline development is shifting the competitive balance among marsh plants by releasing plants from nutrient competition. This shift is leading to the displacement of natural high marsh plants by low marsh cordgrass. On the terrestrial border of these same marshes, shoreline development is also precipitating the invasion of the common reed, Phragmites, by means of nitrogen eutrophication caused by the removal of the woody vegetation buffer between terrestrial and salt marsh communities. As a consequence of these human impacts, traditional salt marsh plant communities and the plants and animals that are dependent on these habitats are being displaced by monocultures of weedy species.     

Unexpectedly high mortality in Pacific herring embryos exposed to the 2007 Cosco Busan oil spill in San Francisco Bay

In November 2007, the container ship Cosco Busan released 54,000 gallons of bunker fuel oil into San Francisco Bay. The accident oiled shoreline near spawning habitats for the largest population of Pacific herring on the west coast of the continental United States. We assessed the health and viability of herring embryos from oiled and unoiled locations that were either deposited by natural spawning or incubated in subtidal cages. Three months after the spill, caged embryos at oiled sites showed sublethal cardiac toxicity, as expected from exposure to oil-derived polycyclic aromatic compounds (PACs). By contrast, embryos from the adjacent and shallower intertidal zone showed unexpectedly high rates of tissue necrosis and lethality unrelated to cardiotoxicity. No toxicity was observed in embryos from unoiled sites. Patterns of PACs at oiled sites were consistent with oil exposure against a background of urban sources, although tissue concentrations were lower than expected to cause lethality. Embryos sampled 2 y later from oiled sites showed modest sublethal cardiotoxicity but no elevated necrosis or mortality. Bunker oil contains the chemically uncharacterized remains of crude oil refinement, and one or more of these unidentified chemicals likely interacted with natural sunlight in the intertidal zone to kill herring embryos. This reveals an important discrepancy between the resolving power of current forensic analytical chemistry and biological responses of keystone ecological species in oiled habitats. Nevertheless, we successfully delineated the biological impacts of an oil spill in an urbanized coastal estuary with an overlapping backdrop of atmospheric, vessel, and land-based sources of PAC pollution.     

Effects of Slotted Water Control Structures on Nekton Movement within Salt Marshes

Water control structures (WCSs) restrict hydrological connectivity in salt marshes and thereby impede nekton movement within the greater habitat mosaic. Transient fishery species, which spawn outside salt marshes and must get past these barriers to reach spawning areas or salt-marsh nurseries, are especially vulnerable to these structures. Water control structures incorporating slots (narrow vertical openings spanning most of the water column) are thought to improve nekton passage; however, few studies have directly examined nekton passage through WCS slots. Dual-frequency identification sonar (DIDSON) acoustic imaging was used monthly (April–September 2010) on diurnal flood tides to examine nekton movement through 15-cm-wide slots at two identical WCSs located in Louisiana tidal marsh channels. Nekton behavior was compared between these WCSs and a nearby natural salt-marsh creek. Examination of 12 h of subsampled acoustic data revealed large concentrations of salt-marsh nekton at the WCSs (n = 2,970 individuals total), but passage rates through the slots were low (≤10% of total observed individuals migrated via the slots). Most migrating fish were observed leaving the managed area and swimming against a flood tide. The mean size of migrating individuals (～25 cm TL) did not differ in relation to swimming direction (going into versus exiting the managed marsh) and was similar to that reported from other studies examining similar slot widths. Nekton formed congregations in the WCS channel, but no congregations were observed in the natural salt-marsh creek, even though nekton species composition and sizes were similar among sites. The WCSs in our study appear to function as ecological hot spots, where large individuals may encounter enhanced foraging opportunities but also fishing mortality and where smaller individuals may experience greater predation rates.

Received July 22, 2014; accepted February 13, 2015     

Critical width of tidal flats triggers marsh collapse in the absence of sea-level rise

High rates of wave-induced erosion along salt marsh boundaries challenge the idea that marsh survival is dictated by the competition between vertical sediment accretion and relative sea-level rise. Because waves pounding marshes are often locally generated in enclosed basins, the depth and width of surrounding tidal flats have a pivoting control on marsh erosion. Here, we show the existence of a threshold width for tidal flats bordering salt marshes. Once this threshold is exceeded, irreversible marsh erosion takes place even in the absence of sea-level rise. This catastrophic collapse occurs because of the positive feedbacks among tidal flat widening by wave-induced marsh erosion, tidal flat deepening driven by wave bed shear stress, and local wind wave generation. The threshold width is determined by analyzing the 50-y evolution of 54 marsh basins along the US Atlantic Coast. The presence of a critical basin width is predicted by a dynamic model that accounts for both horizontal marsh migration and vertical adjustment of marshes and tidal flats. Variability in sediment supply, rather than in relative sea-level rise or wind regime, explains the different critical width, and hence erosion vulnerability, found at different sites. We conclude that sediment starvation of coastlines produced by river dredging and damming is a major anthropogenic driver of marsh loss at the study sites and generates effects at least comparable to the accelerating sea-level rise due to global warming.     

Forecasting sudden changes in environmental pollution patterns

The lack of reliable forecasts for the spread of oceanic and atmospheric contamination hinders the effective protection of the ecosystem, society, and the economy from the fallouts of environmental disasters. The consequences can be dire, as evidenced by the Deepwater Horizon oil spill in the Gulf of Mexico in 2010. We present a methodology to predict major short-term changes in environmental contamination patterns, such as oil spills in the ocean and ash clouds in the atmosphere. Our approach is based on new mathematical results on the objective (frame-independent) identification of key material surfaces that drive tracer mixing in unsteady, finite-time flow data. Some of these material surfaces, known as Lagrangian coherent structures (LCSs), turn out to admit highly attracting cores that lead to inevitable material instabilities even under future uncertainties or unexpected perturbations to the observed flow. These LCS cores have the potential to forecast imminent shape changes in the contamination pattern, even before the instability builds up and brings large masses of water or air into motion. Exploiting this potential, the LCS-core analysis developed here provides a model-independent forecasting scheme that relies only on already observed or validated flow velocities at the time the prediction is made. We use this methodology to obtain high-precision forecasts of two major instabilities that occurred in the shape of the Deepwater Horizon oil spill. This is achieved using simulated surface currents preceding the prediction times and assuming that the oil behaves as a passive tracer.     

Effects of Coastal Urbanization on Salt-Marsh Faunal Assemblages in the Northern Gulf of Mexico

Abstract

Coastal landscapes in the northern Gulf of Mexico, specifically the Mississippi coast, have undergone rapid urbanization that may impact the suitability of salt-marsh ecosystems for maintaining and regulating estuarine faunal communities. We used a landscape ecology approach to quantify the composition and configuration of salt-marsh habitats and developed surfaces at multiple spatial scales surrounding three small, first-order salt-marsh tidal creeks arrayed along a gradient of urbanization in two river-dominated estuaries. From May 3 to June 4, 2010, nekton and macroinfauna were collected weekly at all six sites. Due to the greater abundance of grass shrimp Palaemonetes spp., brown shrimp Farfantepenaeus aztecus, blue crab Callinectes sapidus, Gulf Menhaden Brevoortia patronus, and Spot Leiostomus xanthurus, tidal creeks in intact natural (IN) salt-marsh landscapes supported a nekton assemblage that was significantly different from those in partially urbanized (PU) or completely urbanized (CU) salt-marsh landscapes. However, PU landscapes still supported an abundant nekton assemblage. In addition, the results illustrated a linkage between life history traits and landscape characteristics. Resident and transient nekton species that have specific habitat requirements are more likely to be impacted in urbanized landscapes than more mobile species that are able to exploit multiple habitats. Patterns were less clear for macroinfaunal assemblages, although they were comparatively less abundant in CU salt-marsh landscapes than in either IN or PU landscapes. The low abundance or absence of several macroinfaunal taxa in CU landscapes may be viewed as an additional indicator of poor habitat quality for nekton. The observed patterns also suggested that benthic sediments in the CU salt-marsh landscapes were altered in comparison with IN or PU landscapes. The amount of developed shoreline and various metrics related to salt marsh fragmentation were important drivers of observed patterns in nekton and macroinfaunal assemblages.

Received September 14, 2013; accepted January 27, 2014     

Does vegetation prevent wave erosion of salt marsh edges?

This study challenges the paradigm that salt marsh plants prevent lateral wave-induced erosion along wetland edges by binding soil with live roots and clarifies the role of vegetation in protecting the coast. In both laboratory flume studies and controlled field experiments, we show that common salt marsh plants do not significantly mitigate the total amount of erosion along a wetland edge. We found that the soil type is the primary variable that influences the lateral erosion rate and although plants do not directly reduce wetland edge erosion, they may do so indirectly via modification of soil parameters. We conclude that coastal vegetation is best-suited to modify and control sedimentary dynamics in response to gradual phenomena like sea-level rise or tidal forces, but is less well-suited to resist punctuated disturbances at the seaward margin of salt marshes, specifically breaking waves.     

Genomic and physiological footprint of the Deepwater Horizon oil spill on resident marsh fishes

The biological consequences of the Deepwater Horizon oil spill are unknown, especially for resident organisms. Here, we report results from a field study tracking the effects of contaminating oil across space and time in resident killifish during the first 4 mo of the spill event. Remote sensing and analytical chemistry identified exposures, which were linked to effects in fish characterized by genome expression and associated gill immunohistochemistry, despite very low concentrations of hydrocarbons remaining in water and tissues. Divergence in genome expression coincides with contaminating oil and is consistent with genome responses that are predictive of exposure to hydrocarbon-like chemicals and indicative of physiological and reproductive impairment. Oil-contaminated waters are also associated with aberrant protein expression in gill tissues of larval and adult fish. These data suggest that heavily weathered crude oil from the spill imparts significant biological impacts in sensitive Louisiana marshes, some of which remain for over 2 mo following initial exposures.     

Air quality implications of the Deepwater Horizon oil spill

During the Deepwater Horizon (DWH) oil spill, a wide range of gas and aerosol species were measured from an aircraft around, downwind, and away from the DWH site. Additional hydrocarbon measurements were made from ships in the vicinity. Aerosol particles of respirable sizes were on occasions a significant air quality issue for populated areas along the Gulf Coast. Yields of organic aerosol particles and emission factors for other atmospheric pollutants were derived for the sources from the spill, recovery, and cleanup efforts. Evaporation and subsequent secondary chemistry produced organic particulate matter with a mass yield of 8 ± 4% of the oil mixture reaching the water surface. Approximately 4% by mass of oil burned on the surface was emitted as soot particles. These yields can be used to estimate the effects on air quality for similar events as well as for this spill at other times without these data. Whereas emission of soot from burning surface oil was large during the episodic burns, the mass flux of secondary organic aerosol to the atmosphere was substantially larger overall. We use a regional air quality model to show that some observed enhancements in organic aerosol concentration along the Gulf Coast were likely due to the DWH spill. In the presence of evaporating hydrocarbons from the oil, NO_x emissions from the recovery and cleanup operations produced ozone.On April 20, 2010, an explosion and subsequent leak beneath the Deepwater Horizon (DWH) drilling platform led to the largest marine oil spill in United States history. The air quality issues arising from the oil spill are different for workers at the site than for the population along the coast. Primary emissions are of more concern near the site and secondary pollutants are more important downwind. The key atmospheric pollutants considered in this paper are hydrocarbons (HCs), particulate matter (PM) or aerosol particles, ozone, carbon monoxide, and nitrogen oxides. Four sources of primary air pollutants attributable to the DWH oil spill are detected in our observations: (a) HCs evaporating from the oil; (b) smoke from deliberate burning of the oil slick; (c) combustion products from the flaring of recovered natural gas; and (d) ship emissions from the recovery and cleanup operations. Here, we examine these primary emissions and the subsequent production of ozone and secondary organic aerosol (SOA). Furthermore, we use aircraft data to derive the amount of atmospheric particulate matter formed per mass of oil that reached the surface. These results can be used to estimate implications for air quality during the DWH spill at other times and locations and can also provide information about effects on air quality by past or future spills.     

A linear relationship between wave power and erosion determines salt-marsh resilience to violent storms and hurricanes

Salt marsh losses have been documented worldwide because of land use change, wave erosion, and sea-level rise. It is still unclear how resistant salt marshes are to extreme storms and whether they can survive multiple events without collapsing. Based on a large dataset of salt marsh lateral erosion rates collected around the world, here, we determine the general response of salt marsh boundaries to wave action under normal and extreme weather conditions. As wave energy increases, salt marsh response to wind waves remains linear, and there is not a critical threshold in wave energy above which salt marsh erosion drastically accelerates. We apply our general formulation for salt marsh erosion to historical wave climates at eight salt marsh locations affected by hurricanes in the United States. Based on the analysis of two decades of data, we find that violent storms and hurricanes contribute less than 1% to long-term salt marsh erosion rates. In contrast, moderate storms with a return period of 2.5 mo are those causing the most salt marsh deterioration. Therefore, salt marshes seem more susceptible to variations in mean wave energy rather than changes in the extremes. The intrinsic resistance of salt marshes to violent storms and their predictable erosion rates during moderate events should be taken into account by coastal managers in restoration projects and risk management plans.The potential of salt marshes to serve as natural buffers against violent storms seems even more important in view of significant threats imposed by climate change, such as increased storminess and higher hurricane activity registered in the past decades (1–12). Recent research results show that salt marshes reduce wave energy during storms and possibly, mitigate storm surges (13–15). These results triggered a flurry of planned coastal restorations centered on the concept of “living shorelines” (14), which use vegetated surfaces to reduce the impact of hurricanes (13–16). However, little is known about the endurance of salt marshes against wave action and whether such ecosystems can survive extreme events.Most marsh erosion occurs at its seaward boundary, where the effect of waves is concentrated (2, 3). Wave erosion constitutes one of the main contributions to salt marsh deterioration, and even very small waves can cause failure of large salt marsh blocks (2, 7, 17). Despite the complexity of the problem, some studies have identified a correlation between wave energy and lateral rates of marsh erosion (18, 19). Erosion of marsh edges by wave action is caused by many different mechanisms, such as the indentation of V-shaped notches into linear stretches of shoreline or cliff undercutting when lower sediment layers are eroded more rapidly than the overhanging root mats (2, 17, 19). Varying resistance to wave erosion can be caused by biotic and abiotic factors, such as geotechnical characteristics of the sediments (7, 20), vegetation characteristics (21), height of the marsh scarp, and presence of mussels or crab burrowing (22).However, existing studies have mainly focused on individual marsh locations and do not provide a universal relationship applicable to multiple ecologically diverse systems. Herein, we combine wave energy and marsh erosion data from eight different locations in the United States, Australia, and Italy (18, 19, 23–26). We show that the data collapse into a unique linear relationship (Fig. 1):E^? = a^?P^?, a^? = 0.67, [1]where E^? = E/E_avg and P^? = P/P_avg are the dimensionless erosion rate and dimensionless wave power obtained by dividing field measurements of erosion rate E and wave power P by the averaged conditions at each site (E_avg and P_avg). Nondimensionalization allows for filtering out of the diverse resistance of marsh boundary at individual locations. Field measurements display a linear behavior (R² = 0.62; p < 0.05), as shown by their average over subintervals (gray dots in Fig. 1). Some of the data also account for the occurrence of major storms. As an example, data for Barnegat Bay, New Jersey and Plum Island Sound, Massachusetts account for the passage of Hurricane Sandy, ranked as a 1/900-y event (27) (SI Appendix, Fig. S1 shows detailed salt marsh erosion measurements immediately before and immediately after Hurricane Sandy).Open in a separate windowFig. 1.Relationship between dimensionless wave power (P*) and dimensionless erosion rate (E*) in salt marshes (R² = 0.6; p < 0.05). Gray circles indicate values obtained by averaging data points over regular intervals to emphasize the overall linear trend. The gray area is the uncertainty of the prediction of E* over a range of coefficients with 95% bounds, which are equal to 0.64 and 0.7. The nondimensionalization has been carried out assuming that, if a linear relationship is valid for individual data points, then a linear relationship is valid for the averages as well, such that E = aP and E_avg = a_avgP_avg. A general relationship, valid for all sites, is then obtained and reads E* = a*P*, where E* = ?E/?E_avg, P* = P/P_avg, and a* = 0.67.Two important observations are behind the linear nature of the relationship. The first observation is that salt marsh erosion continuously occurs, even under low wave energy conditions, suggesting the absence of a critical threshold in wave energy below which no erosion is expected. This result underlines the importance of relatively low wave energy conditions for salt marsh lateral retreat. The second observation is that, as wave energy increases, salt marshes do not respond with a catastrophic collapse (e.g., absence of exponential growth in erosion rates), highlighting the intrinsic endurance of salt marshes against extreme events. Scatter in the data arises from several sources of uncertainties, such as different methods used for the calculation of wave power and the estimation of erosion rates.We use this general relationship to investigate long-term salt marsh behavior under realistic wave energy conditions. For this purpose, we collect meteorological data for a 23-y period (from 1991 to 2014) at eight different salt marsh locations in the United States and compute the corresponding wave energy time series (Methods, Fig. 2, and SI Appendix, Figs. S2 and S3). The areas taken into account were chosen to maximize the occurrence of major hurricanes (SI Appendix, Fig. S4). We use wave energy and Eq. 1 to estimate yearly salt marsh erosion rates (Fig. 2). The erosion rate maintains a similar value in different years and at different locations. Moreover, the years characterized by the occurrence of extreme events, such as hurricanes or tropical depressions, do not necessarily correspond to peaks in erosion rate.Open in a separate windowFig. 2.Dimensionless wave power P* (blue) and dimensionless erosion rate E* (pink) for each study site. Wave power values, P*, are daily averages. Yearly erosion rate values and bounds (pink) were obtained using the regression coefficients calculated for the linear relationship between wave power and erosion rate. Major storms affecting the areas of interest are indicated (SI Appendix, Fig. S4 and Table S1 show storm category and date).We further categorize wind data according to the Beaufort wind scale and assess the contribution of each wind category to the total erosion rate of the entire period of record (Fig. 3 and SI Appendix, Fig. S5). The highest contribution to marsh edge erosion comes from moderate but frequent weather conditions (wind speed ranging from 10 to 40 km/h), whereas violent storms and hurricanes (wind speed above 65 km/h) contribute less than 1% to the total marsh edge erosion. This result is because of the linear nature of the relationship between wave power and erosion rate and the short duration of extreme events. In fact, although the action of moderate weather conditions spans most of the study period, the erosion potential of extreme events is concentrated within a few days per year.Open in a separate windowFig. 3.Average contribution of different wind categories to salt marsh erosion rates: calm, 0.1% ± 0.05%; light air, 4.0% ± 1.9%; light breeze, 5.0% ± 2.7%; gentle breeze, 36% ± 8.3%; moderate breeze, 18.0% ± 3%; fresh breeze, 24.0% ± 5.7%; strong breeze, 7.0% ± 2.5%; near gale, 5.0% ± 3%; gale, 0.2% ± 0.1%; strong gale, 0.2% ± 0.1%; storm, 0.2% ± 0.07%; violent storm, 0.2% ± 0.05%; and hurricane, 0.1% ± 0.05%. Plots refer to the entire period of record (SI Appendix, Fig. S5 shows the contribution of each wind category to a specific field site).This behavior can be well-explained in terms of geomorphic work. Following a magnitude–frequency analysis (28), we can multiply the magnitude of marsh retreat for a given wind event by the event’s frequency to find the wind event that does the most geomorphic work. This product attains a maximum, indicating the frequency at which the largest portion of the work is accomplished (28). Our test cases show that the maximum erosion is attained for frequent and low-wave energy conditions, occurring with a return period of 2.5 ± 0.5 mo (Fig. 4 and SI Appendix, Fig. S6). Our results suggest that events occurring with a monthly frequency, such as, for instance, winter storms associated to cold front passages in the Gulf of Mexico in the United States, lead to more marsh erosion than hurricanes occurring at a decadal timescale.Open in a separate windowFig. 4.For the Virginia Coast Reserve, frequency–magnitude distribution of dimensionless wave power f(P*) (dashed black line), total erosion (black line), and dimensionless erosion rate E* (dashed blue line) as a function of P* and its return period, T, in months. For the Virginia Coast Reserve, the return period of wind waves causing maximum erosion is 3 mo. The average return period for all sites is 2.5 ± 0.5 mo (SI Appendix, Fig. S6 shows the plot of geomorphic work for individual bays).Therefore, extreme storms are not the dominant threat to salt marsh stability, such as they are to other coastal environments. As an example, beach dunes generally dissipate wave energy during mild storms, whereas they can collapse during hurricanes (29). Moreover, although the response of sandy beaches to external drivers presents multiple stable states and the effect of storms is amplified or mitigated depending on environmental conditions (29, 30), the response of salt marshes is constant across different geographic regions and for different climatic conditions. Our analysis is only valid for salt marshes and might not be applicable to brackish or freshwater intertidal vegetation, which sometimes fail during hurricanes given their weaker root system (20). The linear relationship between wave energy and erosion and the fact that salt marsh erosion rates vary little from year to year enable the prediction of the long-term fate of these environments and the estimation of their lifecycle (3). Even if salt marshes are constantly deteriorating at a slow rate, their predictable response to a wide range of storms and the possibility of forecasting both their lifespan and mitigation effects make these landforms well-suitable for ecosystem-based coastal defense.     

Rapid shoreward encroachment of salt marsh cordgrass in response to accelerated sea-level rise

The distribution of New England salt marsh communities is intrinsically linked to the magnitude, frequency, and duration of tidal inundation. Cordgrass (Spartina alterniflora) exclusively inhabits the frequently flooded lower elevations, whereas a mosaic of marsh hay (Spartina patens), spike grass (Distichlis spicata), and black rush (Juncus gerardi) typically dominate higher elevations. Monitoring plant zonal boundaries in two New England salt marshes revealed that low-marsh cordgrass rapidly moved landward at the expense of higher-marsh species between 1995 and 1998. Plant macrofossils from sediment cores across modern plant community boundaries provided a 2,500-year record of marsh community composition and documented the migration of cordgrass into the high marsh. Isotopic dating revealed that the initiation of cordgrass migration occurred in the late 19th century and continued through the 20th century. The timing of the initiation of cordgrass migration is coincident with an acceleration in the rate of sea-level rise recorded by the New York tide gauge. These results suggest that increased flooding associated with accelerating rates of sea-level rise has stressed high-marsh communities and promoted landward migration of cordgrass. If current rates of sea-level rise continue or increase slightly over the next century, New England salt marshes will be dominated by cordgrass. If climate warming causes sea-level rise rates to increase significantly over the next century, these cordgrass-dominated marshes will likely drown, resulting in extensive losses of coastal wetlands.     

Changes in Red Snapper Diet and Trophic Ecology Following the Deepwater Horizon Oil Spill

Red Snapper Lutjanus campechanus were sampled at 33 natural and 27 artificial reef sites in the northern Gulf of Mexico prior to (2009–2010) and after (2010–2011) to examine potential diet and trophic shifts following the Deepwater Horizon (DWH) oil spill. We dissected 708 stomachs for gut content analysis and processed 65 muscle tissue samples for stable isotope ratio-mass spectrometry analysis of δ¹³C, δ¹⁵N, and δ³⁴S. Forty-eight percent of stomachs contained identifiable prey, which we grouped into seven categories: fish, decapods, cephalopods, stomatopods, gastropods, zooplankton, and other invertebrates. Based on these categories, Red Snapper diet was significantly different following the DWH oil spill, and was differentially affected by fish size. The interaction between habitat (natural versus artificial reefs) and DWH oil spill effects was also significant. Significant differences in diet among Red Snapper size-classes were due to low trophic position prey, such as pelagic zooplankton, being more abundant in the diet of larger (>500 mm) Red Snapper, while decapods and fish constituted a higher proportion of the diet of smaller individuals. Red Snapper consumed higher amounts of decapods at artificial (21.9% by mass) versus natural (14.8%) reef sites, but the habitat effect on diet was not significant. The habitat × DWH timing interaction was driven by a decrease in zooplankton consumed at both habitat types, increased benthic prey at natural reefs, and increased fish consumption at artificial reefs in post-DWH oil spill samples. Stable isotope data indicated a postspill increase in Red Snapper trophic position (¹⁵N enrichment) and an increase in benthic versus pelagic prey (³⁴S depletion), both consistent with observed dietary shifts. Overall, results indicate shifts in Red Snapper diet and trophic position occurred following the DWH oil spill, thus the relative abundance of prey resources likely changed.

Received May 30, 2014; accepted February 3, 2015     

Spatial response of coastal marshes to increased atmospheric CO2

The elevation and extent of coastal marshes are dictated by the interplay between the rate of relative sea-level rise (RRSLR), surface accretion by inorganic sediment deposition, and organic soil production by plants. These accretion processes respond to changes in local and global forcings, such as sediment delivery to the coast, nutrient concentrations, and atmospheric CO₂, but their relative importance for marsh resilience to increasing RRSLR remains unclear. In particular, marshes up-take atmospheric CO₂ at high rates, thereby playing a major role in the global carbon cycle, but the morphologic expression of increasing atmospheric CO₂ concentration, an imminent aspect of climate change, has not yet been isolated and quantified. Using the available observational literature and a spatially explicit ecomorphodynamic model, we explore marsh responses to increased atmospheric CO₂, relative to changes in inorganic sediment availability and elevated nitrogen levels. We find that marsh vegetation response to foreseen elevated atmospheric CO₂ is similar in magnitude to the response induced by a varying inorganic sediment concentration, and that it increases the threshold RRSLR initiating marsh submergence by up to 60% in the range of forcings explored. Furthermore, we find that marsh responses are inherently spatially dependent, and cannot be adequately captured through 0-dimensional representations of marsh dynamics. Our results imply that coastal marshes, and the major carbon sink they represent, are significantly more resilient to foreseen climatic changes than previously thought.Coastal marsh extent and morphology are directly controlled by rate of relative sea-level rise (RRSLR) and the soil accretion rate, the latter associated with inorganic sediment deposition and organic soil production by plants. Previous studies observed that CO₂ fertilization increases marsh plant biomass productivity through increased water use efficiency and photosynthesis (1), and hypothesized that, as a consequence, marsh resilience should increase via increased organic accretion (2, 3). However, this hypothesis has not yet been tested, and the observed increased plant productivity in response to the CO₂ fertilization effect has not been translated into its actual geomorphic effects. In fact, direct CO₂ effects on vegetation and marsh accretion (as opposed to its indirect effects, e.g., via the increase in temperature) have not yet been incorporated into marsh models, and their importance relative to other leading forcings of marsh dynamics (e.g., inorganic deposition, RRSLR, nutrient levels) remains unknown. Here we use existing data and a 1D ecomorphodynamic model to assess the direct impacts of elevated CO₂ on marsh morphology, relative to ongoing [e.g., RRSLR, and suspended sediment concentration (SSC)] and emerging [nutrient levels (4–6)] environmental change.     

A trophic cascade regulates salt marsh primary production

Nutrient supply is widely thought to regulate primary production of many ecosystems including salt marshes. However, experimental manipulation of the dominant marsh grazer (the periwinkle, Littoraria irrorata) and its consumers (e.g., blue crabs, Callinectes sapidus, terrapins, Malaclemys terrapin) demonstrates plant biomass and production are largely controlled by grazers and their predators. Periwinkle grazing can convert one of the most productive grasslands in the world into a barren mudflat within 8 months. Marine predators regulate the abundance of this plant-grazing snail. Thus, top-down control of grazer density is a key regulatory determinant of marsh grass growth. The discovery of this simple trophic cascade implies that over-harvesting of snail predators (e.g., blue crabs) may be an important factor contributing to the massive die-off (tens of km(2)) of salt marshes across the southeastern United States. In addition, our results contribute to a growing body of evidence indicating widespread, predator regulation of marine macrophyte production via trophic cascades (kelps, seagrasses, intertidal algae).     

林业血防工程对环境生态因子影响的研究

目的了解林业血防工程对相关环境生态例子的影响。方法在长江安徽段上、中、下游各选择一个实施林业血防工稃的环境,并在其邻近选择一个有螺草滩或芦苇滩环境作为对照观察点,分别观察环境草本高度和草本盖度,测定土壤水分、温度、硬度及有效磷、有效钾、碱解氮等的含量,比较分析林地和草滩有螺环境中柑芙环境因子的差异。结果3个试点林业血防工程环境平均草本高度和土壤硬度均高于草滩环境,但平均草本盖度、土壤温度和土壤水分均低于草滩环境。林地环境土壤pH值趋于碱性化,土壤中的有效磷、有效钙、有效硫、有效锌等成分含量均低于芦苇滩环境,有效锰成分林地环境则高于芦苇滩环境。结论林业血防工程对环境生态因子产生一定的影响,促使环境因子向不利于钉螺孳生繁殖的方向发展。     

Long-term impacts of the Exxon Valdez oil spill on sea otters, assessed through age-dependent mortality patterns

We use age distributions of sea otters (Enhydra lutris) found dead on beaches of western Prince William Sound, Alaska, between 1976 and 1998 in conjunction with time-varying demographic models to test for lingering effects from the 1989 Exxon Valdez oil spill. Our results show that sea otters in this area had decreased survival rates in the years following the spill and that the effects of the spill on annual survival increased rather than dissipated for older animals. Otters born after the 1989 spill were affected less than those alive in March 1989, but do show continuing negative effects through 1998. Population-wide effects of the spill appear to have slowly dissipated through time, due largely to the loss of cohorts alive during the spill. Our results demonstrate that the difficult-to-detect long-term impacts of environmental disasters may still be highly significant and can be rigorously analyzed by using a combination of population data, modeling techniques, and statistical analyses.     

A coupled geomorphic and ecological model of tidal marsh evolution

The evolution of tidal marsh platforms and interwoven channel networks cannot be addressed without treating the two-way interactions that link biological and physical processes. We have developed a 3D model of tidal marsh accretion and channel network development that couples physical sediment transport processes with vegetation biomass productivity. Tidal flow tends to cause erosion, whereas vegetation biomass, a function of bed surface depth below high tide, influences the rate of sediment deposition and slope-driven transport processes such as creek bank slumping. With a steady, moderate rise in sea level, the model builds a marsh platform and channel network with accretion rates everywhere equal to the rate of sea-level rise, meaning water depths and biological productivity remain temporally constant. An increase in the rate of sea-level rise, or a reduction in sediment supply, causes marsh-surface depths, biomass productivity, and deposition rates to increase while simultaneously causing the channel network to expand. Vegetation on the marsh platform can promote a metastable equilibrium where the platform maintains elevation relative to a rapidly rising sea level, although disturbance to vegetation could cause irreversible loss of marsh habitat.     

Health Consequences among Subjects Involved in Gulf Oil Spill Clean-up Activities

Background

Oil spills are known to affect human health through the exposure of inherent hazardous chemicals such as para-phenols and volatile benzene. This study assessed the adverse health effects of the Gulf oil spill exposure in subjects participating in the clean-up activity along the coast of Louisiana.

Methods

This retrospective study included subjects that had been exposed and unexposed to the oil spill and dispersant. Using medical charts, clinical data including white blood cell count, platelets count, hemoglobin, hematocrit, blood urea nitrogen, creatinine, alkaline phosphatase (ALP), aspartate amino transferase (AST), alanine amino transferase (ALT), and somatic symptom complaints by the subjects were reviewed and analyzed.

Results

A total of 247 subjects (oil spill exposed, n = 117 and unexposed, n = 130) were included. Hematologic analysis showed that platelet counts (× 10³ per μL) were significantly decreased in the exposed group compared with those in the group unexposed to the oil spill (252.1 ± 51.8 vs 269.6 ± 77.3, P = .024). Conversely, the hemoglobin (g per dL) and hematocrit (%) levels were significantly increased among oil spill-exposed subjects compared with the unexposed subjects (P = .000). Similarly, oil spill-exposed subjects had significantly higher levels of ALP (76.3 ± 21.3 vs 61.2 ± 26.9 IU/L, P = .000), AST (31.0 ± 26.3 vs 22.8 ± 11.8 IU/L, P = .004), and ALT (34.8 ± 26.6 vs 29.8 ± 27 IU/L, P = .054) compared with the unexposed subjects.

Conclusion

The results of this study indicate that clean-up workers exposed to the oil spill and dispersant experienced significantly altered blood profiles, liver enzymes, and somatic symptoms.     

Health effects of exposure to oil spills

The sinking of the oil tanker Prestige off the coast of Galicia was not only the worst ecological disaster ever to affect Spain, it also led to thousands of people who participated in the cleanup of the contaminated areas being exposed to potentially dangerous toxic substances. As the airway is one of the principal routes of entry into the body of these toxic compounds, the possible effects of exposure to such spills is of particular interest and concern to respiratory specialists. The paucity of clinical information available on the subject was the motive for this paper, which reviews the scientific studies undertaken in the aftermath of other accidents involving oil tankers and concludes with a summary of the clinical and epidemiological data published to date on the Prestige oil spill.     

Using Movements and Habitat Utilization as a Functional Metric of Restoration for Estuarine Juvenile Fish Habitat

Resource managers use habitat restoration to offset estuarine habitat loss; however, there is limited information about how functionally successful restorations have been, particularly with respect to their use by mobile marine predators. Restoration monitoring efforts typically use point-of-capture metrics to assess fish community recovery and habitat use, but this provides little insight into how fish habitat use changes through time. Using translocation experiments, we integrated the movements of California Halibut Paralichthys californicus, a conservation target species, into a point-of-capture monitoring program in a restored tidal creek estuary. Large halibut (>25 cm) were captured more frequently in the main stream channel, while small ones (<25 cm) were typically caught in the innermost marsh creeks. We actively tracked these fish (n = 20; size range = 26.6–60.5 cm TL) acoustically to identify their preferred habitats and challenged these habitat associations by means of translocations to a different habitats. Large fish tended to have small localized convex hull activity spaces, remaining in areas with high water flow and sandy substratum near eelgrass Zostera marina beds. Individuals that were translocated to marshes returned to the channel and exhibited movements over long distances from their initial locations to their last tracked positions; however, fish that were translocated from marshes to the channel remained in channel habitat and moved smaller distances between their first and last tracked points. Large halibut likely selected the channel because higher water flow would lead to higher concentrations of prey. Small halibut used marshes more frequently, likely because marshes have temperatures thought to maximize growth rates. Our study can serve as a proof of concept that linking point-of-capture and tracking data provides valuable information for habitat restoration, including the fact that California Halibut utilize estuaries in a size-segregated manner based on environmental conditions. This suggests that tidal creek estuaries with a variety of channel types and morphologies—like our study site—are well-suited to support this species.

Received August 18, 2015; accepted March 1, 2016