Tracking the dynamic ecological history of a tropical urban estuary as it responds to human pressures.

Coastal cities in tropical areas are often low-lying and vulnerable to the effects of flooding and storms. San Juan, Puerto Rico is a good example of this. It is built around a lagoon-channel complex called the San Juan Bay Estuary (SJBE). A critical channel in the estuary, the Caño Martín Peña, has filled in and now frequently floods the surrounding communities with sewage-enriched waters, causing a series of human health and ecological problems. Sediment core analyses indicate that portions of the SJBE now function as settling basins. High urban and sewage runoff to the Caño contributes nitrogen (N), but stable isotope and sediment nutrient analyses indicate that this runoff may also enhance conditions for coupled sulfate reduction-nitrogen fixation. The amount of 'new' bioavailable N created from inert atmospheric N2 gas may meet or exceed that from the runoff into the Caño Martín Peña. The ecological consequences of this appear to extend beyond the ponded channel, potentially contributing to the poor water quality of the SJBE, greater than contaminated runoff alone.

Ciudades costeras en los trópicos generalmente se encuentran localizadas en lugares de baja elevación y vulnerables a los efectos de tormentas e inundaciones. San Juan, Puerto Rico es un buen ejemplo de esto. Esta ciudad fue construida alrededor de un sistema de lagunas y canales que se conoce como el Estuario de la Bahía de San Juan. Un canal crítico en este sistema es el Caño Martín Peña que en el pasado fue rellenado con sedimentos causando inundaciones en las comunidades vecinas. Estas aguas de escorrentía incluyen aguas residuales y aumentado el riesgo a problemas de salud pública y del ambiente. Análisis de los sedimentos indican que porciones de este sistema funcionan como lagunas de sedimentación. Gran flujo de aguas residuales y escorrentía urbana hacia el Caño aportan nitrógeno (N), pero el análisis de sedimentos y nutrientes por isótopos estables indica que esta escorrentía también aumenta las condiciones por procesos acoplados de reducción de sulfato y fijación de nitrógeno. La cantidad de 'nuevo' N biodisponible creado del gas nitrógeno inerte atmosférico podría lograr o exceder esa fijación del nitrógeno derivado de la escorrentía hacia el Caño. Las consecuencias ecológicas de esto parecen extenderse más allá de este canal estancado afectando así la calidad del agua en el Estuario, mayor aún que los contaminantes encontrados en la escorrentía pluvial por sí sola.

Are Tidal Salt Marshes Exposed to Nutrient Pollution more Vulnerable to Sea Level Rise?

Over the past four decades, Long Island, NY, USA, has lost coastal wetlands at a rate of 4% per decade due to submergence. In this study, we examined relationships between the rate of tidal salt marsh loss and environmental factors, including marsh elevation, tidal range, and wastewater exposure through analysis of stable isotope ratios of marsh soils and biota. Our goal was to identify factors that increase vulnerability of marshes to sea level rise, with a specific emphasis on the potential role of poor water quality in hastening marsh loss. Our results suggest that wastewater exposure may accelerate loss of intertidal marsh, but does not negatively impact high tidal marsh resilience to sea level rise. And while marsh elevation and tidal range were statistically significant predictors of marsh loss, they similarly displayed opposite relationships among marsh zones. This study suggests that different functional zones of coastal salt marshes may not respond similarly to global change factors, and that elevation may be an important factor mediating eutrophication effects to coastal salt marshes.

Anthropocene survival of southern New England's salt marshes.

In southern New England, salt marshes are exceptionally vulnerable to the impacts of accelerated sea level rise. Regional rates of sea level rise have been as much as 50% greater than the global average over past decades: a more than four-fold increase over late-Holocene background values. In addition, coastal development blocks many potential marsh migration routes, and compensatory mechanisms relying on positive feedbacks between inundation and sediment deposition are insufficient to counter inundation increases in extreme low turbidity tidal waters. Accordingly, multiple lines of evidence suggest marsh submergence is occurring in southern New England. A combination of monitoring data, field re-surveys, radiometric dating, and analysis of peat composition have established that, beginning in the early and mid-twentieth century, the dominant low marsh plant, Spartina alterniflora, has encroached upwards in tidal marshes, and typical high marsh plants, including Juncus gerardii and Spartina patens have declined, providing strong evidence that vegetation changes are being driven, at least in part, by higher water levels. Additionally, aerial and satellite imagery show shoreline retreat, widening and headward extension of channels, and new and expanded interior depressions. Papers in this special section highlight changes in marsh-building processes, patterns of vegetation loss, and shifts in species composition. The final papers turn to strategies for minimizing and coping with marsh loss by managing adaptively and planning for landward marsh migration. It is hoped that this collection offers lessons that will be of use to researchers and managers on coasts where relative sea level is not yet rising as fast as in southern New England.

Contrasting decadal-scale changes in elevation and vegetation in two Long Island Sound salt marshes.

Northeastern US salt marshes face multiple co-stressors, including accelerating rates of relative sea level rise (RSLR), elevated nutrient inputs, and low sediment supplies. In order to evaluate how marsh surface elevations respond to such factors, we used surface elevation tables (SETs) and surface elevation pins to measure changes in marsh surface elevation in two eastern Long Island Sound salt marshes, Barn Island and Mamacoke Marsh. We compare marsh elevation change at these two systems with recent rates of RSLR and find evidence of differences between the two sites; Barn Island is maintaining its historic rate of elevation gain (2.3± 0.24 mm yr-1 from 2003 to 2013) and is no longer keeping pace with RSLR, while Mamacoke shows evidence of a recent increase in rates (4.2 ± 0.52 mm yr-1 from 1994 to 2014) to maintain its elevation relative to sea level. In addition to data on short-term elevation responses at these marshes, both sites have unusually long and detailed data on historic vegetation species composition extending back more than half a century. Over this study period, vegetation patterns track elevation change relative to sea levels, with the Barn Island plant community shifting towards those plants that are found at lower elevations and the Mamacoke vegetation patterns showing little change in plant composition. We hypothesize that the apparent contrasting trend in marsh elevation at the sites is due to differences in sediment availability, salinity, and elevation capital. Together these two systems provide critical insight into the relationships between marsh elevation, high marsh plant community, and changing hydroperiods. Our results highlight that not all marshes in southern New England may be responding to accelerated rates of RSLR in the same manner.

Varying Inundation Regimes Differentially Affect Natural and Sand-Amended Marsh Sediments.

Climate change is altering sea level rise rates and precipitation patterns worldwide. Coastal wetlands are vulnerable to these changes. System responses to stressors are important for resource managers and environmental stewards to understand in order to best manage them. Thin layer sand or sediment application to drowning and eroding marshes is one approach to build elevation and resilience. The above- and below-ground structure, soil carbon dioxide emissions, and pore water constituents in vegetated natural marsh sediments and sand-amended sediments were examined at varying inundation regimes between mean sea level and mean high water (0.82 m NAVD88 to 1.49 m NAVD88) in a field experiment at Laws Point, part of the Plum Island Sound Estuary (MA). Significantly lower salinities, pH, sulfides, phosphates, and ammonium were measured in the sand-amended sediments than in the natural sediments. In natural sediments there was a pattern of increasing salinity with increasing elevation while in the sand-amended sediments the trend was reversed, showing decreasing salinity with increasing elevation. Sulfide concentrations generally increased from low to high inundation with highest concentrations at the highest inundation (i.e., at the lowest elevations). High pore water phosphate concentrations were measured at low elevations in the natural sediments, but the sand-amended treatments had mostly low concentrations of phosphate and no consistent pattern with elevation. At the end of the experiment the lowest elevations generally had the highest measures of pore water ammonium. Soil carbon dioxide emissions were greatest in the sand-amended mesocosms and at higher elevations. Differences in coarse root and rhizome abundances and volumes among the sediment treatments were detected with CT imaging, but by 20 weeks the natural and sand-amended treatments showed similar total belowground biomass at the intermediate and high elevations. Although differences in pore water nutrient concentrations, pH, salinity, and belowground root and rhizome morphology were detected between the natural and sand-amended sediments, similar belowground productivity and total biomass were measured by the end of the growing season. Since the belowground productivity supports organic matter accumulation and peat buildup in marshes, our results suggest that thin layer sand or sediment application is a viable climate adaptation action to build elevation and coastal resiliency, especially in areas with low natural sediment supplies.