Nathaniel B. Weston

Declining Sediments and Rising Seas: an Unfortunate Convergence for Tidal Wetlands

The availability of suspended sediments will be a dominant factor influencing the stability of tidal wetlands as sea levels rise. Watershed-derived sediments are a critical source of material supporting accretion in many tidal wetlands, and recent declines in wetland extent in several large river delta systems have been attributed in part to declines in sediment delivery. Little attention has been given, however, to changes in sediment supply outside of large river deltas. In this study, significant declines in suspended sediment concentrations (SSCs) over time were observed for 25 of 61 rivers examined that drain to the East and Gulf Coasts of the USA. Declines in fluvial SSC were significantly correlated with increasing water retention behind dams, indicating that human activities play a role in declining sediment delivery. There was a regional pattern to changes in fluvial sediment, and declines in SSCs were also significantly related to rates of relative sea level rise (RSLR) along the coast, such that wetlands experiencing greater RSLR also tend to be receiving less fluvial sediment. Tidal wetlands in the Mid-Atlantic, Mississippi River Delta, and Texas Gulf especially may become increasingly vulnerable due to rapid RSLR and reductions in sediment. These results also indicate that past rates of marsh accretion may not be indicative of potential future accretion due to changes in sediment availability. Declining watershed sediment delivery to the coastal zone will limit the ability of tidal marshes to keep pace with rising sea levels in some coastal systems.

Gas Exchange Rates in the Parker River Estuary, Massachusetts

Gas exchange rates between natural waters and the atmosphere are an important component of our understanding of the dynamics of biologically active gases. For example, estimates of whole system metabolism must account for the transfer of gas (OJ between the atmosphere and water. Previous studies in estuaries have relied on dome measurements to estimate O2 exchange rates ( 1,2). In this study we measured gas exchange rates using sulfur hexafluoride (SF,) as a tracer. SF6 is well suited for this application because it is chemically and biologically nonreactive, occurs at low background levels, and extremely low concentrations are readily detectable (3). The gas exchange coefficient, k, calculated for SF, can be related to the exchange coefficient for other gases (4). We injected -0.004 moles of SF, into the Parker River estuary, Newbury Massachusetts, and monitored the evasion of the gas over time by the decrease in its total mass. The tracer was allowed to mix for one tidal cycle and we then sampled SF, concentration in the water at each of seven successive high tides. Surface water samples were drawn into loo-ml glass syringes, transported, submerged in river water, to a field laboratory, and analyzed within 6 h using gas chromatography with electron capture detection. Windspeed and precipitation data were recorded continuously. Several previous surveys were used to determine cross-sectional reas along a IO-km stretch of the estuary. The total mass of SF, in the estuary was calculated by integrating concentration and water volume by estuarine distance. The distribution of the tracer changed over time in relation to processes controlling mixing and loss to the atmosphere (Fig. la). After the initial tidal cycle the tracer plume measured 5.2 km in length, and the distribution was gaussian. The exchange coefficient, k, and mass are related by the function k = In (M/Mo)h/t where M is the measured mass of SF, in the estuary, M. is the previously measured mass of SFs, h is the depth, and t is the time between samplings. Calculated values for kSF6 range from 1.1-6.2 cm. h-‘. Fluctuations in k are well correlated with wind velocity [in agreement with previous studies (4, 5)] and with precipitation (Fig. 1 b). These values are lower than those predicted from wind relations established from dome studies (Fig. 1 c). For estuarine systems with complicated geometry (e.g., channel longitudinal direction, marsh grass, and high tidal range), direct measurement of SF, evasion may be a more accurate determination of gas exchange rates. The importance of gas exchange as a process influencing the determination of system metabolism was determined by applying our measured gas exchange coefficient for SF6 to the calculation of O2 gas exchange. System respiration was calculated by mass loss of dissolved oxygen between dusk and dawn (Fig. Id) and corrected for gas exchange with the atmosphere. The gas transfer velocities of O2 and SF6 are related by the function k,,

Population growth away from the coastal zone: Thirty years of land use change and nutrient export in the Altamaha River, GA

We used more than thirty years of water quality monitoring data collected by the United States Geological Survey at several stations in the Altamaha River and its tributaries to examine the relationship between population density, agricultural land use, and nutrient export from the watershed. Population densities in the Altamaha River watershed increased during the study period, most notably in the upper watershed near metropolitan Atlanta, while agricultural land use declined throughout the watershed. NO(x), TN and P in rivers were related to human population densities, while OC and NH(4)(+) concentrations in rivers were apparently related to agricultural land use. A general pattern of increasing NO(x) and TN and decreasing NH(4)(+), P and OC over time throughout the watershed reflected changing population and land use. The overall average load from the Altamaha River to the coastal zone during the study period was 1.1, 5.6, 16.9, 0.9 and 262 kmol km(-2) yr(-1), delivering 40, 197, 596, 30, and 9213.10(6) mol yr(-1) of NH(4)(+), NO(x), TN, P and OC, respectively, to the coastal zone. The nutrient export patterns suggest that N and P loading to rivers in the Altamaha River watershed was greatest in the upper watershed where high population densities were found, and in-stream processing, dilution, and only moderate inputs during transit through the lower watershed resulted in relatively low export from the watershed to coastal waters.

Estimating Denitrification in Sediments of the Parker River Estuary, Massachusetts

This research was partially supported by grants from WHOI Sea Grant, the Plum Island Sound LMER, the Sweetwater Trust, and the NSF-REU program. We gratefully acknowledge the help of Jim Ledwell, who graciously provided both his expertise and equipment. 2. Balsis, B. R., D. W. M. Alderman, I. D. Buffam, R. H. Garritt, C. S. Hopkinson Jr., and J. J. Vallino. 1995. Biol. Bull. 189: 252254. 3. King, D. B., and E. S. Sal&man. 1995. .I. Geophys. Res. 100: 7083-7088. 4. Upstill-Goddard, R. C., A. J. Watson, P. S. Liss, and M. 1. Liddicoat. 1990. Tellus 42: 364-377. Literature Cited 5. Wanninkhof, R., J. R. Ledwell, and W. S. Broecker. 1987. J. Geoph~x Res. 92: 14,567-14,580. 1. Marino, Roxanne, and R. W. Howarth. 1993. E.rfuaries 16: 4336. Clark, J. F., R. Wanninkhof, P. Schlosser, and H. J. Simpson. 435. 1994. Tellus46: 274-285.

Preliminary Evaluation of Sedimentation Rates and Species Distribution in Plum Island Estuary, Massachusetts

Tidal salt marshes exist in estuaries throughout the temperate zone of the world. Their presence in the future is threatened by accelerating rates of sea level rise associated with the temperature increases predicted by models of global climate change. In northern Massachusetts, sea level has been increasing at a rate of about 2.4 mm yr-’ since 1921 (calculated from data obtained from National Ocean Survey/NOAA Boston tide gauges). If marshes are to remain common features of the coastline, salt marsh accretion must occur at rates greater than the rising sea. Marshes can increase in elevation by the accumulation of plant material within the sediment profile and by the deposition of organic and mineral matter on the surface. Deposition requires the availability of suspended material and a transport mechanism, such as tides and floods, to distribute the material onto the marsh surface (1). The importance of these processes is likely to vary along a range of elevations that determines both the frequency and duration of tidal inundation and the distribution of plant species. Here we report measures of deposition, elevation, and species distribution in salt marshes of the tidal Rowley River in the estuary of Plum Island Sound, Massachusetts. Five 150-250-m transects were established across Rowley River salt marshes that varied in elevation, species composition, flooding frequency, and distance from the ocean and from freshwater inputs. We measured variables at three sites representative of the range of elevation and species along each transect. Relative elevation for each site was determined by measuring water depth at high tide. Short-term sedimentation rates were estimated by deploying three sets of ashed, pre-weighed 9-cm glassfiber filters placed on inverted petri dishes secured to the marsh surface at each site from 17 June to 14 July 1998. Filters were collected every two weeks; dry weight was determined after drying at 60°C (1, 2, 3) and organic matter content by dry weight loss on ignition (4). The weight of salt has not been accounted for, so results represent maximum deposition (3). We used duplicate quadrats at each site on 1 July to measure the species composition, density, and biomass of marsh vegetation (5). We also determined bulk density and organic matter content of sediment by loss on ignition from 50-cm cores collected at each site. The relative elevation range of the sites was 85 cm. Deposition rates decreased with increasing elevation (Fig. la). Sites with