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1 Amherst College, Amherst, MA
2 The Pennsylvania State University, University Park, PA
3 Marine Biological Laboratory, Woods Hole, MA
* Corresponding author: JRCavatorta{at}amherst.edu
Salt marshes survive sea-level rise by an accompanied increase in elevation. This increase in elevation results from the accumulation of organic matter produced by marsh plants and of sediment transported to the marsh platform by tidal activity, storm events, and rafts of ice (1). Sea level is currently rising and is predicted to continue doing so (2); if marshes cannot increase in elevation at an equal or greater rate, inundation may eventually occur (3). Marsh inundation initially occurs by an increase in internal marsh ponds (4). We traced changes in internal marsh ponds and identified the spatial distribution of suspended solids in tidal waters and marsh sedimentation in the Parker River estuary in northeastern Massachusetts.
Changes in marsh ponds were qualitatively assessed by comparing a 1953 topographic map of the area, compiled from early 1950s aerial photography, with 2001 color orthophotography. The topographic map was obtained from the National Oceanic and Atmospheric Administration (NOAA) and georeferenced (i.e., given a geographic location) using ArcGIS (version 8.3.0) software.
We determined the distribution of total suspended solids (TSS) along the main axis of the estuary as well as along three third-order tidal creeks within marshes of the lower estuary. We collected known volumes of water at high water on a spring tide adjacent to where sedimentation was examined and filtered it through pre-weighed 0.7-µm glass fiber filters. We deployed 94 sediment traps along the estuary; these consisted of pre-weighed 9-cm glass fiber filters secured with rubber bands to upside-down plastic petri dish covers (5). Marsh grass was cut away, and a galvanized nail was inserted through the petri dish covers to secure the structures to the marsh. At West Creek, Club Head Creek, and Nelson Island Creek, we set up transects of sediment traps perpendicular to a mosquito ditch and a first-, second-, and third-order stream (Fig. 1, inset). Each transect consisted of three sites: 4, 20, and 50 m in from the creek edge to investigate the amount of sediment reaching the marsh interior. Eleven sites were also selected along the Parker River (Fig. 1). We deployed two replicate sediment traps at each site. We recovered two sets of sediment traps on 2 and 23 July 2003, after they were exposed to several spring tidal cycles. Several samples (n = 5) with anomalously high weight increases were moderated by rinsing with distilled water to reduce high salt concentrations.
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Internal marsh ponds have increased in number and area since the early 1950s. Many areas that are largely ponded today (several of which cover thousands of square meters) were not ponded on the 1953 topographic map. Few ponds appear (during either year) in the upper 10 km of the estuary. Broad marshes adjacent to the lower third of the estuary are densely ponded. Pond drainage and vegetative recovery seem to have occurred only rarely.
TSS along the main axis clearly show the influence of Parker River runoff: solids in the water column are most concentrated in the upper estuary and decrease towards the sound (P = 0.003), which receives regular exchange with the ocean (Fig. 2B). TSS were also low at the mouths of tidal creeks adjacent to the sound and increased in the second- and third-order reaches, where TSS is about double the amount in the sound (Fig. 2A).
Nelson Island Creek received 0.050 g of sedimentation per 9-cm filter, significantly more than West Creek, which is located farther upstream (P = 0.009). Club Head Creek, which is located between them, received an intermediate amount of sediment (Fig. 2E). We did not observe a strong relationship between TSS and sediment accumulation; their maximum values, however, overlap somewhat in the creeks (Fig. 2A, C). The slight overlap is not surprising since TSS measurements were taken only once. More TSS measurements would perhaps yield mean values more strongly correlated with sedimentation. We did not observe a significant difference between sediment accumulation 4 m and 50 m from the creek.
The three creeks considered in this study are fed from water with relatively low TSS. Our data indicate that sediment accreting on the marsh platform may be generated within the creeks themselves, presumably from eroded riverbanks. Slumping banks deliver large amounts of sediment to stream channels, where it may become suspended and deposited on the marsh surface during flood tides. Examination of 1953 and 2001 imagery supports this conclusion, as creek bank erosion is pronounced near the mouths of these creeks, especially at Nelson Island Creek where the most sedimentation was observed.
The spatial distributions of sedimentation and TSS along the main axis of the estuary are complex, but generally decrease with distance down the estuary. Parker River sediments seem to originate at the headwaters of the river and to remain in its system. This sediment source may explain why internal ponds are rare along the river’s upper reaches (although a higher concentration of mosquito ditches, dug to drain the marsh, also prevent pond formation). Greater sedimentation may not have been observed along the Parker because of the brevity of the study and the fact that summer is typically the season with the lowest sediment transport (5). Generally low sedimentation may also explain why decreasing sedimentation was not measured farther from the creek bank. Sediment sources from other nearby creeks may also have inflated interior marsh accretion.
Because of limited sediment source in the creeks, belowground plant production may be more important than sedimentation in marsh accretion, even though softer organic sediments may be greatly compacted. This supposition is corroborated by the fact that the Parker River marsh sediments consist of 55% organic matter or more. Because plant productivity is higher on creek banks (7), depressed internal areas may develop because subterranean accretion rates are slower away from the creek bank.
The increase in size and number of internal ponds, which coincides with observed patterns of sedimentation, may be a useful indicator that marshes are not maintaining elevation relative to a rising sea level. Because the marshes studied are typical of New England macro-tidal marshes, similar marsh degradation is likely occurring in other areas.
This study received support from The Woods Hole Marine Sciences Consortium, the Plum Island Sound LTER NSF #OCE-9726921, and the Atlantic Coast Environmental Indicators Consortium EPA grant number R828677. Special thanks to Hap Garritt and W. McDonald Lee for their assistance.
Literature Cited
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