FREQUENTLY ASKED QUESTIONS
What’s wrong with the status quo? Why can’t the marsh just stay the way it is?
The reality is that the marsh won’t just stay the way it is. Because the watershed dynamics have been changed by the emplacement of the dike and by subsequent ditching, the natural systems of the Herring River estuary and marsh are in an ongoing struggle to establish a new state of equilibrium. Over the last century such a balance has not been achieved, and so the ecosystems continue to evolve.
Sediment cores retrieved from the Herring River system indicate that it had been a stable salt marsh for approximately 2000 years. The presence of salt water in the system, inhibiting freshwater plant colonization and the balance between deposited sediment and rising sea level maintained this salt marsh ecosystem.
The emplacement of the dike and ditches has artificially induced vegetational succession, creating a strange upland ecology located at elevations below mean high tide! Within our lifetimes, large regions behind the dike have progressed rapidly from a marsh to open meadow to an upland forest ecosystem.
The Herring River estuarine-marsh system currently suffers from episodically severe water quality problems related to this change. There is no inexpensive and practical way to freeze the evolution of the Herring River at this current ecologically and geologically unstable point in its succession. Even if nothing is done at the dike or elsewhere in the Herring River, this area will continue to change.
Management action will be necessary to stabilize the system. The most practical and economical management alternative to restabilize the Herring River would be the restoration of a tidal salt marsh in the lower reaches of the system.
Why are there fewer fish upstream of the Herring River dike? (What is the cause of summertime oxygen depletion and how can the problem be fixed?) (What is the cause/significance of the decaying organic matter within the system?)
Although the Herring River in Wellfleet is a complex system, there are several clear causes for a reduction in fish individuals and species in the waters above the dike. The dike’s small opening restricts water flow in and out of the upper reaches of the estuary and marsh, decreasing mean tidal range. This reduces the submerged and intertidal habitat available to fish for shelter, forage, and spawning areas.
Over the past century, this has made it harder for fish to successfully reproduce and survive in their original numbers. Some species may have disappeared from the marsh entirely.
Farther up the system, in the freshwater portion of the marsh, serious water quality issues pose more problems for fish species. The low pH (high acidity) of the water leaches aluminum out of clays in the marsh sediments, and aluminum is toxic to fish in very low doses in the water column (>3ppm).
The drained marsh peats in this area decompose more rapidly in the air than they would if they were submerged twice a day by tides, and so release more organic material and nutrients into the water column. The chemical breakdown and biological consumption of this debris uses up much of the dissolved oxygen, contributing to generally low levels of oxygen in the water column.
At times, particularly in the summer, this demand is so high that it uses up all of the available oxygen, and there is none left for the fish to breathe, causing fish kills. The high organic content of marsh deposits is natural, and an unaltered marsh can handle the high volume of nutrients.
A normal marsh is “flushed” twice daily with seawater. The chemical composition of seawater contains elements that react with and effectively neutralize some of the nutrients (a process called denitrification), and the physical action of the daily tides washes excess organic debris out to sea.
The reason there is a problem with organic debris “building up” in the Herring River system is that the natural chemical buffers have been removed and the decomposition of the organic debris into easily digested nutrients has been accelerated.
What is the relationship between water impoundment, sea level rise, and marsh surface subsidence?
A natural unaltered salt marsh, such as nearby Blackfish Creek, can compensate for gradual rise in sea level through the deposition of organic material layered with sediments washed into the marsh system on flood tides. The Herring River dike blocks the influx of inorganic sediments (sand and silt), handicapping the marsh’s ability to compensate for rising sea level.
After dike construction, decaying organic matter became the dominant remaining marsh deposit. As the water table dropped further in response to ditching and channelization, which increased the discharge of water to the sea from the marsh region, these organic deposits (peats) began to dewater and shrink.
The individual pore spaces between grains of sediment and organic debris had been supported by water, but as the peats dried out these pores collapsed under their own weight, and the marsh surface subsided still more. In the drained section of the marsh, organic material began to decompose more quickly due to the increased oxygen present in air compared to water, also contributing to subsidence.
Presently, the restricted marsh surface elevation upstream from the dike is 70 cm (over two feet) lower than the natural marsh surface just downstream. A casual observer can note the differing elevations from the hill above the dike, as well as the large difference in tidal range.
What is the cause of acidification and what are its impacts?
The high acidity of water in parts of the Herring River system is one link in a chain of events that began with the building of the dike in 1908-09. The dike effectively eliminated salt water intrusion and therefore greatly reduced the tidal prism upstream.
Without the pressure of the tidal prism in place, the local aquifer increased its outflow (discharge) into the marsh, replacing salt water with fresh. The increased outflow, together with the lowered mean high tide mark, caused the local water table to begin to drop in the wetlands upstream from the dike. Subsequent ditching and channelization of the river and marsh have further lowered the water table.
The marsh surface upstream from the dike has subsided, but the water table has dropped even more. Large areas of salt marsh peat have become completely drained, and are now located well above the restricted high tide elevation. This dried-out peat is the source of the acid that finds its way into the water column in the Herring River.
Salt marsh peat contains high levels of the mineral pyrite, which is composed of sulfur and iron. In normal marshes, the peat is consistently flooded daily by high tides, and an anaerobic (low oxygen) environment is maintained. When this peat is dried out, however, the pyrite is exposed to air, which has significantly more oxygen in it than does water.
The iron in the pyrite essentially rusts out, liberating the sulfur, which enters the water column as sulfuric acid. This acid lowers the pH of the water. Values as low as 3.5 pH have been measured following summer rainfalls that washed large amounts of the dried-out peat into the water. This highly acidic water then leaches toxic minerals, especially aluminum and ferrous iron, out of the clays and organic material in the salt marsh deposits and they wind up in the water column as well.
What are the possible sources of nitrogen loading in the Herring River system?
There are currently no known or suspected point sources (this is generally understood to refer to human waste) for nitrogen loading in the Herring River system—all septic systems should be properly maintained. The plume from the Wellfleet landfill at Cold’s Neck is being monitored but is currently not part of the system.
There is limited agriculture in the watershed, so non-point sources of nitrogen pollution are minimal. Natural sources of nitrogen, however, are abundant in a salt marsh, including animal waste and the decomposition of the impressive plant biomass produced by the marsh.
In a salt-water marsh, denitrification is a natural chemical process involving seawater that limits the amount of nitrate available for primary production. In the current Herring River system, this buffering agent has been restricted for many years, and as a result the region has an overabundance of nitrates (as well as phosphates) stored in the drained sections of the former marsh.
When rain falls on the marsh these minerals are washed into the water column where they may cause algal and bacterial overproduction. In a normal salt marsh such as Nauset Marsh or Blackfish Creek, daily tidal flushing of the marsh peats removes or chemically buffers (neutralizes) much of the available nitrogen and phosphorus.
Describe the comparative value of salt vs. fresh water marshes:
Both salt and fresh water marshes support unique ecosystems and add to the biodiversity of Cape Cod. Salt water marshes also act as nurseries for a wide variety of salt and brackish-water species, providing shelter and feeding grounds.
Geologically, they protect inland areas by absorbing the energy of storm waves as they approach the shore, reducing inland erosion and trapping sediment.
Hydrologically, all marshes are groundwater discharge areas that ultimately return rainwater to the sea. When considering the Herring River system, it is important to realize that its lowland regions are not functioning as a normal and healthy freshwater marsh even above the limited influx of salt water.
The upper reaches of the Herring River, for example Bound Brook and most of Duck Harbor, would now be a healthy freshwater marsh environment if not for the lowering of the regional water table which occurred after diking and was accelerated by the channelization, ditching and drainage of the upper marsh.
Even if the dike had never been built, these upper marshes would have continued to freshen after the natural closure of the inlets at Bound Brook and Duck Harbor. These areas will remain fresh even if the town ultimately decides to take management action and open the gates of the dike, due to their mean elevations and spatial distance upstream from the dike.
Reopening the dike would serve to raise the water table in the upper marshes, improving the health of the ecosystem, but would not cause the upper marsh to return to a salt marsh environment.
The Herring River, before diking, was more than just a salt marsh—it was a complex system grading from estuary to salt marshes and brackish-to-fresh water marshes. The lower reaches of the system were estuarine. Unaltered estuaries are the single most productive environments on the planet in terms of biomass.
Expanding the size of the estuary, which currently is restricted to the Harbor area adjacent to the dike, will increase the productivity of the many species that rely on this environment for spawning and nursery grounds.
In the last few decades, recognition of the economic and environmental importance of estuaries and their associated salt marsh communities has spawned global restoration efforts to remediate these heavily impacted ecosystems. Wellfleet is not alone in its position as a community investigating the health of its wetland environments.