With manmade openings at the Edgartown Great Pond a long-established practice, I would like to raise a question about the more recent practice of dredging.
With manmade openings at the Edgartown Great Pond a long-established practice, I would like to raise the question of whether the more recent practice of dredging near the openings constitutes sound environmental management.
It is my understanding that the scientific advice on managing the Great Pond is that we should maximize the exchange of water between ocean and pond. I assume that this flushing should take place during the bio-active months since the intent of the openings is to influence the bioactivity in the pond.
The pond rises to the height necessary to make an opening by the addition of fresh water. A fourth of this water comes from direct rainfall and three fourths come from groundwater flow. This is the annual fresh water budget, and it limits the amount of water available for pond recharge.
After an opening is cut, most of the pond’s water is exchanged in the first week of tidal flow. By the fourth day, more than half the pond’s water has been exchanged. The rate of exchange diminishes daily, and by the 12th day the exchange is negligible. Significant pond-ocean exchange will not occur again until after the next opening. This is the reason to close and recharge the pond as quickly as possible.
The dredge should be deployed only if the obstruction is such that an opening cannot be made using the traditional mechanical method: digging out the cut with an excavators, by hand or both. That does not seem to be the case here. It appears that we are being asked to dredge so that the duration of the opening is extended. This is the opposite of sensible water management. A long duration opening drains the reservoir of groundwater that is needed for the next opening. Not only is the date of the next opening postponed by delaying the start of the recharge, the duration of the recharge is prolonged by the loss of the groundwater that has been spent for negligible gain.
The Edgartown Great Pond is not a sea-level pond. Natural openings happen about every 20 years. During the decades between those events, if the town did not intervene, the pond would remain in its natural state; always closed, three feet higher than sea level, slightly brackish, and quite eutrophic. Hence the openings.
But these two things should not be confused. A natural opening cannot be used as the model that defines a successful, intentional opening. A natural opening is the small visible part of a much larger animal — a weather event that changes the sediment flow along the entire south-facing beach. It can take months for the beach and its offshore bars to settle into a new equilibrium after the extreme storms that open the pond.
By contrast, a deliberately cut opening will not change the fundamental integrity of the barrier beach or the balance of its offshore bars. It is a temporary structure and it has a specific purpose: exchange between ocean and pond. An opening’s service life is brief. By the 12th day it has ceased to be an instrument of ocean-pond exchange, and has become instead a pond-to-ocean drainage canal.
Beach cuts are a tool for managing a pond that is higher than sea level. They are not capable of transforming it into a pond that is sea level. The ecological spasms the Edgartown Great Pond has suffered in the last decade are the result of attempts to do exactly that. A long time ago, Hiram Jackson, the legendary late Edgartown shellfish warden who worked on the waterfront all his life, told me that people have the hardest time getting through their heads the fact that the Great Pond is higher than sea level.
The optimal duration of an opening is a question I would like to understand more accurately.
Dudley Levick lives in Edgartown.
Comments
Thank you for your
Nelson Sigelman Vineyard HavenThank you for your interesting perspective. I'd be interested to know where herring creek fits into this equation and efforts to restore the once robust herring population.
The herring creek and the
Dudley LevickThe herring creek and the herring are two different questions. The herring creek is being extinguished by the encroachment of the barrier beach.
The herring, like blue crabs, white perch, white mullet, and striped bass, require openings to enter and leave the pond. Pond management that increases the frequency of the openings would be to their benefit.
The crux of the matter is the limited amount of fresh water. Dredging succeeds in extending the duration of the openings. It also reduces the number of openings that can be made. If the opening site is dredged, we can expect two openings in a season, each 95% effective in pond-ocean exchange. If the opening is not dredged, we can expect four openings in a season, each of 70% effectiveness.
During the summer of 2017, I
Pondfront Fisher EdgartownDuring the summer of 2017, I began building a new home on Edgartown Great Pond. I had the luxury of observing the Pond on a daily basis every day during the 360 days of the home's construction. Each day I would walk down to the pond, look for fish and take notice of what had changed. During the 2017 Fall and 2018 Winter, the Pond had crystal clear visibility and I was able to go out several times over the winter in my tin boat and catch stripers. As Spring 2018 approached, I noticed an extreme rise in the water level of the Pond as it creeped closer up into the backyard of the property. Obviously many folks on the pond experienced the same thing and many had water issues in their homes.
As you know, the cut was opened in March of 2018 and the drop in water level was very extreme. I contacted Emily a few times by email to offer her my first hand observations on the status of the Pond and expressed my concern over what I feared might happen due to the length and severity of the opening.
We received an email during the summer from the Foundation that stated: "At this time we do not know what caused this bloom, but there was likely some sort of disturbance to the natural balance of the estuary that caused the dramatic growth of the algae".
There were three obvious changes I noticed over that past year:
1. The constant rise in water level of the Pond over the winter.
2. The dramatic and severe decrease in water level once the cut was opened and the fact the cut remained open for so, so long.
3. An explosion of vegetation growing along the shoreline of the Pond due to extreme low water level and the size of newly exposed mud flats.
Of the three events outlined above, the single one to give me most worry was item #3, the rapid and dense growth of vegetation during the Spring on the exposed mud flats due to low water. In front of my home, where there once was 2'-3' of clear water, there were now mud flats that extended into the pond 20'-30' further out than the traditional shoreline of the previous Summer and early Fall. As these mud flats were exposed and left open to the sun for so long, there was a massive growth of vegetation.
Within days of the cut closing, the mud flats got covered with water and I began to see the beginning stages of the decaying process of all that new vegetation along the shorelines. Each day there was less and less visibility. The once clear water was rapidly changing into a murky brown and the bottom was now covered with dark brown, dead plant material. I feel strongly that this is the single event that upset the delicate balance of the Pond and resulted in the 2018 summers algae blooms.
As an avid sportsman, I have spent countless hours on the water chasing gamefish and I knew that this new vegetation when covered by water once the cut closed, would result in the death and the subsequent decaying of the vegetation as it lay on the bottom of the Pond. To this day, there remains a layer of dead vegetation and brown algae coating the bottom of the pond in many areas.
I'm not sure what the answer is but I'm convinced that had the pond been closed much sooner, the vegetation along the shoreline would not have had a chance to reach the height and depth of that past Spring and perhaps the water quality would have remained consistent with the past few years. I agree that much more research is needed to evaluate just how long the cut needs to remain open to achieve optimal results. If the cut can be opened my man, I see no reason why man can't control the length of the opening.
I grew up in Edgartown, was a
J Klingensmith Naples FloridaI grew up in Edgartown, was a boy scout and many times we camped out near Edgartown Great Pond. I received many merit badges while performing different tasks and completing them, which in turn increased my knowledge about the land around me. My times spent at the pond on the camping trips learning from my Scout Leaders about the land i grew up on are times I will always cherish. Because of the interest I accumulated by my time spent at Great Pond generated by my time in the Boy Scouts, I have read many books and even gone to a few seminars to increase my knowledge on the preservation on pond estuaries. One thing that I would like to happen is for the Home Owners around Great Pond to seek some alternative type fertilizers for their yards so it doesn't run into the pond and kill the natural balance of the water.
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