Vineyard Great Ponds Impose Natural Limits on Growth
By Sam Low
Vineyard Gazette – July 1, 2003
To me, the limit has always been the point at which we truly begin to destroy the environment that we all love. But what is that limit?
One answer came in a seminar last Friday at the Old Grange Hall hosted by the Conservation Partnership of Martha's Vineyard. The seminar focused on threats to our great ponds - specifically Tisbury Great Pond, Long Cove Pond and Big Homer's Pond - and the limit to growth came in the form of a simple number.
To understand that magic number you first must know a little bit about nitrogen and how it affects a great pond. Nitrogen is food for phytoplankton - tiny organisms living in the pond - so increased nitrogen levels means increased "blooms" of these critters which use up oxygen needed by shell and finfish and create a screen that reduces sunlight needed by plants such as eel grass. The plants, in turn, provide cover for various animals, so when they die off the animals are subject to increased predation.
To put it simply - too much nitrogen flowing into a great pond will kill it.
Ecologists tell us that the current nitrogen loading in the three ponds is 29,000 pounds a year - or 29 one ton pickups backing up to the ponds, fully loaded, and dumping their chemicals. The point at which the ponds begins to die is a load of 33,000 pounds per year - or just 4 more pickup trucks. Another way of saying it is that we are 88% of the way to destroying one of our most treasured environmental assets - a magnet that draws many of us here in the first place.
Of all the nitrogen entering the ponds, 42% comes from acid rain - which we can do little about in the short term - but 35% of it comes from septic tanks, 20% from agriculture and 3% from tending our lawns. This nitrogen flows into the ponds with the groundwater in a huge chunk of real estate - an area of 12,000 acres, one sixth of the sland's total land mass - defined as the ponds' watershed.
So how much more development within the watershed will it take to reach those 4 pickup truck loads of nitrogen?
The answer is complex because there are so many variables involved - such as the nature of the soils, type of septic system and the flow of groundwater - but a house occupied year round by four people with a standard septic system introduces about 20 pounds of nitrogen a year into the groundwater - so we can afford to build only 200 such houses in the pond's watershed. Only 200 houses in an area of 12,000 acres. And that's assuming no further commercial or agricultural development.
If we stop all development in the watershed now we will have a tiny buffer, those four pickup trucks of nitrogen, to protect us from degrading the pond. This particular limit to growth is as sobering as it is real.
What can we do to protect Tisbury Great Pond and other great ponds on the island? On a planetary scale we can support efforts to reduce acid rain. On an island scale, we can help our conservation organizations to buy land and reduce the number of new buildings erected in our watersheds. On a personal scale, we can install more efficient septic systems - modern systems can reduce the flow of nitrogen into groundwater by about 66% - and we can learn how to maintain our existing septic systems properly. We can also become more aware of the chemicals we use daily to wash our clothes and our bodies and fertilize our lawns and gardens.
If we wish to preserve our unique island heritage - we must learn to become stewards of our environment, both globally and locally. From the perspective provided by Friday's seminar - there is no other alternative.
Sam Low is a filmmaker and journalist residing in Oak Bluffs, he is a frequent contributor to the Gazette .