Great Ponds Impose Natural Limits on Growth
By Sam Low
Vineyard Gazette - July 1, 2003
problem of determining the limits of growth on our island has
always been simply defining the limit. Is it the point at which
most of us get fed up and leave? Too subjective. Is it when
we achieve "build out" according to zoning regulations?
Too political - who sets those limits anyway?
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
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.
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
put it simply - too much nitrogen flowing into a great pond
will kill it.
tell us that the current nitrogen loading in the three ponds
is 29,000 pounds a year - or 29 half-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.
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 island's total land mass - defined as
the ponds' watershed.
how much more development within the watershed will it take
to reach those 4 pickup truck loads of nitrogen?
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.
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.
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
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.