Without management, the Mill Pond off Edgartown Road in West Tisbury will look more like wetlands than a pond within the next decade, according to the senior biologist in charge of a town-commissioned report released last week.
"The dense vegetation, if left unmanaged, will continue to degrade water quality, impact the pond's fish and wildlife populations, and further contribute to the already seemingly accelerated sedimentation/filling in of the pond," the report says of the 2.5-acre pond beside the West Tisbury police station.
The Mill Pond is actually a man-made impoundment of Mill Brook, reportedly created prior to the early 1800s to power a small family-owned textile mill. Now the pond is used recreationally for fishing, non-motorized boating and wildlife viewing.
In recent years, however, the pond has become increasingly shallow - its average depth is 1.7 feet - and in the last few years, a weed problem has become apparent.
"All of a sudden in 2003, it was really noticeable, and I think that's where people starting getting really concerned," said William M. Wilcox, the Martha's Vineyard Commission's water resource planner. "It was a worry that if something's really wrong here and we don't know what it is, we have to figure it out and do something - we don't want the pond to turn into a swamp."
Biologist Keith Gazaille and Aquatic Control Technology, the Sutton-based company commissioned to do the study, concluded that dredging is the first step in addressing the issues facing the pond. The report says that the pond's vegetation later should be managed with chemical herbicide treatments or mechanical Hydro-Raking - a paddle-wheel-driven barge that can operate in water less than two feet deep and focus on very specific areas.
"Dredging is the only technique that will address, at least in part, both the excessive growth of rooted vegetation and the significant accumulation of sediment," the report says. "...Dredging is certainly the only technique that can appreciably restore the pond's water depths."
The board of selectmen will meet with the conservation commission on Wednesday, Jan. 17 to discuss the report's conclusions. It is natural for ponds to gradually fill in, Mr. Wilcox said, but for man-made ponds, that process is accelerated. Even natural ponds require some level of management in order to keep them in good condition, according to the report. The Mill Pond was last dredged 40 years ago.
Although the town's original motivation for seeking a pond management plan was aesthetic, the report notes that restoring the water depth will do more than improve the recreational value of the pond. It will also improve the habitat for fish and could better protect Tisbury Great Pond from unwanted nutrient loading if Mill Pond's ability to retain nutrients improves. Mill Pond feeds into Town Cove of Tisbury Great Pond through two separate water outlets.
West Tisbury commissioned Aquatic Control Technology for $3,500 last September to study the state of the pond, the reasons for its increasing vegetation and accelerated sedimentation, and present immediate and long-range management options with price estimates.
In mid-September, two Aquatic Control biologists in a 12-foot flat-bottomed boat collected five kinds of data on the pond: a vegetation inventory, water depth mapping, sediment measurement, water quality sampling and a critical wildlife habitat assessment.
The team found that the pond's submerged vegetation is in excess of what is recommended for a warm-water fishery. Thinning the plant population would be better for trout species and herring, the report says. Local efforts last spring to restore Mill Brook and Mill Pond as a viable herring run by installing a fish ladder at the Mill Pond dam had little success.
The biologists also found that phosphorus - which supports algae - was above desirable levels, but the report notes that these levels fluctuate widely and more sampling would be needed to draw conclusions. E.coli was slightly above the maximum levels recommended for swimming, although the Mill Pond is not used for swimming. The tests did not show whether the E.coli is due to waterfowl or other animal waste from the watershed.
Aquatic Control Technology also carries out the kinds of projects that the survey recommends. Depending on what the town decides to do, the company could take care of the "complex multi-tiered permitting process" the report says is necessary for most projects - particularly dredging. Local, state and federal permission are required. The permitting expenses for a conventional dredging project the size of the Mill Pond would be $20,000 to $30,000, the report estimates. The operational dredging costs would be in the range of $400,000 to $600,000.
Dredging would reduce the amount of rooted vegetation in two ways: by removing the nutrient-rich sediment it grows in and, if dredged to depths of eight to 10 feet, by limiting the amount of light that reaches the pond bottom. To dredge to those depths, some of the pond's hard-packed refusal layer, made from sand, gravel or clay, would have to be removed in addition to the "organic muck" sediment layer.
Regardless of what management options the town chooses to pursue, the report encourages periodic testing to monitor regrowth and the introduction of any nonnative or invasive species. The biologists reported not observing any non-native or invasive species in the pond.
After deepening the pond, the report says that chemical herbicide treatment is the most cost-effective and least disruptive way of controlling nuisance aquatic vegetation control. The herbicides are species-specific and the report details which products it recommends for the dominant plants in the pond, which include ribbon-leaf pondweed, coontail and bur-reed.
Before implementing any management techniques, the report recommends studying the land use and watershed delineation more closely to identify potential watershed management issues. The pond is fed by surface water flowing from Mill Brook and direct runoff from its large watershed area, which is relatively undisturbed by development, according to the report. Mill Brook originates 3.5 miles north of the pond and feeds three other ponds.
The report says that it is important to limit potentially high risk land uses, like industrial, commercial and even high-use agricultural land use, as well as residential activities that might increase the level of nutrient transport to the pond, such as lawn fertilizers or faulty septic systems. The report also gives several strategies for limiting nutrient-loading in the pond, including the promotion of natural filtering of storm water, not feeding waterfowl, and preserving natural woodland areas.