Short answer: Because it has taken time, scientific inquiry, and significant site-specific data to demonstrate this fact in the scientific literature. Midge control for Straits Pond was treated as a yearly, isolated event without considering longer-term solutions such as increasing salinity.
Background:
New Data about salinity above 18-22 parts per thousand (ppt):
A very thorough and scientifically rigorous study was performed regarding the salinity of Chironomidae midges as part of the East Harbor/Pilgrim Pond restoration project. Tim Smith, Coastal Zone Management, described this project during the Straits Pond Forum Series. This study supports a similar conclusion stated by Linda Beres, et. al. in the 1990s Straits Pond studies that salinity maintained above approximately 18-22 parts per thousand (ppt) inhibits midges from pupating, or becoming free-flying insects. It is important to note that midge larvae can persist for a considerable period of time at high salinities so it is necessary to maintain tidal flow and high salinity levels throughout the season.
New Data about Salinity above and below the tide gate:
Although midges do occur down stream from the West Corner Culvert, their existence appears to be insignificant. A significant variable between the Weir River and Straits Pond is salinity. During the spring the salinity in the Weir River is typically above the 20s ppt range and the salinity in Straits Pond is in or near single digits ppt. The low spring salinity in Straits Pond allows the midges to pupate.
New Data about "communication:"
Previously it was thought that it took two tidal cycles to clear the Weir River water from the West Corner Culvert area. Recent data indicates that only one tidal cycle is necessary. Significant data has been collected during the past year, which data indicates that there is good "communication" (that is, exchange of waters) between Straits Pond and the Weir River. With the tide gates in a full-open position the outgoing tidal flow sufficiently transports water from the pond through the estuary into Hull Bay. The incoming flood tide water is significantly different from the previous ebb water as proven by higher salinity readings. Springtime salinity of the flood tide water on the Weir River side of the culvert is similar with respect to salinities observed in Hull Bay (near the pier). There can be significant salinity benefit to Straits Pond by improving exchange between Hull Bay and the pond with each tidal cycle.
New Data about Stratification:
Although previously it was thought that there was stratification in Straits Pond, data from recent water quality sampling indicates that there is little if any stratification in the Straits Pond water. Any stratification is likely to be short lived due to the shallow depth of the pond and mixing via wind. With the equal disbursement of the salinity, if the ppt were to be 20+ ppt, this would significantly impair the ability of the midges to pupate.
New Data about Flushing:
We now know that with the current culvert and tidal gates the maximum tidal exchange of water in Straits Pond during one tidal cycle is approximately 1', depending on the tidal range in the estuary. Greater flushing would have a positive effect on all of the sources of the pond's problems. The sources of the pond's problems are low salinity, as well as high water temperature, low dissolved oxygen, and high nutrient overload. It is important to stress that salinity is only one indicator of Strait Pond's water quality impairment.
Questions for this Season:
The Straits Pond Watershed Association hopes to find the answers to these questions from tests this coming season.
Lawry Reid, president - Straits Pond Watershed Association - 781.925.8659 - viaspeech@comcast.net