Black Pond and Hawksnest Pond are very different. Black Pond is shallow, filled with vegetation, and high in nutrients, while Hawksnest Pond is deep, mostly free of aquatic plants, and nutrient poor.
Yet Black Pond is about a foot higher than Hawksnest. When water levels are high, a strong current flows from Black to Hawksnest. This probably imports a lot of nutrients into Hawksnest.
In the 1950s tthrough the 1970s, the two ponds were connected only during very high water. Even then, I did not notice much flow between them. High bushes on the isthmus must have filtered any water that did flow from Black Pond.