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The bay is still alive……

The bay is still alive.  There are crabs walking about, flounders swimming between our feet, and a dense assemblage of baitfish residing in the bay.  The baitfish have attracted loons, mergansers, king fishers, and a number of young Bonaparte Gulls.  There is also a small harbor seal residing along the piers and floats, feasting on the baitfish, and attracting humans.  What’s going on?

Well, all of this coincides with the recent news that this past December was the second warmest recorded by the Blue Hill Observatory (about 20 miles away in Milton, MA) which has been monitoring meteorological conditions since 1885.  The warmest December on top of that hill was in 2006.  This past December was preceded by a relatively mild fall, so the bay remains fairly warm this winter…so far.  In fact, the bay water was, on average, about 5 degrees F warmer throughout the first two weeks of December this year compared to last year.  You’re not going to want to jump into it, but it probably explains the rigmarole of life out there.

We keep track of the temperature in the bay and it is often discussed among the farmers.  We talk about the weather a lot but we are more keyed into the water temperature.  It is the temperature of the bay that often determines oyster growth, the availability of food and oxygen, and the likelihood that adverse conditions like disease, freezing, or ice scour may affect any individual farm.  But the dynamics behind the bay temperature is not as simple as you might think.

Let’s start with the summer:  the leaves are out and the warm southwest breezes blow over the bay while the gin and tonics are prepared on tailgates.  You would assume that these lovely, warm breezes correspond to warm water in the bay, but it often results in the opposite.  What happens is that the wind essentially blows the top layers of Cape Cod Bay over toward the outer cape (Provincetown).  Due to laws of conservation, the water below this warm surface layer must rise up, as if it were a conveyor belt, to replace the exported surface water.  This bottom water is cold in comparison to the surface layers (cold water is heavier than warm water) and when the tide comes into Duxbury Bay during these summer conditions it is relatively cold.  It is much colder than the warm, sun baked water that had just left the flats.  So, due to these dynamics, the water is significantly warmer around low tide than at high tide in the summer.

In the winter it is the exact opposite.  At low tide, when there is not much water in the bay, heat is rapidly radiated out and the temperature plummets, often below zero Celsius.  But the incoming tide infuses the bay with relatively warm offshore water.  When there is ice covering the bay the difference in temperature between high and low tides is small due to the insulating effect that the ice provides.

It gets more complicated.  The timing of spring (big) and neap (small) tides has an additional effect on water temperature trends throughout the year.  But this may require a separate blog entry.

It’s been mild so far.  This could easily change in the next week or two.  But by then we’ll start to notice longer days and with the sun’s angle increasing each day, we might make it through without any serious ice.  And maybe the baitfish, birds, and seals will continue to thrive at the waterfront for a while.


 John W. Brawley or Dr. John uses science  to cheat at fishing.

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