What’s your favorite sign of spring?

Thursday, April 06, 2017 3:5PM

A favor. I have started a poll of sorts, asking people what their favorite sign of spring is. Outdoors, in the natural world.

I wrote last time about phenology — nature’s timing: ice-out on the pond; blooms (first crocus?); first summer blueberries picked; return of hummingbirds. For the latter, sometimes we see a male hovering where the sugar-water feeder has always hung. He remembered. We hurry to put it out.

Phenology is an important concept, but as I ask people about their favorite signs of spring, it turns out the term is not a familiar one, or the concept. As climate shifts, data about firsts is important information. There is a lot of data that has been collected over the decades.

“Pay attention,” a favorite teacher often said. A lot of people are paying attention.

My request that you send me your favorite sign or signs of spring (to vonmertens@myfairpoint.net) doesn’t relate directly to phenology. I have a theory that needs testing and your help is needed.

The nature writer Hal Borland calls this time of year “spring-hungry.” What seasonal happening do you notice that lifts your sprits, feeds that hunger? If you have more than one sign, please put them in the order that they lift your spirits. If you announce a first to the household, certainly add that detail. That’s a good indication of spirits lifted.

I mentioned ice-out as a phenology event. Over the years I’ve heard about “spring lake turnover,” surely an odd concept, and recently learned it’s an essential process that give a charge of oxygen to lakes. It sometimes arrives in the nick of time as aquatic plants and animals have used up a lot since ice shut off their main supply.

Trout are the most oxygen-sensitive, and they hang out by springs and stream inflows that add oxygen in winter.

Here’s my understanding of lake turnover.

It’s about the weight of water. Water at 39 degrees F is the densest, and therefore the heaviest. It sinks.

On either side of that pivotal temperature, water is less dense and lighter.

It took me a while to understand that 39 degrees F is a pivot point.

As lake ice melts, sun warms surface water to 39 degrees F and that water sinks, displacing cold water at lake bottom. That surface water picked up oxygen from the atmosphere and reintroduces it to the lake depths. Fresh air at last!

This process continues as new surface layers warm and sink. In time, the lake is uniformly 39 degrees F, or thereabouts. Meanwhile, prevailing winds are blowing. Somehow that’s key to the turbulence and mixing, as well as to oxygenating lake water that in time reaches lake turnover.

The end result I do understand. I think. Lake water reaches peak oxygenation, and then the process of oxygen depletion begins as aquatic vegetation grows and animals (fish, turtles, newts, dragonfly larvae, etc.) use it up, too, over the warm months.

Temperature layers form that we all experience diving into lake water: warm on top, colder the deeper we go, until we reach — yes — 39 degrees F at great depths.

Brilliantly, in fall, lake turnover happens all over again as surface waters cool to—yes—39 degrees F and sink, reaching peak lake oxygenation before ice forms to cut off the main supply and oxygen depletion begins again.

All freshwater aquatic plants and animals need oxygen. Give thanks on their behalf to lake turnover. And our behalf, too, for healthy waterways.

And give thanks to shoreland and wetland buffers. Runoff from yard and agriculture fertilizers creates vegetation blooms that deplete oxygen. A lot of lake and river water is tested for dissolved oxygen by volunteers. Another reason for thanks.

Continuing the water theme, last Saturday’s annual Connecticut River duck prowl was postponed until this Sunday by the April Fools’ Day snowstorm.

Ducks and geese push north with ice-out along major north-south river corridors. It’s a foolproof system as they rely on open water for food, and their trip north stalls if ice-out is slow to arrive.

Spring comes earlier in the Connecticut River lowlands. The annual trip usually delivers the first song sparrow songs, a week or more before they’re heard here in our Monadnock Highlands. Duck variety is greater, too.

I did a mini-duck prowl locally on Sunday.

Canada geese were easy to find including this pair in Dublin with Mud Pond in the background.

“When I was young. . .” I remember the excitement spring and fall of true wild geese honking and flying high overhead in V formation on their way to Canada. Those were the only times we saw geese.

Today, New Hampshire hosts what’s called Resident Canada Geese, descendants of live decoy birds released along the Massachusetts flyway when market gunning was outlawed. When they multiplied and became problems along the coastal flyway, they were transplanted inland — and spread. For the same reason a bunch were transplanted way north to Lake Umbagog to great excitement of hunters and non-hunters. Until they multiplied and became a problem. As migrants, they don’t go far. The call of wild Canada got extinguished long ago.

They are not one of my favorite signs of spring, but they are easier photo subjects than the wary wood ducks and mergansers at Mud Pond.

A true, spirit-lifting sign of spring circled over open water on the Contoocook just south of ConVal. An osprey. A long-distant migrant. Transmitter-tagged ospreys indicate flights from South America to Cuba, Cuba to Florida, and then on north arriving on New England nests early in April.

It made my day.

Backyard Birder by Francie Von Mertens runs every other week in the Monadnock Ledger-Transcript.