Putting the Lakes to Bed

As the days grow colder, the lakes are getting ready to turn over. This phenomenon, which happens every fall, is related to the unique chemistry of water.

In summer, the water in a lake is typically separated into two distinct zones, separated by a boundary called the thermocline. Near the surface, closer to air and sunlight, the water is relatively warm and oxygenated. Deeper, below the thermocline, it’s colder and contains less oxygen. Because of the different densities of water at different temperatures, these two zones don’t mix much – think of oil and water. If you’ve ever gone swimming in a lake you might have even felt where the water abruptly becomes colder at a certain depth.

I’m sure I could find a perfectly good public domain illustration of this online, but it’s so much more fun to make my own in MS Paint.

As winter approaches, the surface water cools along with the air. When it gets cold enough and dense enough, it sinks. The two distinct zones break up and all the water in the lake circulates and mixes freely. This is the “turnover.”


Here’s what’s really cool. Almost any other liquid becomes denser and denser as it cools and gets densest of all as a solid. Water is densest at 4°C or 39°F, several degrees above its freezing point. This is why ice floats – and why lakes freeze from the surface down than from the bottom up. The water under the ice, freshly re-oxygenated from the fall turnover, remains at 4°C all winter long, allowing the fish and other aquatic live to survive until the spring thaw.

Yes, this lake is inhabited by goldfish. Stop judging me.

There’s nothing like walking over the frozen surface of a lake in the dead of winter and imagining all the sleeping life sealed beneath your feet. This morning we woke up to another dusting of snow… it won’t be long.

I Can Now Die Happy

A few months ago I wrote about camping for a night at Manatee Springs State Park in northern Florida, in a fruitless attempt to see manatees in the wild. Turns out I didn’t need to go Florida.  Yesterday I saw one a five-minute drive from where I live and work.

Yesterday morning I wasn’t teaching, but instead was preparing for the new school groups arriving later in the day, putting together binders and posting schedules and calling to confirm reservations. Around ten a coworker came back from taking a class to the dock and excitedly told everyone in the office they’d seen manatees. “The kids were bonking them on the heads with their dip nets accidentally! That’s how close they were!”

Me: “Wait, this was just now? Do you think they’re still there?”

Her: “Sure, probably!”

Me: (bolts out of office to get my camera, binoculars, and car keys)

I have the best boss in the world, because when I breathlessly explained to him that there probably manatees at the dock RIGHT NOW and I’d already finished all the really crucial preparation for the groups coming and I’d be back in twenty minutes and could I please please please PRETTY PLEASE go look for the manatees, his response was basically “of course you can.” Seriously, how many people have a job where they’re allowed to leave in the middle of the work day to go look for interesting wildlife? Soon I was on the road driving faster than I probably should I have as I zipped around the island’s southern tip toward the dock overlooking the salt marsh.

Manatees in the U.S. are strongly associated with Florida, and many people – including people who live on the coast of Georgia – are not aware that during the warmer months of the year manatees from the Atlantic coast population migrate north into Georgia’s salt marshes. I certainly didn’t know this before I moved here. The problem is that the water here is naturally murky with sediment, making them impossible to see except in the brief moments when they surface to take a breath. As a result, very little is known about the movements and habits of manatees in Georgia.

I was thankful when I arrived at the dock to see that there was no one else around – I knew if I actually saw one I would probably start jumping up and down and making high-pitched noises, and I’d just as soon there not be any crusty local boaters or fishermen around to witness that. I’d seen images before of the characteristic swirls left on the water’s surface by manatees passing below, and almost as soon as I’d stepped out onto the dock I saw what looked like those same patterns on the water in front of me, but with the water so opaque there was no way of really knowing what was down there. So there was nothing I could do but walk slowly back and forth along the length of the floating dock, keeping my eyes peeled.

At the dock’s far end I turned around and looked back, and my eyes were caught by a round gray shape protruding from the water near the end of the boat ramp, in the direction I had just come from. Hang on, there wasn’t a big rock in the water in that spot, was there–NO. OH MY GOD. I did then what I am always telling my students not to do and ran down the length of the dock, because the round gray shape had a snout… and it was slowly moving.

Guys, I literally was almost in tears. I’m not sure why finally seeing a wild manatee made me so emotional but it did. It quickly submerged again but after a few more minutes of waiting I was rewarded with a second glimpse of its whiskery nose when it came up to take a breath. I would have stayed longer, but I really did have to get back to work, so I went back to my car and drove away bouncing around in the seat with happiness.

Both times the manatee surfaced, it did so too briefly for me to get my camera on it, so unlike some of my previous epic wildlife encounters I don’t have a sweet video clip to share with you. Instead you’ll have to settle for my artistic interpretation of what I saw.

In conclusion: MANATEE.

P.S. Later in the day I told an eighth grade student I’d seen a manatee at the dock that morning. He responded, “What’s a manatee?” Sometimes I despair for the future of humanity.

Spring Tide in the Salt Marsh

If you don’t live on the coast – or if you do, but don’t actually spend a great deal of time on the beach or in the salt marsh – you probably don’t give much thought to tides.  I certainly didn’t before I moved here.  I knew they were caused by the gravitational pull of the moon, but I didn’t know the specifics of how it works.

Now, of course, tides play a role in my daily life.  If I’m teaching a salt marsh ecology class, what the tide is doing affects where I can take my students (and how wet we’ll get getting there).  Last Friday, right around the new moon, I forgot to check the tide schedule ahead of time and arrived in the marsh with a class to discover that pretty much the entire thing was under water.  Jekyll Island really looked like an island for once!  (Normally the marsh separating us from the mainland looks mostly like a grassy plain.)

I was, of course, witnessing a spring tide.  Turns out the moon isn’t the only thing that affects how the water comes and goes: while the moon is the major player, the sun has a role as well.

Twice a month, at the new moon and the full moon, we experience what are called spring tides, times when the high tides are extremely high and the low tides unusually low.  This happens because the sun and the moon are lined up and working together, with the sun augmenting the normal tug of the much closer moon.

The pull of the moon (combined, here, with the sun) causes a bulge of water on the side of the Earth facing the moon, and because the Earth is spinning we also get a corresponding bulge on the opposite side – don’t ask me to explain why exactly, it involves more complicated physics than what little I can recall from my junior year of high school.  In any case, you can see clearly why we have two high tides and two low tides every day.

About a week later, we’re now at the first quarter of the lunar cycle, a.k.a. a half moon.  This means that instead of spring tides we’re getting on toward neap tides.

Now the sun and the moon are working against each other.  The moon wins, because it’s much, much closer, but the sun still has enough effect to smooth out those bulges quite a bit.  At this time of the month the difference between low and high tide is not nearly as great.  Between this and the fact that some of my favorite night walk activities to do with kids don’t work so well in the glare of the full moon, the lunar calendar has been on my mind a lot lately!  Hope someone out there finds this interesting.

Laaaateral Liiiiiine, Operculum and Gills…

To the tune of “Head, Shoulders Knees and Toes”:

Dorsal, pectoral, pelvic, caudal!  Pelvic, caudal!
Dorsal, pectoral, pelvic, caudal!  Pelvic, caudal!
Lateral line, operculum and gills!
Dorsal, pectoral, pelvic, caudal!  Pelvic, caudal!

…And don’t forget the anal fin!

This has been stuck in my head all afternoon.  You’re welcome.  This would be a longer, less random post, except I have to get my stuff together to lead a night walk on the beach.  The season has started, I’m teaching again, and all is right with the world.

P.S.  Why yes, I do have mad skillz with MS Paint.