One of three things is happening in pools now depending on the water.  If the pools are dry then the resting eggs of invertebrate inhabitants are safe and secure.  They will be stimulated by some set of cues in the spring.  The order of hatching is not at all random and careful observation will reveal that the sequence is the same every year.  The resting eggs are in suspended animation and can survive for years or even decades until the appropriate time.

The other thing that happens is that the ponds remain cold and wet.   I think this is one of the best times.  Again, with careful observation you can see that the zooplankton community is thriving.  In earlier posts I wrote about Daphnia ephemeralis, a species that is a cold stenotherm.  That means they survive only over a narrow range of cold temperatures.  Freezing marks one extreme and the other is about 50°F.  Much warmer and the animals are stressed by the heat!  The species is reported from New England but I haven’t been out myself to collect it but anyone with the curiosity might find them happily swimming around even now.

Cold and wet ponds are also interesting because they have a seasonal succession of species.  There are copepods and cladocerans in the cold but they are not the same ones that you will find once it gets warmer.  These cold-water forms are often larger than their warm water relatives.

The other option is that the pond could freeze.  Even then all activity has not stopped.  There are any number of invertebrates that live in the unfrozen muck.  I know this because I have been out in the late winter-early spring and collected large Daphnia covered in algae and muck swimming about in a pond that I know was frozen but a few days before.

So I write this as encouragement to all you vernal pool enthusiasts.  We may not have the seed catalogs to comfort us as gardeners do, but our little habitats are safe, secure and even active for the winter.

I spoke with John at the last such meeting before he left Kansas and I could tell that the parting was more than a little bittersweet.  I think that it is remarkable that the loose network of colleagues in the loosely defined “Great Plains” could maintain the meetings for so many years.  I think the persistence of the group is a tribute to his tenacity and his desire for everyone to share their knowledge, and perhaps a beer or two.   I wish I had known John better.

Perhaps one of the readers of this could offer me and others a little more about John and Stanley.  Although I hadn’t seen either in so many years, I miss them.

I knew Stanley better, even spending a few weeks in his lab at the University of Wisconsin.  My favorite story about Stan was the time he was invited to the University of Windsor to give a seminar.  It was the middle of the winter and bitterly cold.  After his talk a large group of us went out to dinner at a nearby Chinese restaurant.  You have to know that I occasionally break into song when I receive even the most fleeting cue.  This usually draws stares, sometimes smiles and I am sure people think I am a bit (or more) strange.  Anyway, after dinner we walked out of the restaurant into the cold and the sign at the bank across the street informed us that the temperature was 85^o That was my cue to break into “Heat Wave.”  I was surprised and pleased that Stan joined in with my off-key, off everything rendition, “We’re having a heat wave, a tropical heat wave, the temperature’s rising, it isn’t surprising…” That’s as much of the song as I knew (and I still don’t know the rest), but just that short bit, with Stanley singing along, caused me and the group to start laughing.  That is a simple moment I’ll never forget.  Stan was killed in a bicycling accident yesterday.

Stanley was a consistent supporter of my career and I am forever grateful for his encouragement.  I offer my condolences to his family and will think of him often as I pray.  The world, and the science of ecology, are all the better for their presence and heartfelt efforts of these two fine gentlemen.

My first intense interaction with the topic of community structure came about as I was introduced to the ponds around Churchill, Manitoba (made semi-famous by the National Geographic feature on polar bears at their garbage dump, and yes, they are scary animals).  There are two broad classes of ponds there.  Ponds in the tundra outside of town and those along the granitic outcrops along Hudson Bay.  These ponds have a distinct biota (sorry, nothing about amphibians here but they do occur occasionally).  Tundra ponds are relatively large (they can be acres but are often smaller) and are acidic due to the peat moss.  They are the home to the interesting cladoceran Polyphemus, one of the few predatory members of the group (most of it’s relatives are marine although their aren’t many species of marine cladocerans).  These ponds are also the home of Daphnia middendorffiana.  The nearby bluff ponds have neither of these species but rather are home to D. magna.  There is no overlap in the distribution of the species even when tundra ponds are within a few meters of the bluff habitats.

Stan Dodson and John O’Brien were among the first people to study this relationship in Alaskan ponds (he had D. pulex instead of magna as his other species).  In the Churchill ponds, factors driving this distinction in distribution were studied by one of Paul Hebert’s graduate students, Jaimie Loaring (now MacIssac).  In both regions the distribution is driven by the predatory copepod, Heterocope septentrionalis. In tundra ponds the copepod lives along with D. middendorffiana but the copepod is absent from bluff ponds where magna is found.  The two Daphnia species are not found together.

There are three distribution issues: 1) Why isn’t the copepod found in bluff ponds?  2) How can one species of Daphnia co-exist with the predator but the other can’t?  and 3) Why don’t the Daphnia species occur together?  The answers will follow.

The stain we used was specific for the cuticle (the waxy coating) of the plant.  We had two species we were trying to distinguish, partly by stomata count and so we stained both.   The species with the fewest stomata stained poorly while the species with the large number of stomata stained well.  The explanation I came up with I think is the most simple (I’m always interested in better explanations if there any out there).

Photosynthesis is all about gas exchange.  Plants need carbon dioxide to turn into glucose and they need to get rid of oxygen as the waste product of that process.  They also need to retain water which is largely the role of the cuticle (wax and water don’t get along; wax, a lipid, is hydrophobic).  So, the species with many stomata had many sites for gas exchange and had little need for gas exchange across the rest of the surface of the plant.  The other species, with few stomata, had a much thinner cuticle because there is a need for gas exchange across the rest of the surface of the plant.

I’m not sure how I would have quantified the difference in staining intensity.  I am sure that there are spectrometers somewhere that could have done the job but they weren’t available.

The moral of the story?  I return to my theme of interesting stories from the smallest of vernal pool life.  What we found was hardly a major breakthrough in science.  But it re-enforces the importance of observation of even the smallest details.  I know most people involved with vernal pools are after the largest species (amphibians) but the vernal pool ceases to be much of a home when there is no supporting ecosystem.  Try raising salamanders in distilled water or even filtered pond water and it is unlikely they will survive for long.  They need something to eat which needs something else to eat.  Think about the relationship between yourself and the size of the bites of food you consume.  Even with hands to manipulate our food, we take in only a small amount with each bite.  A salamander has similar constraints (and no hands) so their food must be that much smaller, the size of Daphnia and other similar food items.  And the food of Daphnia has to be that much smaller again.

I started this story with observations about “duckweed” but I think there are lessons here about more than these tiny plants.  My admonition, again, is to keep your eyes open and appreciate all that is going on in these tiny worlds.

But…this is the sort of minor point that can be irksome for scientists.  A robin is not a goldfinch.  I know they are both birds but they don’t behave the same way or feed in a similar manner.  I put out different bird feeders for the two birds.  So, just like a birder might be bothered by pictures of the two being captioned incorrectly, I feel a minor irritation in the same way.  Again, this has absolutely no negative connotations on the great work done by Mass Audubon but just gives me a chance to be on the soap box.

I thought I would resume my blog by writing about a topic that is both different yet similar to my earlier posts. I have written extensively about Daphnia, the parthenogenetic crustacean common to vernal pools (and lakes but that will have to be discussed at another time). A few years ago, (okay, more like 15 years) I realized that there was more to a vernal pool than just being a home for animals that can make clones. Vernal pools can also be the homes for plants that make clones. That vernal pools host such a diversity of organisms that require sex rarely, if ever, is interesting in itself but I’ll save that discussion for later as well.

First, I thought I would introduce everyone to the world’s smallest flowering plant, Wolffia (water meal; and you thought it was Lemna, or duck weed). As small as duck weed is, Wolffia is even smaller (a nice comparison photo is at http://www.uwgb.edu/BIODIVERSITY/herbarium/wetland_plants/wolcol_aspect01.jpg). What is interesting about both of these genera is that you only need one individual to start a population. I have to admit that while growing duck weed is pretty easy I have had little luck with Wolffia although it grows in vernal pools so how can it be to grow? Anyway, unlike Daphnia which use eggs to produce clones, the plants do so by producing buds which separate from the parent. Like Daphnia, at some point the population is induced to become sexual and flowers are formed (see http://waynesword.palomar.edu/plaug96.htm for some cool pictures of the flowers and seeds).

My interest in Wolffia came about while hunting for Daphnia around Floyd County, Georgia while a faculty member at Berry College. I came across a few ponds that were covered with the stuff and thought they might make an interesting study subject for those students that were not so much into Daphnia (I know that is hard to imagine). I tried to find guides and keys to the species, but this was before the internet made it so easy to find information.  I learned there are a few species and they could be identified, in part, by the number of stomata on the surface of the plant. For those of you that have forgotten your botany, stomata are the openings on leaves that allow plants to breath. They are necessary because plants are covered with a waxy cuticle that restricts water loss but also limits gas exchange. Plants need carbon dioxide for photosynthesis and need to get rid of oxygen, the waste product of that process.

I set a couple of students to look for the same sorts of clonal indicators used with Daphnia using allozyme electrophoresis. We had marginal luck and nothing that I would feel comfortable using for a study of population genetics. As I mentioned, I had read that the species could be distinguished by the number of stomata on the surface so I had students counting these openings on samples from the different habitats. As it turned out, we had more than one species growing in the area and the populations each consisted of a single species.

That is sort of interesting but not what caught my attention.  What caught my attention is the topic for tomorrow.  Glad to be back.

I’ve been writing about water, the stuff without which we would have no ponds.  Their meager depth hides the complexity of water in vernal pools.  I’ll provide one last bit of data from our studies in Oklahoma and then give my take on the relevance for everywhere else.

 My first post on this topic includes a photograph of a student standing in a turbid pond.  This is the common appearance of ponds in that part of the country.  As we were studying the vertical profile of many water parameters we included total suspended solids, a measure of turbidity.  I was surprised by the results – ponds are not uniformly turbid.  As you can see from the graph they are actually less turbid at the top and more turbid as you descend.  Given that the Secchi disc depth (a measure of water clarity used by aquatic scientists everywhere – see the photo) was measured in centimeters as opposed to meters as it is in clear lakes, I never would have guessed that the water is even more turbid at depth.

Secchi disc - and this water isn't particularly turbid!

 The explanation is pretty simple.  Heavier particles settle leaving lighter particles suspended in the water.  The soils of Oklahoma contain lots of clay that is made up of exceptionally small particles that settle slowly so the water stays turbid.  What about the winds sweeping down the plains and keeping the water mixed?  Just as I showed in the graphs of my last post, the wind doesn’t make any difference.  The reason for this is not anything revolutionary.  The closer you get to the ground, the slower the wind speed.  For ponds to mix, the winds would have to be howling.  It is windy for sure at head level but your feet rarely sense it.  Ponds don’t get disturbed by the wind enough for much to happen to the layers established by changes in temperature.

 My guess for the global significance of our results?  Ponds in New England are probably stratified more than people have previously expected.  A single scoop of water from a pond is in no way adequate to tell what is actually going on in the water at the scale that organisms experience the water.  Ponds in the woods, sheltered from the wind are probably even more stratified than those in the middle of the plains making that scoop of water particularly meaningless, again at the appropriate scale.  I could be wrong but until there is adequate data in opposition to what I suggest I think I have hit on a great, unexplored area of vernal pool studies.   

 Finally, I wrote a number of posts about how water serves as a medium to transmit messages between predator and prey and within species populations as well.  These organisms have poorly developed sight and rely on water to carry information about their surroundings.  It should come as no surprise that habitats with compromised waster quality, not necessarily toxic but enough to screw up signals, can lead to all sorts of problems for pond inhabitants.  I’d like to tell you some stories proving this point but unfortunately, I don’t know any.  Perhaps one of the readers of this post can pass along such a story to me.  I know they are out there but the number of people looking at vernal pool water chemistry is small and finding these stories may be difficult.  But incredibly important.

 I’ll tell what I think is a great story about the response of populations to differing water chemistries tomorrow.  These differences are natural but point to the types of intricate responses we miss by ignoring the base of the food web.  

Using the device I described yesterday we were able to get a fairly accurate picture of what happens in vernal ponds, what limnologists call a vertical profile, at least in central Oklahoma.  I see no reason why these events should be different anywhere else.  Using the device as pictured yesterday we determined a profile for temperature, dissolved oxygen, pH, conductivity (a measure of ions in the water), and the oxidation-reduction potential (ORP; what I was taught as the redox potential as a measure of the availability of certain substances that I can explain as we go).  Using a light meter we measured light attenuation in the field and in the lab we measured total suspended solids (TSS – turbidity), total organic carbon, phosphorus and nitrogen.  The impression I am trying to make is that we were fairly thorough in trying to describe what happened in the pond.  And we did this for two ponds, as you’ll see in the graphs below.

I won’t bore you with all the details, just some of them, and I’ll tell you the end of the story first.  My first impression, that shallow waters are not like lakes, was wrong.  I had the idea that in Oklahoma, where the winds come sweeping down the plains, ponds were thoroughly mixed most of the time.  Wrong.  I originally thought a sample of water from anywhere in a pond would tell the entire story of the pond.  Wrong. I had it all wrong and I am the first to admit it.

Ahh, but for me to be wrong there must be a good story to explain the errors of my way.  As it turns out ponds are like lakes and have similar vertical profiles, just highly compressed.  What occurs in meters of water in a lake, takes only centimeters in a pond.  If you just see the pattern and not the scale, you wouldn’t know if you were looking at a pond or a lake.  In my naiveté what I had failed to take into account was that there could be so much compression in the profile.  Again, I won’t bore you with all the details but just give you a couple of examples to make my point.

Here’s the vertical profile for temperature.  Without knowing the scale of depth you might think this is a typical lake with warmer, less dense water, on top of cooler, denser, water.  Happens in lakes (and swimming pools) everywhere. 

Another example would be pH.  Same pattern, with more basic water on top of acidic water.  The reason for this is that there is a bunch of photosynthesis going on at the surface removing carbon dioxide whose removal makes the water more basic.  The amount of photosynthesis is hidden in the photographs and even standing near the ponds.  But the pH profile matches the profile for chlorophyll.  As it turns out these turbid systems are loaded with small algae that we can’t detect because of all the turbidity.

 I’ll try and put this all together tomorrow.