A few more thoughts on Drs. John O’Brien and Stanley Dodson

I read over what I wrote yesterday about John O’Brien and Stan Dodson and I realized I had left out mentioning one of the lasting tributes to John O’Brien. I know I don’t remember the entire story but in the late 1970’s John and a few of his colleagues in the region thought it would be nice if the aquatic scientists in that part of the country would get together and share the doings in their labs.  I don’t know the conversations but I do know that the birth of the Great Plains Limnologists had much to do with John’s encouragement and support.  The group, with no officers and no central administration of any sort, has managed to hold a meeting every year for at least 30 years. Students have a chance to present papers to a friendly audience and perhaps a few libations are shared in the evenings.  As a graduate student I had the good fortune to attend these meetings at Kansas, Missouri and Nebraska.   It was a treat for me to return to the meetings more than 20 years after my first attendance and see the group stronger than ever.

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.

In Memoriam, Drs. John O’Brien and Stanley Dodson

I just learned of the passing of two of the pre-eminent zooplankton ecologists of the last 50 years, John O’Brien and Stanley Dodson.  I knew John since 1977 when he was a faculty member at the University of Kansas and I was a graduate student at the University of Nebraska.  I later did a post-doc with Paul Hebert.  Paul and John had a run in when John gave a talk at the University of Michigan and Paul gave him a hard time during the question period after the talk.  I wasn’t there but I got to hear from each of them about the details of the incident and why each of them was right and how the other was at fault.  Being on the outside I got a chuckle out of their tiff.  John died of pancreatic cancer last week.

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.

You are where you are. Biogeography 1

A common observation among those studying vernal ponds is that ponds in close proximity don’t share all the same species.  As I have discussed before, even when they share the same species the populations are often genetically distinct.  I can clearly state the conclusion of the next series of posts by saying that vernal pool species don’t get around much.  A newly created vernal pool may fill quickly with species, or not, and two new vernal pools within few meters of one another won’t necessarily have the same species composition.  Okay, that’s the end of the story without all the meaningful and fun part.  And maybe the reader has had the experiences to reach the same conclusion.  Or perhaps after hearing some of my stories…

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.

Big Stories From Tiny Life

I ended the last post with as much of a cliffhanger as I could muster.  I had students in my lab stain Wolffia to make their stomata (the tiny lines on the photograph; they are actually easier to see with a regular microscope) more visible for the purpose of counting and identifying the species.  The results were interesting but hard to quantify with the equipment on hand so the project never got much further.

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.

Psychic

So I haven’t posted anything for months and today I finally wrote a piece about Wolffia, the smallest flowering plant, often confused with duckweed (Lemna).  I just got around to reading yesterday’s Boston Globe, and, lo and behold, there is an article about cleaning nutrients from a pond that fostered the growth of duckweed at Drumlin Farm.  Unfortunately, the web based article doesn’t include the photos that were in the paper.  If you have the paper from Sunday look at the last page of the Globe West section.  The picture at the upper right shows two frogs poking their heads through the “duckweed”.  And there are indeed a few individuals of duckweed but the vast majority of plants are Wolffia.  The difference in size is clear, even in black and white.  The incorrect identification has no bearing on the story or the excellent effort being made at Drumlin Farm to control pollution.

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.

Clones of a different sort, sort of

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.

Shallow water chemistry

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.  

Shallow waters, tiny lakes

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.

The not so unique limnology of vernal ponds

 Having wrtitten about biological aspects of shallow waters for the first 31 posts, I thought I’d switch and write about another critical aspect of these habitats, water.  There is no pond,lake or river without the stuff so understanding it is pretty important.  No water, no pond, no salamanders, toads or zooplankton.  The formal study of water is called limnology.  Although I have taught limnology for a long time (it was the first independent class I ever taught) I am not a limnologist.  I am an aquatic ecologist but there is just way too much specialized knowledge for me to call myself a limnologist.   This was made clear to me during the 18 months I spent at the now defunct Kinneret Limnological Lab in Israel.  They had real limnologists there and I wasn’t one of them.

Not know much about a topic never stopped me from having an opinion so sometime during the 1980’s I decided that shallow waters were not anything like lakes but behaved in some unique way that required special understanding that limnologists did not give to ponds and pools.  When I finally had the opportunity to collect limnological data, I of course discovered that I was absolutely wrong.  What threw me off initially was scale.  Everything that happens in the water of a lake happens in shallow waters but in a much compressed way.

 My graduate students at Oklahoma State came across a pond a few miles from campus that became our limnology study site.  A typical sampling scheme for a lake depends on a mechanism whereby water is collected at specific depths and then brought to the surface for analysis.  There are Nansen Bottles and Van Dorn collectors (similar items at http://www.aquaticresearch.com/discrete_point_water_samplers.htm) designed to sample discrete masses of water.  These devices work fine in meters of water but less well in centimeters of water.  They were originally designed for ocean studies and work well there but ponds are different, obviously,

 

The first challenge was to come up with a mechanism that would allow us to collect unique samples of water that were centimeters apart.  I think we came up with a good system.  In the pond we placed a ringstand to which was attached a meters stick (to accurately measure the depth of the water).  The assembly was carried into the pond and placed as upright as possible.  After waiting a short time the meter stick assembly was slowly lowered through the water column by a grad student standing nearby (photo at left show Chad Boeckman who collected most of the data you’ll be reading about; he changed projects so the data has been dormant for a few years).  We were able to clearly see differences at distances only a 2 cms apart.  You can see from the photograph that the typical central Oklahoma pond is rather turbid and this makes for some interesting differences between Oklahoma and ponds in New England where  turbidity is usually absent .

 

Attached to the meter stich was a length of Tygon tubing that had a horizontal fitting on the end for collecting water at a specific depth (at left).  By placing holes throughout the end we hopefully disturbed upper and lower water as little as possible.

 

 

 The assembly in the pond was connected by a long length of tubing to an assembly on shore that consisted of a an automated water tester attached to a special sampling vessel and a hand operated pump.  Normally the device would be lowered through the water of a lake or ocean but these systems are much too shallow so we brought the water to the unit.  Turbidity was determined by weight in the lab so a measured  aliquot of water from each depth was brought back to lab.  

I know all of this seems like a great deal of trouble but as you’ll see in the next post or two it revealed some neat things about vernal ponds inn Oklahoma which should translate into ponds everywhere.

 

Benthic Predators and Planktonic Prey

When I made it to Churchill in 1984 I conducted a series of experiments with Mesostoma lingua, a common member of the fauna of ponds in the granitic outcrops around Hudson Bay.  If you really want all the details you can read them in the published paper (Schwartz, S.S. and P.D.N. Hebert. 1986.  Prey preference and utilization by Mesostoma lingua (Turbellaria, Rhabdocoela) at a low arctic site. Hydrobiologia 135:251-257).  The results of the experiment were clear to me: these benthic flatworms had the ability to shape the zooplankton community above them.  They showed a distinct like for the tube-building larvae of chironomids (midges) around them, but they had no aversion to eating whatever came their way.

One easy to replicate experiment involved a cafeteria in which I offered all the different available food equidistant to the flatworm.  Although the freshly killed prey (by me) were randomly placed the flatworm chose the chironomids twice as often as anything else.  They must have sensed the presence of the prey and crawled to it each time.  Once there, they spent much longer with the chironomid as any other prey as well.  My experiments comparing chironomids in their tubes as compared to those I removed demonstrated the effectiveness of the tubes.  Tube-building chironomids are common in vernal ponds and I’d suggest that predator defense at least partially explains this behavior.  The test is easy enough to do yourself (maybe in a classroom with kids).

Even though Mesostoma preferred chironomids, they still ate Daphnia and probably did so regularly.  The demonstration of choice among the remaining prey indicated to me that these benthic organisms could play a pivotal role in structuring the zooplankton community by selectively feeding on some species and ignoring others.

I studied Hydra at about the same time as I have described previously.  It seemed to me that in shallow ephemeral ponds benthic organisms might play a major role in structuring the zooplankton community. 

A few years later the point was re-enforced while I was a lecturer at the University of Houston.  While there, I had looked at ponds in a reserve between Houston and Galveston.  I was surprised to see amphipods (scuds) grasping, and apparently eating, mosquito larvae.  This led to another set of experiments with a new predator but the same sort of zooplanktonic prey.  Again, I wouldn’t be telling the story if the results didn’t make my point.  In this case, it was amphipods that loved to eat Daphnia and mosquitoes and lots of them.

The moral of these stories about flatworms, amphipods and Hydra is that vernal ponds, shallow waters, are much more complex than we imagine.  And the lack of depth means that animals that we consider to be planktonic, in the open water, really aren’t.  The bottom is so close to the top that animals that the benthos may have profound effect on animals on the plankton.  Understanding these systems takes an appreciation of the fact that there might be lateral distinctions in a pond (the particularly shallow shore vs. the slightly less shallow “pelagic” realm) but even the open water is so shallow that planktonic near the bottom are potential meals for the animals that dwell there. Daphnia and other plankton are never free of some sort of predator from below or within the water.

To me, it makes the entire system that much more exciting.  People driving by or even walking through a wetland area are unaware of the drama that is played out daily in vernal ponds.  We just haven’t appreciated all that is going on in these tiny ecosystems.

Next, a slight departure as I discuss why chemically at least, ponds are tiny lakes.