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.

More on Flatworms

My experiments with flatworms were pretty simple.  I fed the flatworms until they were all satiated (none were hungry) then I starved them for 24 hours so they were all equally hungry.  Then, I put them into cups each with a different food.  The results were clear.  They preferred small Daphnia (1.2 mm) but not the much smaller Ceriodaphnia (0.65 mm).  Larger prey, like adult Daphnia and Simocephalus (each around 2.5 mm), were also not consumed as much as the mid-sized animals. I didn’t try feeding them copepods because they were just too fast.  A flatworm could consume daphniids a day with ease, so I concluded that Mesostoma could be substantial predators in ponds and was a selective feeder. 

My prediction proved prophetic with a few months.  One of the main interests of the lab were the ponds around Churchill, Manitoba.  These ponds are ephemeral but really aren’t vernal because the ice doesn’t melt until well into the summer.  There are two general types of ponds in this region, those in the granite rocks surrounding Hudson Bay and those inland in the tundra.  They have completely different zooplankton even though they are in some places a few meters apart.

One of the grad students was doing an experiment where he took a large tube made of plankton netting, a few feet across and a few feet long and suspended it in a pond.  To the best of my recollection (and I could be wrong), the idea was to suspend the tube in one pond type and place Daphnia from the other in the tube.  It would then be possible to determine if the two could co-exist and the separation was due to factors other than chemistry.  I was in Windsor when the call came from the student that the experiment had started successfully but within a few days every animal in the tube was gone.  The mesh was too small for them to escape and preliminary experiments had shown that they could survive in the water.  The student was perplexed and worried because much of his thesis depended on these experiments.

I had been reading about flatworms in the subarctic and had come across a report of Mesostoma lingua (I couldn’t find any pictures on the web; it looks like a tiny, black tube tapered on both ends) in these ponds.  I suggested that he have a look inside the tube to see if perhaps there were any of these in the tube.  I wouldn’t be telling the story if I was wrong, so when the student looked in the tubes they were filled with hundreds of worms.  They were small enough to crawl through the mesh and then had a trapped meal.  Every time a daphniid swam against the tube it was a meal for a flatworm.

While the student was distraught over these results I was excited.  I was in my predator-prey mode and was anxious to travel to Churchill and do some experiments on these flatworms.  Later that summer I did just that.  And, as it turned out, one of the determining factors for distribution of Daphnia species in Churchill was an invertebrate predator, but not a flatworm.

More soon.

Flatworms and vernal ponds

I know I promised my flatworm stories a few weeks ago but better late than never.

 When I first arrived to start my post-doc at the University of Windsor I thought it would be interesting to start a small project to get my feet wet, so to speak.  The Hebert lab had study sites all across southern Ontario and I often went on collecting trips for Daphnia to be used in population genetics studies being conducted.  I like that stuff but I was much more fascinated by the diversity of animals that came in with each haul of the plankton net.  The diversity was striking to me because the sites I had used in Nebraska for my PhD were boring by comparison.  Collections in any pond in Ontario were far richer in species diversity than those near Lincoln.

 I was intrigued by the many animals on the bottom of our sorting trays (9” square, white, opaque plastic trays; I highly recommend them).  I recognized planarians but the other flatworms were unknown to me.  One of the most interesting animals I came across was a nearly transparent flatworm, 10 mm or longer, that I identified as Mesostoma ehrenbergii (these drawings  doesn’t do it justice but the image of it cradling a Daphnia is fairly accurate; this photograph  is better).  They are about the same size and shape as a leaf of Lemna trisulca (star duckweed)  and the color of a decaying leaf matches the flatworm closely.  The name of the genus is descriptive of the location of the mouth being somewhere near the middle of the animal.  American and European members of the species are apparently distinguished by the number of chromosomes, which speaks to the size and ease of extraction of chromosomes as this was demonstrated more than 50 years ago.

 I mention all this detail because of what I noticed about the prey capturing behavior of the flatworm.  One method they use is by producing sticky mucus that ensnares the prey.  Even a large Daphnia (2.5 mm; remember “large” is relative) would be trapped.  Their hunting method was to make their way to the surface of the tray climbing along the side, glide along the surface (perhaps using the surface tension of the water?) and then release them selves to float, and hunt on their way to the bottom.  When they contacted a Daphnia they released nematocysts and their prey was instantly paralyzed.  One interesting myth was that the flatworm ate Hydra and was able to store their nematocysts and employ them in capturing their own prey.  I raised many, many flatworms that never saw a Hydra and all were perfectly capable of killing their prey so I have to at least partially discount that story.

 Once a daphniid was captured the behavior of the flatworm was always the same.  It would crawl over the animal searching for the suture between the head and body.  Once found, the flatworm would extend its proboscis and puncture the suture and proceed to pump the inisdes of the Daphnia into itself.  The operation took a few minutes and it was possible to see Daphnia innards inside the gut of the transparent predator.  Often the Daphnia eye would remain intact and you could see it through the predator staring back at you. 

 I’ll explain the experiment and the results of the experiments tomorrow.

 

Of Current Interest

I just posted on my other blog about baseball, Fenway Park and vernal pools.  Have a read if you dare.

Kinky Sex

I described some of the difficulties aquatic invertebrates face in finding mates.  These are pretty much the same in ponds, lakes and oceans.  It is more intriguing in highly turbid ponds but the method of doing the process by scent and taste works whether there is sufficient light or not.   Not being able to see your mate also means that you can’t see if someone has already found her.  The result is that, in Daphnia at least, the ménage a trois is not uncommon with two males attached on either side of a female that is desperately swimming trying to keep all three from sinking to the bottom.  Both males are doing their best to get their sperm where they can be used for fertilization, but remember that males have no sexual appendage to insert nor is there any place to insert it.   That would explain why both males have a fair chance of their sperm being used to inseminate the eggs.  I am unaware of anyone looking at the sibs that hatch from an ephippium, but I suppose it is possible that there could be half-sisters from any one ephippium.

While the Daphnia ménage is kinky it is tame compared to fairy shrimp in arctic ponds I looked at on Igloolik Island.  Those animals are really freaky.  Male fairy shrimp have large grasping mouthparts by which they attach to females.  There are photographs and even videos of brine shrimp (which are just fairy shrimp living in salty water) at http://www.captain.at/artemia-mating.php.  At least in Daphnia the females are larger than the males they have to carry around.  Female fairy shrimp have no such luck and males are often much larger than their mates.  Worse yet is the fact that males have a behavior called mate guarding in which they stay attached to their mate for long periods to make sure that no other male comes along to mate with the female.

Mate guarding is interesting but not kinky.  The kink comes from finding the ménage a trois again but this time the second male is grasping the first male and not the female.  Kinkier yet is the ménage a quatre with yet a third male attached.  And the poor female, smaller than even one of her suitors, has to carry all of them along.  Want even kinkier stuff?  I have seen a number of instances where at least one of the males in line was dead and partially decomposed with another male attached, unable to reach the female because of the original suitors body.  That is kinky.

Mate guarding occurs in other pond dwellers as well.  Amphipods, also called scuds, have a similar behavior.  Much larger males will cover a much smaller female and essentially ride her even while she is carrying already fertilized eggs.  At least scuds are bottom dwellers and the females don’t have to try and keep the couple afloat.   It is not unusual to see seemingly many legged individuals that are actually the mating pair

Other species have kinky sex as well (snails in particular) but I’ll stick to telling about what I’ve seen.  Tomorrow I’ll write about flatworms, their sex and other hungers.

Sex in the Pond

Now that I’ve described male and female Daphnia separately, it is time to bring them together.  Yes, I come to the topic of sex.  It is not as simple as it might seem.  Remember, the majority of invertebrates that live in vernal ponds have rudimentary eyes at best and finding a mate is potentially difficult.  Also, vernal pools in many places are highly turbid (visibility measured in millimeters, almost liquid mud) so even with good eyes they wouldn’t be able to find their mates.   

How do they find one another?  Although there are few studies on this topic, the best explanation is that females produce chemical signals for males to taste in the water.  These chemical signals are called pheromones.  I haven’t specifically mentioned any particular invertebrate because this is likely true for many of them.  Jeanette Yen at Georgia Tech has shown this to be the case for copepods and Terri Crease looked for such pheromones in Daphnia.  I return yet again to the idea that the medium is the message. The consequence for conservationists (even sex has important consequences for them!) is that water carries many messages and this is another.  I don’t know how many arguments one needs to establish that the watershed of vernal ponds need to be protected.  Water quality is so important to so many aspects of the lives of inhabitants – detecting predators, the taste of food (haven’t written about this yet), finding mates – that a deterioration in water quality by any sort of contaminant has the potential to really screw things up.  Even sex.

Once a male finds a female, then what?  This is not so easy to answer and, as usual, it depends.  Flatworms have one set of issues (I haven’t told many flatworm stories but I will soon) and rotifers and crustaceans have others.  As usual the stories I know best are about cladocerans.  Like other crustaceans they can’t see their mates so even after finding them there are issues.  Copulation is not internal in cladocerans, coupled with the observation that sperm are amoeboid, means the male has to get his equipment really close to where it needs to be in the female.   But he can’t really see where to go.  The best answer to this dilemma comes from studies of copepods and rotifers conducted by Terry Snell at Georgia Tech.  He detected the presence of a concentration gradient of surface glycoproteins on females.   Once a male finds a female he grabs on and then tastes his way to the best spot to orient himself to optimize the chance of a successful copulation.

I worked with Terry for a short while when I lived in Georgia and we tried to find a similar factor in Daphnia with no luck.  In all likelihood we were searching for the wrong kind of chemical signal.  But it is interesting that in all that mating couples I have seen the males are oriented exactly the same way.  There must be one best position that gives the highest degree of successful mating.    

Tomorrow:  kinky sex 

Where Do Little Boys Come From?

I don’t want to get mired in the genetics of Daphnia (although fascinating) but rather move on to other topics about vernal pools.   But before I do, I want to finish a few last thoughts on invertebrate sex.  That means a few words on males, where they come from, and how they do what they do.  This is not as obvious as it may seem.

Firstly, what is the source of male Daphnia?  I’ve already mentioned that females produce clones by generating eggs that are identical to the mother.  That’s the whole point of parthenogenesis.  Most animals use chromosomes to determine gender.  There are many variations on the theme, but most people remember from high school biology that humans females have two X chromosomes (XX) and males an X and a Y (XY) with the genes on the Y chromosome making men what they are.  This is called chromosomal sex determination. As female Daphnia make identical copies of themselves, there is no way to use this method.  So, as I began the paragraph, where do male Daphnia come from?

Cladocerans, of which Daphnia is one, have evolved a different mechanism, called environmental sex determination.   There are no sex chromosomes but rather conditions in the environment trigger changes in the expression of DNA so eggs develop into males instead of females.  Male Daphnia are distinct from females in look as well as behavior, but they are genetically identical to their mothers.

What triggers the changes in how the genome is expressed?  That leads to my favorite answer to all questions: it depends.  The best answer is stress and you should now ask, “what sort of stress?”  The answers get more and more murky as I tell you that stress can be not enough food, the wrong kind of food, not enough oxygen, too much Daphnia waste, or all the biochemical changes that come from being too hot or too cold.

This explanation is based on the observation that cladoceran males are rarely seen early in the season, when there is plenty of food  and oxygen in the cool waters of vernal ponds. Males are absent until the pond is crowded and then they become relatively common, approaching 50% of the population just as in sexual species.  No stress, no males; lots of stress, lots of males.  How much stress is needed?  Some.  I don’t feely badly in not knowing the answer because even cladocerans don’t know.  Depending on the degree of stress, Daphnia respond by producing a continuum of gender confused offspring that range from mostly female all the way to true males.  These intersex individuals may produce eggs or sperm depending on how far along they are in the continuum.

The other neat observation is something I have mentioned before, the hatching of males from resting eggs in Daphnia ephemeralis.  I believe this rare example is the result of the fact that this species only tolerates cold water (less than 50F) and has such a narrow time period in the ponds that there is isn’t sufficient time to produce a few generations of females before producing males.  Producing males from the start ensures that mates are available if there is a heat wave that shortens the life of the pond, at least as D. ephemeralis experiences it.

Tomorrow: sex in the pond.