clad

CLADOCERANS
This group of "zooplankton" animals I find the most exciting to study. From the samples taken over the years I can not consider these animals planktonic, or if they are planktonic than the definition of plankton is meaningless. Also from the data gathered I would put forward the point that if the animals population fluctuates the way they appear to in the following notes than all studies of the plankton taken in the past are flawed if they do not take the "swarming" or "schooling" of the cladocerans into account. But more important than whether or not these animals are or are not planktonic is the questions of how they come together and why. These are questions I cannot begin to answer.

The Cladocerans are animals that do not lend themselves to analysis by monthly averages. They have wide swings in their population density from sample to sample. It is as if they move in swarms, though a far better study than this would have to be done in order to prove that.

The numbers for the graph below were generated by the same 12.5-liter samples described earlier (see the section on monthly averages for copepods).
The graph is cluttered with what look like to many lines, but I did this for reason. I wanted to show that this bunching of the Bosmina is not an isolated occurrence. Year after year the same pattern is followed (though sometimes it happens a little earlier or later).

series 1 1994 Sample 15 is mid may

series 2 1995 Sample 35 is the end of July

series 3 1996 Sample 40 is mid August

series 4   1997                        Sample 60 is the end of October
     series 5   1998
    series 6   1999

The graph below omits one sampling date from mid June of 1996 when 1273 Bosmina were found in the sample. I did this to make the numbers more read able on the graph. ( a graph of the year 1996 with the June date included is found immediately below the 1994-1999 graph).

From May to the end of June the Bosmina have almost appears daily swings in population density. Remember these are all samples taken in the morning so diurnal migration can said to be a factor.

Below is a graph for 1996 with the mid-June sample added.

What clearly is shown by the graph is that Bosmina has an increase in density in late Spring/early Summer and in the Fall.

Below are graphs for the years 1999 and 2000. They are remarkably similar .

Below are graphs for the other major cladocerans found at Avon Point, Daphnia. These numbers are also hard to interpret. They show an increase in density in the spring and a smaller increase in the fall. Like the Bosmina the numbers widely fluctuate from sample to sample. Though again it is hard to interpret the numbers it seems as if the population density of these cladocerans has decreased over the years

Sample 15 is mid May

Sample 35 is the end of July

Sample 40 is mid August

Sample 60 is the end of October

Before leaving the Genus Daphnia I would like to give data regarding swarming of these animals. Below are notes concerning two dates when swarming was witnessed. The data here brings up a whole range of fascinating questions concerning the movement of zooplankton in the water.

August 27, 1995. The time was between 9 and 9:30 in the morning. Being underwater I can only estimate that the sample was taken somewhere between 100 yards and 1/4 mile from shore. There were no waves. The animals, like in the July 13, 1994 sample, could be seen as a white cloud in the water. Sample was taken in 10 to 12 feet of water. The swarm was no more then 10 feet wide, but I have no idea how long the column was.

Tropocyclops prasinus mexicanus 123

Cyclopoid copepods 75

Regretfully I wasn't able to take a plankton sample on the 27th but did take one on the 26th, so the comparison is not as strong as the sample taken in July of 1994.I think the plankton sample still strongly points to swarming behavior of all of the crustacean zooplankton except for T. mexicanus.

Cyclopoid copepods 3

Below for the purpose of comparison is the count of the sample taken on July 13, 1994. While there are differences in both the number of animals found and the species making up the sample what is striking is the similarities between the 2 samples. What is especially noteworthy is the presence of so many L. kindtii in the sample. These large zooplanktoners are never present in this large of a number in any of the regular 12.5-liter plankton samples.

Daphnia retrocurva 674 1 to 3 mm in length

Cyclopoids 63 .75 to 1.5 mm.

Calanoids 3 1.25 to 1.5 mm.

Nauplii1 6 .10 to .25 mm.

Diaphanosoma 3 1 to 1.5 mm.

Bosmina

A PERSONAL NOTE

If I had learned nothing else over all these years of studying this part of the Lake, coming to know that these exquisitely beautiful crustaceans would have made the time spent worth while. To me they appear as living jewels and I have become wealthy watching them. Once I was asked,when talking about these animals, "What purpose do they serve?". I could ask the same question of us humans, even now at this moment, listening to a Bach's Brandenburg Concerto what purpose do we serve? There is no purpose, we are just expressions of the wonderous bounty of evolution, our only point of seperation is that our evolutionary linage has given the capability to grasps the beauty of both a string of music and the swimming of a living jewel.