The two photographs below are of the same rotifer, this animal is NOT a member of the two major genus Synchaeta and Polyarthra). My best shot at identifying what genus this animal does belong to is she is a member of the genus Euchlanis. You may have noticed I use the feminine, when dealing with rotifers even if you have never seen one you can safely bet that any you are shown will be a female. In this phylum males are almost never seen. In some groups of rotifers no male has ever been seen. In others  the males are so diminutive that they are thought of a nothing more than mobile sex organs. The rotifers are the one phylum of all earths animals that it is believed evolved in freshwater.
      Luckily both the Synchaeta and Polyartha are easily identifiable animals (at least to the genus level) but, and this is the reason there are no photographs of them here, they are very fast moving animals. I haven't yet figured out a way to get them to hold still for a picture. The animal below moves nice and slow.

 
 
 
 
 
 
 

I have years worth of data that I hope to some day work through. Below is an incomplete picture of the number of rotifers found at Avon Point. The picture is incomplete and of limited value because the data is not broken out by genus. The two major genus of rotifers (Synchaeta and Polyarthra) have different life cycles. Synchaeta being a spring rotifer and Polyarthra being a summer one it is necessary to list the data by at least genus. But not yet having that data ready let me at least present the data on all the rotifers found.

                         ROTIFERS

     The following chart list the monthly average for the total number of rotifers found in samples taken at Avon Point. I'm presenting them but I have come to wonder if they have any value. Unlike the crustaceans or more precisely to a greater degree than the crustaceans, the rotifers populations seem to achieve their population peaks at different times of the year. Whether this is due to temperature, food supply, or some other factor, I have no idea. But it does seem that trying to say anything about the rotifers as a whole may be misleading as each species responds to the environment in different ways.

     All that being said I present the data because:

1) Well I did the work so might as well present it,

2) Perhaps the data is useful and I'm just too ignorant to see its value.

   The only blanket statement I can make from the data on the rotifers and crustaceans is the while the total number of crustaceans seems to be going down the number of rotifers is going up. It may be possible that there is a direct correlation here. Some species of crustaceans feed on the rotifers and their reduced numbers may be responsible for the rotifer's increase. But a better study than mine would be necessary to show this for there might be many other causes for such population changes.

   Plankton sample size 12.5 liters

   In the chart below the number column is the total number of all rotifers found in the month. The average is the total rotifers divided by the total number of samples taken that month. Generally there were two samples taken every week or 8 to 9 samples a month

 

                                     1994                         1995                      1996

                            number   average       number average      number  average

April                        60          10              75         8.33          250      31.25

May                        78            8.6          660       82.5        *1362    151.33

June                       532         48.36         491      54.55         1335    148.33

July                    *4144        460.44        501       55.66        2787     348.37

Aug                      2065        229.44        176       19.55        2697     385.28

Sept                       124          15.5          379       47.38        1217    135.22

Oct                          82            9.11        279       31              267      29.66

Nov.                        66            7.3          102       11.33         103      12.87

Dec.                          1              .12           3         0.6             10     1.11

                                    1997                         1998                          1999

                           number   average       number     average

Jan                           13        2.2                 3          0.5

Feb                            0         0                   4          0.6

Mar                           6         .7                   3          0.3

Apr.                         36        4                310          38.8

May                       726       38             2594         324.3

June                     2229     247.7           1274         141.6

July                      1271     141.2           1610         178.9

Aug.                       707     101              1793         224.3

Sept                      1245    155.6              970         121.5

Oct.                        759      84.3              925         102.8

Nov.                       11         1.2

Dec.                           3          .3
                                  2000                             2001
Apr                                                                  59
May                                                                377.6
June                                                                  41.7
July                                                                  172.6
Aug                                                                  105.6
Sept                                                                   24.9
Oct                                                                      1.4
Nov                                                                      .2
 

* The July 1994 and the May 1996 are so high because in both these month on one date there was an enormous number of rotifers.  July 20 1994 there were 3043 rotifers in just one 12.5 liter sample.

  May 26 1996 there were 1073 rotifers in the sample.

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   To understand the dynamics of the changes in the rotifers population densities it is necessary to look at the individual species found. Here I'm at a big loss since I haven't the equipment or knowledge necessary to allow me to identify the following two groups to the species level and will have to be content working at the genus level.    What is interesting is that the Syncheata population rises first and then declines as the Polyarthra's population increases. I don't know whether there is a direct correlation such as they both occupy the same niche and the Polyarthra is able to out compete the Syncheata or whether there is no relation at all; that the food of the Syncheata disappears early and the different food supply of the Polyarthra arrives latter. It is also possible that the different genus are sensitive to the Lakes increasing

temperature in an inverse way. There are no doubt several other possible reasons way the one population should rise as the other falls. Figuring out what is driving these changes would make a fascinating study.

    Below are the monthly averages using the same samples used in the chart of the total rotifers.

                             1996        1997     1998              1996      1997        1998

Avg                                   Syncheata                                Polyarthra

Jan                           0            1.5         0.2                 0            0             0

Feb                          0.3          0.3         0                    0            0             0

Mar                        11.7          0.2         0.1                 0            0.3          0.1

Apr                          5.8          2.3        32.1                 0.1         0.4          0.8

May                     144.3         86         283.1              17.4          0.3       17.5

Jun                         64.4       160.7          0.2              42.1       56.9      135.7

July                         61.1        34.9         5.6              274.6       98.7     135.8

Aug                         10             4          53.9              301.3       80.3     145.3

Sept                        23.8        20.9         7.3                 72.3     100.6     101.4

         DIFFERENCES IS SURFACE AND BOTTOM SAMPLES OF ROTIFERS

     There are many sampling dates when significant differences are found between the surface samples and the benthic samples. Problem is what is a significant enough difference to allow me to say that the animals are actually at different levels in the water column or the differences are sampling errors.  Here I am assigning significance only to those dates that have a ratio of more then 2 to 1.   Under the second column, rotifers, are the number of animals found in a 5 liter sample. Under the column X-2.5 are the number of rotifers in the sample times 2.5 to have that sample equal the plankton sample of 12.5 liters. The last column, Plankton, is the number of rotifers in the 12.5 liter sample. The bottom and surface samples were taken about one-half hour apart.

          The numbers for these dates in 1996 were:

        Benthic Rotifers            X-2.5              Plankton

6/2              8                         20                    110

6/9              2                          5                       15

6/30         135                      337.5                  726

7/27           45                      112.5                  302

8/18         235                      587.5                  167

9/1          166                       415                      43

9/5            66                       165                    485

10/6            3                          7.5                   19

10/16         25                        62.5                   13

   Bear in mind that these samples were all taken in the morning so vertical migration due to light does not appear to be a reason for the animals distribution. Though in the case of these animals wave currents alone may account for the animals distribution. But even if the animals are distributed by currents these wide variations present real problems in coming to terms with the animals density.

    Again, as in the last set of notes I posted I want to stress my disdain for the use of "patchiness"

to explain the uneven distribution of animals in the waters of the Lake. Whether rotifers are members of the plankton (using the strict definition of "plankton") may be a function of wave length. On days of calm

these animals may, like the crustaceans, be animals of the nekton.

     The Rotifers are maintaining their high population density as the chart below attests
ROTIFERS