Expert: Walter Hintz - 3/20/2007

Question
QUESTION: I just completed my A2 fieldwork on the decrease in the population density of Mayflies with an increase in depth (I said that this decrease in the population density is due to a decrease in the oxygen dissolved in water with increasing distance from the surface). Now I am wondering what the biological significance of such an experiment could be!
Could you help me?
ANSWER: Hi kim
 I believe that you are correct in assuming a lower O2 at greater depths but I am not sure this is the only factor in a population decrease.  Did you test dissolved O2?  What body of water were you studying and what time of the year did you sample.  The reason I asked is because in some lakes there is a large emergence in late summer of adults.

---------- FOLLOW-UP ----------

QUESTION: Dear sir,
The fieldwork I undertook was in Thailand in the space of one day. The river stretch under investigation was 30 meters. Both the limited time frame and limited river stretch under investigation allowed me to control several factors, for example pH, human effluent, temperature etc. The only thing....well not quite the only thing, but theoretically the only thing, that changed was (or should have been) the depth. The substrata was also controlled (rocks on gravel without leaf litter) and the flow rate was fairly constant!
I almost finished my right up and came to the conclusion that oxygen could be the only factor influencing the mayfly population at the sites I chose. This is because I made a comparison between the population density at different sites, using Spearman's Rank to test for significance. Therefore, I believe, the bigger picture i.e. the season etc. has indeed and effect on the mayfly population but not on the change in the mayfly population with depth.
My hypothesis is: there is a decrease in the Mayfly population with an increase in depth of the river :)....seems very simple but....well not quite!
The most difficult part for me though is the final little bin in which I have to state what the biological significance of this experiment is, i.e. what good does such an experiment due in the wider context of biological investigations....well....I have no idea what good such seemingly senseless investigations have other than broadening our knowledge about invertebrate...
I hope I am not taking up too much of you time.
Thank you very much!
yours sincerely,
Kim

Answer
Hello again Kim:
 I did not realize that you were working with stream mayflies in Thailand but I assume that you are talking about members of the Order Ephemeroptera. I did some studies of Ephemeridae in Lake Erie in Ohio many years ago.  The limiting factors of lake and stream mayflies differ somewhat.
 From what you are telling me your conclusions seem to be logical.  Does the population of the species you are studying have normal population cycles?
 I do not believe that biological investigations of invertebrates is senseless.  Our lives depend upon all other living organisms and on the basis of the food pyramids.