Subject: How and why is the heart rate of daphnia effected by high and low pH?

I am currently doing my dissertation for my HND in Biology. It is about 
the physiological effects of pH on the heart rate and mortality rate of 
daphnia. I want to understand my results. It shows that the heart rate of 
the daphnia in extreme high pH (pH8,9 & 10) and extreme low pH's (5,4,3etc)
has decreased significantly and the mortality rate of these daphnia is 
also higher. Comparing these results to the daphnia in pond water of 
around 5.6-7.8, as their heart rates remained roughly the same and the 
mortality was low (they lived for 2-3 weeks)

Re: How and why is the heart rate of daphnia effected by high and low pH?