Daphnia is an acuatic microarthropod which depends on the environment to perform its metabolic tasks. Arthropods are poikolotermic, which means their body temperature depends in the environment's temperature. Considering the above, it would be important to measure the following variables for some period fo time, let's say 90 days: 1) temperature, 2)salinity, 3)organic matter and 4) pH. Number of days is important, so we could give Daphnia time to acclimate to its new environment. We should measure at least to points, in the beginning, to measure the immediate to response to the abiotic change, and at the end, to measure the effect of the measured variable in Daphnias heart.
The design could comprise treatments with normal values of this variables where Daphnia lives, and with a couple of extreme values (both upper and lower extremes. For example, imagine Daphnia uses to live in lakes with mean temperature of 19 C, then this would be the control case for temperature and you could also use 14 C and 24 C, as the treatments to explore the effects of temperature variation. In any case the choice of values should be informed by literature. For example with ongoing climate change, you could use the projected rises in temperature to explore how Daphnia would perform in those cases. After performing the individual variable assays (I mean, temperature fluctuates, the rest of parameters are hold the same). It would be pretty informative to combine variables. Let's say, how could Daphni behave with warming and acidic pH. Another example, how could Daphnia behave with warming and organic matter enrichment/oligotrophy (poor concentration of nutrients). In this way you could asses more realistic scenarios, since nature has these and much more variables playing simultaneously. According to the problem, heart rate is the variable of interest (that is of our response variable), therefore we should measure heartrate in all the conditions mentioned above. As all organismis search to keep their metabolism/functions (that includes heart rate, among many other) with major changes, we should see some fluctuations in the begginng of the assay, but their will tend to stabilize if Daphnia is able to cope with the altered environment conditions.