For my project, I am studying how carbon dioxide levels affect the growth rate of phytoplankton and the grazing rate of microzooplankton. I am using the dilution method to study this, and I am working closely with Mike Foy to accomplish my project. Every other day, I am out on the dock performing the dilutions after the morning samplings have been taken. The dilutions are conducted on mesocosms 4 and 5, one control and one high treatment. I am making two dilution levels, one of 100% whole seawater and the other of 20% whole seawater with 80% 0.2 micron filtered particle free seawater. The particle free seawater is filtered so that there are no organisms whatsoever, including phytoplankton, microzooplankton, and even bacteria. Nutrients are added to the dilution bottles to ensure that the phytoplankton have enough nutrients to grow. The bottles are incubated off the dock by mesocosms 4 and 5 for 24 hours at a depth where they receive similar light levels as the mesocosm bags. After 24 hours of incubation, the samples are filtered and kept in the freezer till the following day when the chlorophyll levels are measured with a fluorometer. Chlorophyll levels are also measured from the samples before the bottles are incubated to compare the measurements. The amount of chlorophyll is used to calculate phytoplankton density.
At the end of the incubation period, the grazing impact on the phytoplankton in the 20% whole seawater dilutions should be 0.2 times the grazing impact in the 100% whole seawater bottles. This is seen because the microzooplankton grazing rate decreases as the concentration of whole seawater is reduced because the microzooplankton encounter their prey, the phytoplankton, by the factor of the dilution level. However, the growth rate of phytoplankton does not depend on the number of phytoplankton present, because they should all have the same amount of nutrients to keep them happy; therefore, dilution does not change the growth rate of phytoplankton.
As you can tell, the dilution method is very complicated and I am still working to understand the mathematical equations to solve for the growth rates and grazing rates. I will make future posts about my project as I learn more, but for now I hope this makes the dilution method a little easier to understand!