I am conducting a mesocosm experiment in 68 L rubbermaid totes examining the differences between gamma irradiated tailings waste (treatment) and non-irradiated tailings waste (control), we believe this tretament reduces toxicity by breaking down recalcitrant napthethnic acids. I am interested in seeing if the net productivity, GPP and R differ after treatment. I collect these measurement through high frequency sonde data (HOBO U26 Dissolved Oxygen loggers). For one of my sampling dates, I see a dramatic drop in DO over a 15 minute period (ca. 7 mg/L). I've attached a photo of the graph of DO. The drop occurs around 2045 on three sampling days which leads me to believe that this isn't an error in the data because of it's consistency. However, the values calculated for this sampling period are odd in that the GPP is negative and the R is positive, I'm wondering if anyone has any suggestions on how to treat these data points (are they valid, should they be thrown out)?
From past experience with various stream detection equipment, sondes and transducers have membranes that are sensitive to interference, such as sediment or abrupt change in concentration, and sometimes there is an overcompensation in the data until it comes to equilibrium. Oxygen sensors are sensitive and seem to do better if sample is mixed, but without air bubbles such as turbulent water. If the oxygen standards are used ans still seem to be giving reliable results, I would probably smooth over the period of decline where it is so abrupt, and say that in the results that an abrupt change was measured in some samples and remove by smoothing the recession curve. You might try purchasing a new probe or changing the fluid, probe membrane and recalibrating..I do not know if particles utlizing oxygen in respiration (dark) could be attached to the membrane, and be abruptly using all the oxygen around the membrane, the oxygen level drops severely, they die off or go dormant, and then the solution concentration is then revealed. That could be another explanation. In either example I suggest (not knowing), the drop is more of a probe membrane concern, local circumstance. I think I would discus with the Onset Hobo technical representatives and see if they have any other ideas. . .
You may check by replicating using two probes per tote with different position and membrane history.. Several others factors may interfere https://books.google.it/books?id=tXzm4nYd4J4C&pg=PA52&lpg=PA52&dq=gpp+mesocosm&source=bl&ots=XaTTSzwRh1&sig=FkS8adB8Bvs1DCHOOGcTIxawEqE&hl=it&sa=X&ved=0ahUKEwjpkoeM2ubQAhWGOxQKHaLuCBUQ6AEIHTAA#v=onepage&q=gpp%20mesocosm&f=false For polynuclear toxicity you may check a sample by mass spectroscopy after chromatography
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