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Strange algae blooms

Thu, 01/01/2004 - 2:13pm — Wally
I'm trying to model pH and cyanobacteria blooms. Right now the initial surface bloom is modeled ok, but later in the simulation i get a large algae concentration and pH spike right where the lake reaches anoxia (~10m). I have my settling rates down to 0, and i assume that my extinction coeefficents are high enough to limit algal growth at those depths. Any guidance as to where to look would be appreciated. [addsig]
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Water Quality
Mon, 01/05/2004 - 8:20am — Tom

Welcome to the "art" of water quality modeling. Without knowing anything about the system you are trying to model, it is next to impossible to provide any meaningful guidance on how to fix this. That said, I will try to tell you what I would do. First, I would determine why the algae are growing in the model. This can be done by looking at the limiting factors output in the snapshot file to determine what is limiting growth at this depth. I suspect what is happening is the following. At the end of summer, there is probably an increase in dissolved inorganic P right at the thermocline as a result of sediment inorganic P release when the hypolimnion goes anoxic. There is then sufficient P to support an algal bloom given high enough light levels and sufficiently warm water. I would start with all default kinetic coefficients. Then I would make certain that hypolimnetic inorganic P concentrations are being reproduced if they are available (probably not). Then I would conduct sensitivity analyses on the impact of varying light half saturation values, algal temperature rate multipliers, and sediment P release rates. Better yet, why don't you zip up all the input files and send them to me so that I can take a look. This is always interesting and I generally learn something about the model, so I don't mind spending some time on it. [addsig]
Thu, 01/08/2004 - 7:25am — Tom

In case anyone is interested, Wally was specifying light extinction coefficients that in some cases were less than the extinction coefficient for pure water. As a result, the model did exactly what he told it to do - allow sufficient light to penetrate such that there is light available for algal growth at a depth of 10 m where there is a lot of P available. So now we know that the model can be successfully applied to Canadian shield lakes that go anoxic.[addsig]