I recently did a nutrient-drip (NH4 and PO4) experiment in a high-altitude stream in the Andes. The stream had quite low background N, and quite high P (probably an N-limited system). We did the drip for 24 hours during which time turbidity fluctuated along with discharge. Any thoughts on why the mass-transfer coefficient (Vf) for NH4 would increase with increasing turbidity?
I'm not sure what exactly is meant by mass-transfer coefficient in your field, but NH4+ uptake due to increased turbidity could be due to sorption to suspended solids. In particular (pun intended), clay minerals are well-known sinks for NH4+ due to uptake into the clay mineral interlayer (within the mineral structure). Otherwise it could be a biological uptake due to increased phytoplankton concentrations. Maybe you need to measure total and volatile suspended solids in your stream? (VSS are taken to be a measure of biomass in environmental engineering.)
Thanks Drew. But wouldn't sorption to the suspended particles (or uptake by phytoplankton) result in less uptake within the stream channel? As a point of clarification, the nutrient drip technique measures uptake by the benthos.
Thanks Ellard: In our particular study NH4 concentrations were very low throughout the 24 hr. sampling period, and fairly consistent, at least relative to uptake which was a lot more variable. That looks like a very useful paper - I'll be sure to get it.
Scott, did you measure flow velocity? When you say "turbidity fluctuated along with discharge", did you mean turbidity increased with discharge? I am not sure if you are aware of the paper by Larned & Atkinson (1997) that reported a positive relation of NH4/PO4 uptake of a marine macroalga with flow velocity (Effects of water velocity on NH4 and PO4 uptake and nutrient-limited growth in the macroalga Dictyosphaeria cavernosa. Marine Ecology Progress Series 157: 295-302). Their explanation for this was that velocity could change DIN source and availability. Hope this somehow helps.
My thinking is that with increase in nutrient load (ammonium-nitrogen and phosphate), there will be increase in microbial load and hence the turbidity.
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