Nitrogen, Phosphorus and Organic Carbon (The Big Three)

I'm a stoichiometry kind of guy (even if I've been relatively unsuccessful lately), and stoichiometry seems to revolve around N, P and C. And really, mostly just N&P. As a result, I've been thinking a lot about how the terrestrial and upstream watershed affects the N, P, and C in receiving waters. So, for instance, if you change the proportion of wetlands, how is the ratio of these nutrients going to change? In this vein, I wrote a paper (still in review) that looks at the relationships between watershed characteristics and dissolved, inorganic N&P in streams (in Michigan and Wisconsin). Sounds like fun right? Yes.

In that context, I was surprised to stumble upon a paper recently that I hadn't previously been aware of that seemed to look at something very similar. I'm referring to Saunders, McClain and Llerena (2006) "The biogeochemistry of dissolved nitrogen, phosphorus, and organic carbon along terrestrial-aquatic flowpaths of a montane headwater catchment in the Peruvian Amazon." I was specifically intrigued by the reference to sampling nutrients along transects. So I hit up my sources for subscription-only publications, and I soon had my hand on the paper.

The paper isn't at all what I was expecting, although it was incredibly interesting. I was expecting longitudinal transects along stream networks. Instead, the authors investigated transects from the upland down to the stream (see Fig 1 from the paper below).

Although this isn't what I was expecting, this is really interesting. Basically, the authors measured the nutrient concentrations in groundwater as it moved from these upland areas down into the stream. There's a lot of interesting data in here, but big-picture, what they found is that nitrogen tends to be high in the upland areas (both organic and inorganic) and then decreases as it moves down into the stream. By contrast, phosphorus is low in the upland areas, but much higher in the stream.

This seems to support a general trend observed in the tropics: In-stream inorganic nitrogen is really low, and fairly constant. Based on this study, it appears that this is driven by the fact that as soil/groundwater is passing through the riparian and hyporheic zones, all the inorganic nitrogen is being stripped out. That's an oft-repeated story (my first encounter was Pusch et al. 1998; pdf), but it is interesting that this seemed to be generally true for the organic nitrogen as well.

I also find it interesting that P behaved in almost the exact opposite way. I actually have a hard time reconciling this. If water coming in to these riparian areas is low in P and high in N, then I would anticipate the biota holding onto that P, since they are presumably P-limited. The opposite seems to be the case, with N being stripped out of the water and P actually increasing. This might indicate that a big part of this story is abiotic (i.e., stuff is sorbing to surfaces in the riparian areas).

Also interesting is the fact that nutrients were just really different in the terrestrial and aquatic areas. Money quote from the authors:

Spatial trends in nutrient stoichiometry highlighted a clear divergence in the nutrient composition of terrestrial and aquatic hydrologic compartments along the Wara transects.

- Saunders et al. p 2559

I have a few quibbles with the paper. The nutrient concentrations are given in graphical form, but nutrient ratios are not provided. Also, the authors froze their DOC samples! I found that freezing could change the DOC concentration slightly (it changes the absorbance a lot), but I don't know the mechanism by which that happens. As a result, I never freeze DOC samples. Without knowing the mechanism, it is pretty difficult to really know how big of a problem this is. I've seen a lot of other authors do this though, so maybe I'm missing some methodological papers that investigated this.

Summary:

Great paper, easy to read and well-written. Documents the pretty dramatic differences in nutrient composition that can occur over a relatively short lateral gradient.

Pusch, M., D. Fiebig, I. Brettar, H. Eisenmann, B.K. Ellis, L.A. Kaplan, M.A. Lock, M.W. Naegeli, and W. Traunspurger. 1998. The role of micro-organisms in the ecological connectivity of running waters. Freshwater Biology 40:453-495.

Saunders, T., McClain, M., & Llerena, C. (2006). The biogeochemistry of dissolved nitrogen, phosphorus, and organic carbon along terrestrial-aquatic flowpaths of a montane headwater catchment in the Peruvian Amazon Hydrological Processes, 20 (12), 2549-2562 DOI: 10.1002/hyp.6215