Testing for nutrient limitation of algal biomass across elevational and permanence gradients in high elevation ponds near the Rocky Mountain Biological Lab
Abstract
We investigated nutrient limitation in high elevation ponds in the Gothic, Colorado, USA area and whether it varied with elevation and/or degree of permanence. Nitrogen (N) and phosphorus (P) are often the key limiting nutrients in aquatic systems that regulate algal biomass (Pick & Lean 1987; Heo & Kim 1995). Rock weathering is the primary way phosphorus enters a system. As in many location, the bedrock at subalpine and alpine ponds such as those at Mexican Cut are have little or no phosphorus, and I therefore predicted that this element would limit algal growth. In contrast, at montane elevations (e.g., Kettle Ponds below the Rocky Mountain Biological Lab (RMBL)) ponds should have a greater phosphorus supply from surface and subsurface water flow, and therefore be more likely to be nitrogen or nitrogen-phosphorus co-limited. We found a significant difference in algal biomass between elevational and permanence gradients, as well as in their interactions. However, even within the same permanence gradient nutrient limitation between the ponds varied. On average, permanent ponds showed no nutrient limitation, while semi-permanent and temporary ponds had lower mean chlorophyll a in the nutrient-addition treatments compared with the control. We believe that high control treatments may be due to extensive grazing on algae growing on nutrient addition treatments, or that the presence of the nutrients actually inhibited algal growth. Studying nutrient limitation is important in understanding the basics of this system, as well as providing useful preliminary data in understanding how this system might change as species migrate up-slope with globally warming temperatures, thereby altering the ecosystem structure currently in place.
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