Document: PAU-3-74-12

Effects of benthic grazers on epilithon C:P stoichiometry in boreal lakes.

FROST, P.C.* ¹, J.J.ELSER ¹ and M.A.TURNER ²

Arizona State University, Tempe, Arizona 85287-1501 ¹
DFO, Freshwater Institute, Winnipeg, Manitoba R3T 2N6 ²

Abstract:
Epilithon (particulate organic matter attached to rock surfaces) elemental composition has recently been shown to be highly variable in boreal lakes. However, causes of epilithon C:P variation remains poorly understood. We examined the hypothesis that variable epilithon C:P ratios are caused by benthic grazers who may remove a different combination of elements than they return. This hypothesis was tested at the Experimental Lakes Area in northwestern Ontario, Canada where we exposed epilithon in small enclosures (700 cm²) to different grazing intensities by caddisfly larvae for 20 days. We sampled enclosures after 10 and 20 days to see how grazers affected the carbon and phosphorus content of epilithon. Caddisfly grazing strongly reduced epilithon total C (p<0.001, day 20) and P (p<0.05, day 20) and by doing so, altered epilithon C:P ratios (p<0.05, day 20) compared to no/low grazer treatments. Our results show that ungrazed epilithon accumulates C but not P in these extremely P-poor lakes. Grazers alter epilithon C:P ratios by removing C faster than P, perhaps by removing large quantities of non-living organic matter characteristic of these benthic biofilms. These results suggest grazers have the capacity to affect multiple aspects of epilithon, especially its elemental make-up. By doing so, benthic grazers may alter aspects of algal and detrital decomposition and the efficiency with which primary production moves into higher trophic levels.

Keywords: stoichiometry, grazing, littoral zone

This abstract is being presented at: 8:30 AM in session:
Oral Session #54: Lake Ecology.