N-P STOICHIOMETRY OF SEDIMENTATION IN LAKES OF THE CANADIAN SHIELD - RELATIONSHIPS WITH SESTON AND ZOOPLANKTON ELEMENTAL COMPOSITION

Stoichiometric theory predicts that zooplankton with low body N:P rati o recycle nutrients at higher N:P ratios than zooplankton taxa with lo w body N:P ratio, an effect that may accentuate P-limitation of phytop lankton growth. However, existing theory assumes that all regenerated nutrients are returned to the dissolved pool and does not account for the counteracting possibility that materials not assimilated by zoopla nkton might be lost from the water column Fia sedimentation. To assess the stoichiometry of zooplankton effects on N and P sedimentation, a survey of sedimentation, zooplankton, and suspended particulate matter was conducted in twelve lakes in the vicinity of the Experimental Lak es Area, Canada. Concentrations and ratios of carbon, nitrogen, and ph osphorus in suspended particulate matter in the mixed layer of each la ke were determined. Zooplankton community composition, biomass, and el emental composition (N:P ratio) for each lake were assessed. Finally, sediment traps were deployed in the hypolimnion of each lake to quanti fy rates of particulate matter sedimentation as well as C:N, C:P, and N:P ratios of sedimenting material. C:P and N:P ratios of sedimented m aterial were considerably lower than those in seston but C:N ratios we re similar. The N:P ratio of sedimented material had no relationship w ith seston N:P but decreased strongly with zooplankton N:P, consistent with stoichiometric recycling theory. However, the residuals of sedim ented N:P versus zooplankton N:P relationship had a negative relations hip with seston N:P, suggesting a joint stoichiometric dependence of s edimentation on consumer and seston elemental composition with consume r N:P ratio of primary importance. These patterns suggest that water c olumn processes integrate the stoichiometry of microbial processing of detrital materials with that of consumer-driven nutrient cycling to d etermine sediment elemental composition.