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 ratio recycle nutrients at higher N:P ratios than zooplankton taxa with low body N:P ratio, an effect that may accentuate P-limitation of phytoplankton 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 zooplankton 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 Lakes Area, Canada. Concentrations and ratios of carbon, nitrogen, and phosphorus in suspended particulate matter in the mixed layer of each lake were determined. Zooplankton community composition, biomass, and elemental composition (N:P ratio) for each lake were assessed. Finally, sediment traps were deployed in the hypolimnion of each lake to quantify 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 material were considerably lower than those in seston but C:N ratios were similar. The N:P ratio of sedimented material had no relationship with seston N:P but decreased strongly with zooplankton N:P, consistent with stoichiometric recycling theory. However, the residuals of sedimented N:P versus zooplankton N:P relationship had a negative relationship with seston N:P, suggesting a joint stoichiometric dependence of sedimentation on consumer and seston elemental composition with consumer N:P ratio of primary importance. These patterns suggest that water column processes integrate the stoichiometry of microbial processing of detrital materials with that of consumer-driven nutrient cycling to determine sediment elemental composition.