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Comparison of the responses of benthic and planktonic communities to enrichment with inorganic fertilizers Cameron, Roderick L.

Abstract

A comparison was made of the responses of benthic and planktonic components of natural water-sediment systems enriched with inorganic fertilizers. Three levels of fertilization were applied to a series of enclosures placed in a shallow (1 m depth) area of Marion Lake, British Columbia and community respiration, primary production, standing algal crop and the distribution of added phosphorus were measured in both the sediment and the water column over a period of two years. A loss of added phosphorus from the water column corresponded to an increase measured in the sediment. Uptake and release of phosphorus by the sediment was proportional to its concentration in the water column, indicating a water-sediment equilibrium. In an undisturbed system, however, there was a net movement of phosphorus into the sediment . A sustained high level of planktonic primary production persisted throughout the period of fertilization. However, an initial increase in total benthic primary production returned to pre-fertilization levels following the establishment of an increased standing crop of epipelic algae. At this point, benthic primary production appeared to be a function of grazing pressure, responding to increased grazing but not to additional fertilization. When grazing was experimentally increased by concentrating grazers in an experimental area, benthic primary production increased. Fertilized sediment appeared to have a much greater ability to withstand increased grazing pressure than normal sediment, maintaining high levels of primary production even at five times normal grazer density. But despite this potential, measurements of grazer and bacteria populations within the experimental enclosures showed no response to the greater standing crop of epipelic algae. As a result, sustained increase in benthic primary production could be induced only experiment-, ally. It was concluded that the benthic community was more stable in its response to enrichment than that of the plankton and by absorbing nutrients from the water column served to dampen the effects of the more pronounced fluctuations of the planktonic community.

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