Fate of benthic primary production

Most periphyton production in flowing waters likely is consumed by herbivores or enters the pool of particulate detritus, either locally or after downstream transport. In addition, cells at times exude organic compounds, and these constitute inputs to the pool of dissolved organic carbon (DOC), although they are relatively small compared to allochthonous DOC sources. A complete reckoning of these quantities is not possible at present, but their inclusion should serve to emphasize that grazing of living plants is not the only fate of plant production.

Substantial downstream transport of sloughed periphyton has been observed in the laboratory and the field. In their year-long study of periphy-ton dynamics in laboratory streams, McIntire and Phinney (1965) recorded considerable export, particularly of filamentous green algae under higher light conditions and during periods of high turbidity of the inflowing water. Sycamore Creek, Arizona, was found to be a net exporter of organic matter during a 63-day recovery between floods (Fisher et al. 1982). Of the 56% of NPP not utilized at the site of production, the major amount was exported downstream and a smaller portion was stranded laterally by drying. Based on experiments conducted in laboratory streams exposed to different irradiance (20, 100, and 400 |imol m2 s"1) and grazing levels (240 snails m2 or no snails), Lamberti et al. (1989) found that increased irradiance and grazing both enhanced algal export. At the highest irradiance algal export represented 72% of total algal biomass in ungrazed streams and 84% of the biomass in grazed streams, while under low irradiance, exports were only 7-24% of total biomass.

Energy budgets constructed for periphyton populations in laboratory streams demonstrate that substantial quantities of dissolved and colloidal matter may be exuded by live cells. Energy budgets (GPP should equal respiration + biomass accumulation + biomass export) revealed a discrepancy of 17-28% (McIntire and Phinney 1965), indicating a substantial export term, which the authors attributed to dissolved matter. Extracellular release by periphyton in White Clay Creek, Pennsylvania, was found to dominate DOC dynamics and resulted in a pronounced daily cycle in DOC concentrations at certain times of the year (Kaplan and Bott 1982). Particularly in the spring, when periphyton biomass was maximal, allochthonous inputs were few and discharge was low, extracellular release by auto-trophs was estimated to account for as much as 20% of daily DOC export. Clearly, a substantial fraction of GPP can be exported as DOC under circumstances of high primary production.

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