Production and respiration of epilithic algal communities in Irish lakes of different trophic status

Metabolism of shallow epilithic algal communities for 21 lakes of different trophic status in Ireland was measured by changes in dissolved oxygen in chambers. Epilithic net community production ranged from 51 to 805 mg O-2 M-2 h(-1) (ca. 16 to 270mg Cm-2 h(-1)), and likely was a substantial contributor to whole-lake production in the more shallow lakes. Stepwise multiple regression using 10 environmental variables indicated that alkalinity and turbidity explained a significant amount of the variance in net community and gross primary production, while color and turbidity were most related to respiration rates. Alkalinity and turbidity were associated with watershed geology and land use, as lakes in limestone areas were more influenced by agricultural inputs. Lower respiration in highly colored takes may have indicated a lower supply of organic matter for heterotrophs. Traditional, pelagic-based, trophic classification parameters for the lakes had weak or nonsignificant linear relationships to epilithic production because some lakes with intermediate nutrient and phytoplankton chlorophyll concentrations had higher production than lakes with greater concentrations but lower water clarity. More of the variance in periphyton production was significantly explained using a derived variable that combined phosphorus concentration and light reducing factors (chlorophyll, turbidity and water color), suggesting trophic indicators that reflect both nutrient loading and light availability may be more applicable for estimating whole-lake production in lakes where benthic processes dominate energy budgets. A conceptual model is proposed for examining how watershed and physicochemical parameters can interact to potentially influence periphyton production in the many shallow lakes in Ireland.