Benthic algal response to pulsed versus distributed inputs of sediments and nutrients in a Mackenzie Delta lake

The Mackenzie Delta is a lake-rich region where the distribution and abunda nce of phytoplankton, macrophytes, and benthic algae among lakes may be con trolled by an interaction between nutrient supply and sediment in the water column, and episodic reduction in light availability associated with river flooding. To simulate the effects of episodic river inflow on benthic alga e, limnocorrals (depth = 3 m) were either not manipulated (control), or rec eived high or low weekly additions of nutrients (N and P) and sediments, de livered as a pulse (1/wk: high-pulsed, low-pulsed) or distributed increment ally through each week (1/3 of high or low load x 3 d/wk: high-distributed, low-distributed) in a balanced triplicated design (15 limnocorrals). Artif icial substrates, enriched to mimic the supply of nutrients at the sediment /water interface, were suspended in the limnocorrals at 2 rn depth for 6 wk beginning in mid-July 1995. Maximum average levels of phytoplankton biomas s (inferred from chlorophyll) occurred in the treatment where light attenua tion was highest (high-distributed), and lowest levels occurred where light attenuation was lowest (control). By contrast, maximum average accrual of benthic algal biomass on the artificial substrates occurred in the control, whereas lowest accrual occurred in the high-distributed treatment. Chlorop hyll content per unit biovolume of benthic algae increased modestly in trea tments experiencing low light relative to treatments with higher light, but was insufficient to explain the 3-fold difference in inferred biomass betw een the control and high-distributed treatment in the case of benthic algae , or the 4-fold difference in the case of phytoplankton. Under equivalent i n situ light conditions, areal net photosynthesis of benthic algae was high est in the control and lowest in the high-pulsed treatment. Net photosynthe tic rate per unit benthic algal chlorophyll was not significantly different among the control and treatments. Last, biomass accrual rates among treatm ents showed a strong inverse relation with average light attenuation among the limnocorrals over the duration of the experiment. These results indicat e that light availability was primarily responsible for maximum biomass acc rual in the control and was the dominant factor affecting accrual rates amo ng treatments. Overall, this study suggests that the abundance of benthic a lgae among Mackenzie Delta lakes may progressively increase as flood freque ncy decreases and light availability becomes greater, whereas phytoplankton abundance may peak in lakes with intermediate transparencies by responding to nutrient additions despite interruption of the light environment by sus pended sediments.