Active fluorometry improves nutrient-diffusing substrata bioassay

Benthic algal nutrient bioassays traditionally have been done by measuring periphytic algal biomass that has grown on fertilized or unfertilized patches of habitat produced by nutrient-diffusing substrata (NDS). This method requires destruction of the accumulated periphyton communities and, thus, does not allow for convenient monitoring through time. Variable fluorescence methods of estimating algal biomass and photosynthetic activity have been used in aquatic environments, but generally not over different nutrient treatments and not for a substantial duration. We evaluated the use of a pulse amplitude modulated (PAM) fluorometer for measuring algal biomass and photosynthetic activity in conjunction with NDS over several weeks in a wetland under several nutrient-addition treatments. We were able to detect a significant fluorometric response as early as 1 wk into the study with addition of both N and P. Wetland periphyton was co-limited by N and P. Dark-acclimated minimal fluorescence was highly correlated with chlorophyll a in different nutrient treatments. Our results suggest that active fluorometry is a useful method for measuring periphytic responses to nutrients and for evaluating the effect of nutrient additions on overall photosynthetic efficiency.