A cautionary tale for in situ fluorometric measurement of stream chlorophyll a: influences of light and periphyton biomass

The use of in situ fluorometry to estimate stream benthic chlorophyll a (Chl a) has the potential to substantially reduce sampling costs and increase the feasibility of spatial and temporal replication. We evaluated the utility of a bbe moldaenke BenthoTorch fluorometer by estimating stream benthic Chl a under different natural light scenarios and by comparing estimates to laboratory measurements. BenthoTorch Chl a increased after rocks measured under direct solar radiation were moved into the shade, but values generally plateaued within 20 to 30 min of being in the shade. A proposed in situ procedure to shade substrates prior to measurements showed mean Chl a estimates between 1.05 and 2.13× greater than when substrates were measured under midday solar radiation conditions. We used this proposed procedure to compare Chl a estimates between the BenthoTorch (Chl a BT) and established laboratory methods (Chl a Lab) at 50 sites throughout a productive basin in northeastern Oregon. A positive relationship existed between reach-scale Chl a BT and Chl a Lab (r² = 0.51, p < 0.001), but this relationship was not 1:1. Chl a BT was greater than Chl a Lab when laboratory values were 4 μg Chl a/cm², Chl a BT was always lower than Chl a Lab estimates. The Chl a Lab:Chl a BT ratio was positively related to reach-scale periphyton ash-free dry mass (AFDM), indicating increasing discrepancies between methods with greater periphyton biomass (r² = 0.68, p < 0.0001). Collectively, our findings suggest that: 1) light conditions typically encountered in natural field settings can substantially alter Chl a estimates when using the BenthoTorch, and 2) at productive sites, the BenthoTorch can underestimate Chl a because of greater periphyton mat thickness.