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 (r 2 = 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 <2 µg Chl a/cm2, but 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 (r 2 = 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.
Other Key Terms: primary producers
Kaylor, M.J., A. Argerich, S.M. White, B.J. VerWey, and I. Arismendi. 2018. A cautionary tale for in situ fluorometric measurement of stream chlorophyll a: influences of light and periphyton biomass. Freshwater Science 37(2):287-295. Online at https://doi.org/10.1086/697239.