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Scientific Report

A Cautionary Tale for in situ Fluorometric Measurement of Stream Chlorophyll a: Influences of Light and Periphyton Biomass

Report

Abstract

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

Authors

Matthew Kaylor, Alba Argerich, Seth White, Brian VerWey, and Ivan Arismendi

Citation

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. https://doi.org/10.1086/697239.

Date

Feb 16th, 2018

Report No.

JournalPost_Kaylor_etal2018

Media Type

Journal Article

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