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Intercalibration of an acoustic technique, two optical ones, and a simple seston dry mass method for freshwater zooplankton sampling

Minna Rahkola-Sorsa, Ari Voutilainen, Markku Viljanen

Limnol. Oceanogr. Methods 12:102-113 (2014) | DOI: 10.4319/lom.2014.12.102

ABSTRACT: A thorough comparison is made of conventional and automated zooplankton sampling techniques. The findings offer tools for complementing netted and weighed zooplankton biomasses with estimates based on acoustic and optic techniques, and thereby fill a gap in our knowledge concerning the suitability of acoustic and optic techniques for freshwater zooplankton sampling. Plankton abundance in six large boreal lakes with varying characteristics was determined by four independent methods: one acoustic (Acoustic Doppler Current Profiler), two optic (laboratory versions of the Optic and Laser Optic Plankton Counter), and a simple seston dry mass (DM) method. The combined DM of Chaoborus and medium-sized zooplankton (300-500 μm) explained 63% of the original variability in the acoustic backscatter strength. The correspondence between the optical measurements and DM was especially good in the case of medium-sized zooplankton. Thus the LOPC-derived biomass (ESD 351-500 μm) explained 90% of variation in the DM of zooplankton of size 300-500 μm. The medium-sized fraction consisted mainly of small Cyclopoida and copepodite stages of Eudiaptomus and Daphnia. Both optical devices, and particularly the LOPC, are reliable for analyzing preserved net samples of freshwater zooplankton, but in situ estimation of zooplankton abundances on the basis of ADCP results is not reliable without simultaneous net or optical sampling. Acoustic results can be calibrated by reference to DM or LOPC-derived biomass estimates. The combined use of acoustic and optical techniques clearly improves the spatio-temporal resolution of zooplankton measurements and can thus provide closer insights into ecosystem dynamics, including the forces which regulate plankton communities.