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

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 LOPCderived 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. *Corresponding author. E-mail: minna.rahkola@uef.fi AcknowledgmentsWe thank the skilful staff of the R/V Muikku for carrying out the sampling in the field, and all the ex-workers of the former Ecological Institute for their assistance in the laboratory. We are grateful to Greta Waissi-Leinonen, Irina Dushkina, and Matti Koistinen for running the OPC and LOPC routines and Kirsti Kyyrönen for drawing the map. We also thank professors (emeritus) Kalevi Salonen and Jouko Sarvala for their helpful comments and suggestions that improved the clarity of the manuscript. Malcolm Hicks kindly revised the English language of the manuscript.

On biogenic turbulence production and mixing from vertically migrating zooplankton in lakes

“…To overcome this limitation, the measured ADCP echo intensity (see Online Resource 1 in the "Electronic supplementary material") can be used as a proxy for the zooplankton abundance, where the highest values correspond to the highest abundance, while the lowest value corresponds to those parts of the water column free of zooplankton or with a negligible concentration. Several studies in the ocean and lakes (Fielding et al 2004;Lorke et al 2004;Rahkola-Sorsa et al 2014;Record and de Young 2006;Huber et al 2011) show that the concentration of different taxonomic groups can be related to the linear volume backscatter strength ( VBS linear = 10 VBS∕10 ) via a physical and acoustic-based model. This ADCP calibration allows us to (1) continuously estimate zooplankton concentration without resorting to multiple net tows; (2) use finer vertical resolution (0.5 m) than zooplankton tows (3 m); (3) provide an estimation of the concentration within the migrating layer during the DVM, as it is not possible to sample the water column with the net during the migration and obtain a concentration profile under reasonably stationary conditions because the concentration changes too quickly during the DVM; (4) understand which taxonomic groups explain and contribute the most to the acoustic signal and to its variation.…”

Measuring the distribution, abundance, and biovolume of zooplankton in an oligotrophic freshwater lake with a 710 kHz scientific echosounder

“…Further, these studies used areal estimates or estimated zooplankton biomass at specific depth intervals. Rahkola-Sorsa et al (2014) noted that their 614 kHz ADCP data were better correlated with Chaoborus and medium-sized zooplankton (300-500 mm). Using the 710 kHz frequency echosounder we were able to both accurately estimate zooplankton biovolume and provide high-resolution information on the zooplankton depth distribution (Table 2; .…”

“…Recent work (Rahkola‐Sorsa et al ) did compare optical, net, and a 614 kHz ADCP measures of zooplankton biomass and concluded that “in situ estimation of zooplankton abundances on the basis of ADCP results is not reliable without simultaneous net or optical sampling.” High‐frequency acoustic surveys in freshwater systems have rarely taken advantage of the temporal resolution of these systems to monitor changes in the distribution of small zooplankton in real‐time or the spatial resolution to fully characterize zooplankton distribution over the entire habitat. Unlike ADCPs, scientific echosounders are designed specifically to quantitatively measure the amount of acoustic energy scattered by objects in the water column.…”

“…More advanced optical and acoustic techniques have also been used to study freshwater zooplankton (Sprules et al ; Schulze et al ; Holbrook et al ; Rahkola‐Sorsa et al ). Optical techniques have the advantage of providing relatively automated data, but they are also limited by their inability to provide high resolution data or any detailed taxonomic identification as well as small sampling volumes.…”

“…A variety of high frequency systems have been used to study spatial distributions and dense aggregations (>10 3 organisms m −3 ) of small (1–2 mm length) freshwater zooplankton including a 430 kHz system to measure copepod spatial distribution in Lake Superior (Holbrook et al ) and a 710 kHz system to detect copepod aggregations over a range of 30 m in a shallow coastal bay (Parks et al ). While similar systems, including a 710 kHz echosounder and a 614 kHz Acoustic Doppler Current Profiler (ADCP), have also been used to detect vertical distributions and migrations of zooplankton (Lorke et al ; Knudsen et al ; Rahkola‐Sorsa et al ), these studies have focused on the gas‐bearing larval insects Chaoborus flavicans that are strong backscatterers and large in size (2–10 mm length). To our knowledge, only two previous studies (Roman et al ; Parks et al ) have used acoustic systems to examine vertical distributions or DVM of small (<2 mm) zooplankton.…”