Estimation of zooplankton abundance, distribution patterns and patchiness in a large humic lake using an Optical Plankton Counter

Abstract

Zooplankton usually behaves in complex and dynamic ways on various temporal and spatial scales and its spatial distribution is usually highly aggregated, as it possesses a large-scale spatial variability similar to that observed in the physical environment in general. We assess here the suitability of the Optical Plankton Counter for estimating zooplankton abundance, as well as the day-to-day temporal and spatial patterns in its distribution during the open water season. The influence of certain environmental variables on zooplankton abundances is also examined. Abundances of mesozooplankton-sized particles were measured with the Optical Plankton Counter monthly at five stations in a large mesohumic lake from May to September 2005 and agreed rather well with zooplankton abundances counted using a microscope. The poorest agreement was in autumn, when the Counter overestimated the zooplankton abundance, and for some samples in July, when it underestimated the abundance. Fluorescence and chlorophyll a had a significant effect on the Counter readings. Both the intercept and the slope of chlorophyll a varied randomly between depths, because the chlorophyll a decreased much more markedly in deeper waters than did the Counter particle counts. This emphasized the stratified nature of sampling. In addition to seasonal variations, there were also marked short-interval variations (day-to-day) in zooplankton abundances in all the sampling periods. These patterns are highly dynamic and can on some occasions change rapidly in response to fluctuations in the weather. There were no clear differences in zooplankton abundance between the sampling stations. The horizontal heterogeneity was less pronounced than the seasonal and short-interval heterogeneity, possibly because there were no trophic gradients in the basin. While the Optical Plankton Counter provided a rapid assessment of temporal and spatial patterns of zooplankton abundances, it does have limitations. At times it either overestimated abundances due to a large contribution from non-zooplankton particles, or underestimated abundances due to coincidence.

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

“…Despite the many potential sources of errors in optical techniques, a good correspondence between OPC/LOPC-derived biomasses and actual seston DM was observed here (see also Rahkola-Sorsa et al 2010). This is slightly surprising, as Patoine et al (2006) have stated that optical techniques are not suitable for zooplankton sampling in oligotrophic boreal lakes.…”

“…Then the sample was gently poured into the sampling chamber without allowing any air bubbles to enter the system. The OPC and LOPC were connected to the same special water circulation system so that the same aliquot flowed through them both at a rate of ~ 14 L min -1 , regulated by a ball valve placed just behind the OPC and LOPC in the system (Rahkola-Sorsa et al 2010). The aliquots were retained with a 50 μm net after they had passed through the devices.…”

“…The ESD for the specimens was calculated both using a L:W ratio of 3:1 (Huntley et al 1995;Wieland et al 1997) and according to Herman (1992): ESD = 2(LW) 0.5 . The specimens measured represented the taxa Bosmina coregoni, Bosmina longispina, Daphnia cristata, Eudiaptomus spp., Mesocyclops leuckarti, and Thermocyclops oithonoides, which form the majority of the zooplankton biomass in Lake Pyhäselkä (Rahkola-Sorsa 2008;Voutilainen et al 2012). …”

“…Whereas these methods provide a rapid assessment of temporal and spatial patterns of zooplankton size distribution and abundance, they do have limitations. The occurrence of other particles in samples may lead to the overestimation of abundance and the coincidence of particles within the sensing zone can cause an underestimation of abundance (e.g., Rahkola-Sorsa et al 2010). However, because the smallest zooplankton, despite their high abundance, constitute only a small proportion of the total zooplankton biomass, the OPC and LOPC techniques appear to provide highly reliable biomass estimates (Liebig et al 2006).…”

“…The collecting and processing of data for the determination of plankton biomass distributions for example using optical methods can be simple compared with laborious microscopic analyses of zooplankton net samples, which also require taxonomic expertise. Optical and laser optical plankton counters (OPC, LOPC) are able to determine both the size distribution and abundance of zooplankton, quickly and effortlessly (Herman 1988;Herman et al 2004;Liebig et al 2006;Finlay et al 2007;Rahkola-Sorsa et al 2010). Whereas these methods provide a rapid assessment of temporal and spatial patterns of zooplankton size distribution and abundance, they do have limitations.…”