A comparison of pre-industrial and present-day changes in Bosmina and Daphnia size structure from soft-water Ontario lakes

Abstract

Understanding the long-term controls on cladoceran size structure has important implications for aquatic ecosystems. Although there has been considerable interest in zooplankton size trends for Canadian Shield lakes, data are not available for zooplankton size structure prior to the period of anthropogenic disturbances. Here, we present pre- and post-impact size data for the common pelagic cladocerans Bosmina and Daphnia for 44 well-studied Shield lakes in south-central Ontario (Canada). We show that Daphnia were larger and that the length of Bosmina body appendages (mucrones and antennules) was longer in pre-industrial times than they are today. The reduction in Bosmina appendage length we observed may suggest a reduction in copepod predation pressure since pre-industrial times. Reduced maximum body size in Daphnia is a predicted response to a warming climate in north temperate lakes; however, we suggest that alternate explanations, specifically acidification and subsequent recovery following emission reductions, should also be explored as the primary drivers of Daphnia size changes in this lake set. Overall, our results highlight the importance of pre-impact data for understanding the long-term controls on cladoceran body size from pre-1850 to present.

Long-term changes in invertebrate size structure and composition in a boreal headwater lake with a known minnow introduction

“…Slides were scanned completely in a series of horizontal transects and each non-fragmented Bosmina carapace, mucro, and antennule was measured according to Korosi et al (2010). Measurements were made using a Retiga digital camera and Northern Eclipse Image Analysis software version 6 (Empix Imaging Inc., Mississauga, Canada).…”

“…Non-DVM chaoborids in western Europe, such as C. obscuripes, are also used to determine historic fish population dynamics from sediment records (Palm et al, 2005;Luoto and Nevalainen, 2009;Palm et al, 2012). The cladoceran Bosmina, a key herbivore and cosmopolitan zooplankton taxon, also responds to the dominant predation regime within lakes and have been utilised as an indirect indicator of predation (Palm et al, 2005;Alexander and Hotchkiss, 2010;Korosi et al, 2010Korosi et al, , 2013. Several Bosmina size attributes, including antennule and mucro length, can vary significantly based upon the dominant predation regime (Kerfoot, 1975).…”

“…Chaoborus), as well as its common prey (i.e. Bosmina), we build upon other paleolimnological studies of historic shifts in zooplankton size structure from eastern North America (Leavitt et al, 1989;Korosi et al, 2010Korosi et al, , 2012. Whereas most paleolimnological studies recognize the importance of top-down controls of predation on zooplankton (i.e.…”

Response of Bosmina size structure to the acidification and recovery of lakes near Sudbury, Canada

“…As lake sediments integrate organisms across whole-lake environments, incorporating seasonal and inter-annual variability, they can provide a holistic view of biotic shifts through time. In a top-bottom (i.e., pre-impact to present-day comparison) paleolimnological survey of 44 softwater Ontario lakes located around Dorset, Ontario, Canada (~200 km SE of Sudbury), Korosi et al (2010) noted decreased antennule and mucro lengths in Bosmina, and decreased postabdominal claw lengths in Daphnia spp. in modern times, compared to specimens collected from pre-industrial sediments.…”

“…in modern times, compared to specimens collected from pre-industrial sediments. The differences in size were attributed to decreased copepod predation, as well as moderate acidification that occurred in the region (Korosi et al, 2010). However, since only two sediment samples are analysed when using a top-bottom paleolimnological approach, this procedure does not allow for the timing of size changes to be linked with the onset of individual stressors.…”