Permafrost thaw lakes and ponds as habitats for abundant rotifer populations

Paschale N Bégin,Warwick F Vincent

doi:10.1139/as-2016-0017

Abstract

Thermokarst lakes and ponds were sampled across a range of permafrost landscapes in subarctic Québec (Nunavik, Canada) to compare their rotifer and other zooplankton characteristics with a set of rock-basin lakes and ponds in the region. A total of 24 rotifer species were identified, with an average of seven taxa per waterbody. Rotifer abundance was an order of magnitude higher in the thaw ponds than in rock-basin waters. In some thaw ponds, rotifers accounted for >50% of the total zooplankton biomass, but for <10% in all of the rock-basin waters. Neither α- nor β-diversity was significantly different between the two waterbody types. Grazing experiments with microspheres (0.5–6 µm diameter) showed that medium-sized particles (2–3 µm) were preferred to smaller and larger particles; clearance rates were <0.05% of the water column per day, implying that the rotifer populations were unlikely to be bottom-up limited by food availability. Rotifer abundance was negatively correlated with cladoceran densities, suggesting possible interference effects. Chaoborus larvae were also present and may exert a top-down control. Thermokarst ponds are currently favorable environments for rotifers, but the rapid change they have begun to experience brings uncertainty about their ongoing capacity to sustain these prolific communities.

Highlights

Small lakes and ponds tend to receive less attention relative to larger freshwater ecosystems, but there is an increasing awareness that small shallow waterbodies are globally important sites for aquatic biodiversity, biological production, and biogeochemical processes (Downing et al 2006; Downing 2009)
Twenty-four rotifer taxa were identified at the surface of the 50 sampled lakes and ponds, with an average of seven taxa per waterbody (Table 1; Fig. 3)
Insect larvae of two species of Chaoborus were observed in 19 of the waterbodies; these were subsequently identified as Chaoborus americanus and Chaoborus trivittatus, which were combined in the enumerations

Summary

Introduction

Small lakes and ponds tend to receive less attention relative to larger freshwater ecosystems, but there is an increasing awareness that small shallow waterbodies are globally important sites for aquatic biodiversity, biological production, and biogeochemical processes (Downing et al 2006; Downing 2009). The degradation and subsidence of ice-rich permafrost give rise to shallow depressions that fill with water to produce thermokarst lakes and ponds (hereafter referred to as thaw ponds; the terms lakes and ponds are both used in the thermokarst literature, with no formal distinction). These are among the most abundant freshwater ecosystems across vast areas of the circumpolar subarctic and Arctic regions (Vincent et al 2013a). Thaw ponds contain high concentrations of photosynthetic cells, spanning a broad range of size classes (Przytulska et al 2016). Despite these varied and potentially large resources for transfer to higher trophic levels, the food web characteristics of thaw ponds have been little studied to date

Results

Discussion

Conclusion