Introduction

Thecamoebians (also called testate amoebae) are protists that form a diverse and important component of the microbial trophic level within the benthic community of lakes and wetlands, and play a critical role in food webs as the intermediate between bacterial and benthic invertebrate communities (Patterson and Kumar, 2000; Beyens and Meisterfeld, 2001). These epifaunal/shallow infaunal benthic protozoans, particularly those belonging to the Superfamily Arcellacea, produce a fossilizable test of pseudo-chitinous material that is variably agglutinated in different species (Medioli and Scott, 1983). Their fossilized tests are found in all freshwater aquatic and moist terrestrial sediments, although the preservation potential varies between species, with some rarely reported as fossils (e.g., Difflugia amphora Wallich, 1864), even though they may be common in community studies of surface sediments (Boudreau et al., 2005; Patterson and Kumar, 2002). Thecamoebians display a rapid generation time and a high degree of sensitivity to environmental conditions at the sediment-water interface and epibenthic zone, and their fossil remains preserve a record of their populations over time (Boudreau et al., 2005; Patterson et al., 2002; McCarthy et al., 1995). Unlike most microfossil groups, thecamoebians do not dissolve in low pH environments, and in comparison to other microfossil types with shells that preserve well (e.g., diatoms, spores and pollen) thecamoebians reflect depositional conditions at the sediment/water interface of lacustrine freshwater and peat environments (Patterson et al., 1985).

Thecamoebians have recently been used to investigate the impact of sulphide mining in acid-sensitive lakes in Ontario (Patterson et al., 1996; Reinhardt et al., 1998; Kumar and Patterson, 2000; Patterson and Kumar, 2002), the impact of road salt runoff (Roe et al., 2010) and reclamation options in the oil sands constructed wetlands in Northeastern Alberta (McCarthy et al. 2008; Neville, 2010). As part of the oil sands study, it was demonstrated that thecamoebians can be used as proxies to monitor varying degrees of impact of oil sands constituents (Neville, 2010). To better apply this tool, an understanding of the natural variability of the population was required. This included investigating thecamoebian assemblages in natural lakes in Northeastern Alberta, as well as identifying their population response characteristics in relation to seasonal environmental changes. The families Centropyxidae and Difflugiidae were found to exhibit different degrees of sensitivity to the major by-products of oil sands mining activity in the Suncor Wetlands, with most difflugiid taxa exhibiting high sensitivity and lower tolerance to oil sands constituents, whereas centropyxid taxa appeared to thrive in all but the most highly impacted sites. Major byproducts created during the extraction and processing of oil sands in Alberta include naphthenic acids and elevated levels of conductivity, both of which are leached from the oil sands during processing (Harris, 2007). Naphthenic acids (NAs), a family of low molecular weight, naturally occurring carboxylic acid surfactants are released from the bitumen into water under the elevated pH conditions used in the oil-sand extraction process. They are important because in process waters they have been shown to be responsible for most of the acute toxicity to aquatic organisms (MacKinnon and Boerger, 1986; Han et al., 2009). Oil sands process-affected water (OSPW) also contains elevated levels of ions relative to regional water bodies. Salt leaching from the oil sand during processing and addition of process chemicals adds to the ion load, so that conductivity in OSPW ranges from about 1000 to 5000 µS/cm, with the primary ions being Na, Cl, HCO₃ and SO₄ (FTFC, 1995).

The study of thecamoebian response in the Suncor Wetlands did not isolate environmental parameters and only focused on water chemistry of the test systems (Neville, 2010). Recognizing that these protists respond to physical as well as chemical aspects of their environment, a study to assess the response of thecamoebians to seasonal environmental variations (temperature, dissolved oxygen (DO), nutrients) under consistent chemical conditions (salinity, NAs) was undertaken. The majority of seasonality studies have been conducted using foraminiferal populations (Murray, 1973; Boltovskoy and Wright, 1976). The first investigation of seasonality using living vs. total populations was conducted by Scott and Medioli (1980). Much less research, however, has been conducted using thecamoebians, the only studies of seasonality to date were conducted on peatlands by Heal (1964) and Warner et al. (2007).