Cyanobacteria are natural constituents of lentic and lotic waters, but they appear to have become increasingly prominent in recent decades, possibly in association with anthropogenic eutrophication (Mur et al., 1999) and climate change (Paerl & Huisman, 2008). In lentic systems cyanobacteria can proliferate rapidly in response to adequate nutrient supply and elevated water temperature, with stratification being a key part of their surface accumulation (Oliver & Ganf, 2000). Cyanobacterial blooms are aesthetically unpleasant and can have serious environmental impacts (Paerl et al., 2001).
During periods when conditions are unfavourable for planktonic growth, many cyanobacteria persist in lake sediments as resting stages, either as short filaments (hormogonia), akinetes (resting stages) or vegetative cells (Head et al., 1998; Verspagen et al., 2004). Benthic populations that survive winter may provide significant inocula for the development of pelagic cyanobacterial populations (Preston et al., 1980; Brunberg & Blomqvist, 2003; Kim et al., 2005). It has been suggested that large overwintering populations are one reason why cyanobacteria with low specific growth rates (e.g. Microcystis) become dominant in summer phytoplankton communities (Reynolds, 1994). Analyses of surficial lake sediments can provide valuable forecasts of the potential species composition of pelagic blooms (Baker & Bellifemine, 2000; Faithfull & Burns, 2006), while sediment profiles may provide information on historical phytoplankton composition and abundance (e.g. Livingstone & Jaworski, 1980; Dickman & Glenwright, 1997; Tani et al., 2002).
Various techniques have been used to analyse phytoplanktons within sediment cores. Livingstone & Jaworski (1980) showed that akinetes from sediments deposited up to 64 years earlier could be germinated when incubated in culture media. Recently, molecular techniques have been successfully used to detect cyanobacteria (Innok et al., 2005) and cysts of eukaryotic algae (Coyne & Cary, 2005) from sediments. Automated rRNA intergenic spacer analysis (ARISA) is a recently developed DNA finger-printing method (Fisher & Triplett, 1999) that exploits the length heterogeneity of the intergenic spacer (ITS) region between the 16S and 23S rRNA genes. In this study, we used both germination experiments and ARISA assays to investigate the cyanobacterial community composition in layers of a sediment core taken from a eutrophic lake of volcanic origin in the Rotorua district of New Zealand.
A light-colored tephra deposited over the Rotorua district in the 1886 Tarawera volcanic eruption provides highly visual differentiation between lake sediments deposited before and after the Tarawera eruption (Nelson, 1983). Within this period, many Rotorua lake watersheds have been subject to European colonization and changes in land use from native forest and scrub to pastoral farming and plantation forestry. Correspondingly, nutrient loads to many of the lakes have increased and, for some lakes, there is a well-documented history of increasing trophic status, for example Lake Rotorua (White et al., 1985) and Lake Rotoiti (Vincent et al., 1984). However, there is little reliable historic information on phytoplankton species composition. Thus, it is difficult to ascertain if bloom-forming species have always been present in the lakes and have proliferated relatively recently, or if there has been a succession of cyanobacteria with new arrivals through recent history.
Lake Okaro is a small, monomictic, eutrophic lake in the Rotorua district of central North Island of New Zealand. It was formed as a hydrothermal explosion crater c. 900 years before present (Healy, 1964). Pastoral farming proliferated rapidly in the Okaro watershed in the 1950s (Jolly, 1968) and by 1970 around 95% of land use in the watershed had been adapted for pastoral farming (McColl, 1972), similar to present day land use. Compared with other New Zealand lakes, Lake Okaro has a long limnological data record, dating back to the 1950s (e.g. Jolly, 1959; Fish, 1969; McColl, 1972; Flint, 1977; Dryden & Vincent, 1986; Forsyth et al., 1988). This extended record includes a period when the lake changed from a continuously oxygenated hypolimnion during the 8-month seasonal stratification cycle, to being devoid of oxygen for all but 1 month. Correspondingly, there have been large increases in nutrient concentrations, increased relative abundance and biomass of cyanobacteria, and decline in diversity of littoral benthos (Forsyth et al., 1988). Since the 1970s, the lake has had seasonally recurrent cyanobacterial blooms (Dryden & Vincent, 1986). Gall & Downes (1997) investigated fossil pigments in a sediment core from Lake Okaro. The pigments myxoxanthophyll and canthaxanthin are specific to cyanobacteria. Neither of these pigments was detected in the core in the period estimated from 210Pb dating to be before 1900. A small increase in both myxoxanthophyll and canthaxanthin occurred between 1900 and 1950, followed by peaks of both pigments around 1965. The pigment analysis unequivocally showed an increase in cyanobacterial concentrations in the lake; however, it provided no information on possible changes in species composition.
The major objective of this study was to reconstruct the historical composition of cyanobacterial communities in Lake Okaro in order to provide a sedimentary record of their presence and long-term succession through a period of rapid progression of eutrophication in the lake. A secondary objective was to evaluate the relative efficacies of germination experiments and ARISA assays in reconstructing the assemblage of cyanobacteria through the sediment profile.