The Te Arawa/Rotorua lakes located in the central North Island of New Zealand have significant cultural, historic, social and economic value. Anthropogenic changes in land use have led to a decline in water quality in some lakes. A number of lakes have accelerated eutrophication with recurring cyanobacterial blooms and periods of bottom-water anoxia. Whilst there has been extensive research undertaken on phytoplankton dynamics in freshwater lakes there is little information on the abundance and activity of viral-like particles (VLPs) and bacteria. VLPs are the most abundant biological entities in aquatic environments and play an important role in carbon and nutrient cycling, reproducing either by cell lysis or replication in the host cell. Bacterial community structure is thought to be closely linked to the viral community and bacterial taxa have a key role in biochemical cycling in freshwater systems.
Two Te Arawa/Rotorua monomictic lakes differing in their trophic status; oligotrophic Lake Tikitapu and eutrophic Lake Okaro, were sampled over a 12-month period at the surface (epilimnion) and bottom (hypolimnion) and through a discrete layer of the thermocline during stratification. The thermocline is a region where there may be steep clines in physiochemical parameters (e.g., dissolved oxygen) which may strongly affect the distribution of, and environmental factors that influence, prokaryotes, protozoa and viruses. Physiochemical variables, nutrients, microbiological and molecular analyses were undertaken on samples in order to compare and contrast changes occurring within and between the two lakes.
Both lakes were strongly stratified for c. 8 months from September 2009 to June 2010 as indicated by Schmidt stability values > 1, which contributed to well-lit but nutrient-limited surface waters for phytoplankton productivity. With increasing duration of stratification a deep chlorophyll maximum formed in both lakes with Chlorophyta and Euglenophyta the dominant phytoplankton.
Viral-like particle abundance in both lakes exceeded bacterial abundance by a factor of c. 100, with maximum VLP and bacterial abundances in both lakes c. 10⁸ cells mL-¹ and 10⁶ cells mL-¹, respectively. Bacterial abundance in both lakes was similar in the epilimnion and hypolimnion during stratification with the exception of a peak (2 x 10⁷ cells mL-¹) in the epilimnion of Lake Okaro in February 2010 (and in the hypolimnion (9.74 x 10⁶ cells mL-¹) in March 2010 of in Lake Tikitapu. Viral-like particle abundance was variable but the epilimnion and hypolimnion tracked in both lakes between August 2009 and January 2010 after which Lake Okaro epilimnion and hypolimnion remained steady with Lake Tikitapu hypolimnion showing considerably higher VLP abundance than the epilimnion before tracking together in June 2010.
The abundance of the following bacterial functional genes was monitored through the study; nifH (encoding the nitrogenase reductase protein), dsrA (encoding the sulphate reductase protein), mcrA (encoding the methyl coenzyme M reductase protein), amoA (encoding the ammonium oxidising protein and nosZ (encoding for the nitrous oxide reductase protein). The occurrence of the nifH gene correlated with increased abundance of cyanobacteria capable of fixing nitrogen in the epilimnion of both lakes while the dsrA gene was more abundant in Lake Okaro, likely due to higher organic matter concentrations and greater duration and spatial extent of reducing conditions in that lake. Abundance of mcrA was expected to be high in the anoxic waters of the nutrient rich sediment of Lake Okaro but there was very low abundance. The amoA genes were detected when concentrations of ammonium were elevated in the bottom waters of both lakes. Both lakes showed the presence nosZ genes with high abundance occurring in Lake Okaro in December 2009 through all levels and also in December 2009 in Lake Tikitapu in the epilimnion and hypolimnion. With denitrification reliant on the availability of nitrate (NO3-N) and dissolved organic carbon levels and performed by obligate and facultative anaerobes, conditions need to be precise for the process to occur
