Water quality trends into and out of Lake Rotorua
DairyNZ recently analysed patterns in water chemistry, clarity and E.coli concentrations on streams across the Bay of Plenty with a 10-year monitoring record (NB: E.coli are common germs or bacteria).
Several things stood out when we looked at the two streams flowing into Lake Rotorua (Puarenga and Ngongotaha), the Ohau channel connecting to Lake Rotoiti and the three stations on the Kaituna River. Firstly, all six stations were pretty similar. As of 2013 these six stations possessed lower E.coli levels, high oxygen levels, greater clarity and lesser nitrogen (N) but greater phosphorus (P) than many others in the Bay of Plenty.
The long-term trends for the period 2004-2013 were also similar between the six stations. Water clarity improved in all, suggesting reduced sediment entering the streams, lake and river. In fact, across the four major indicators for sediment, nutrients (N & P) and E.coli only 1 of the 24 indicators worsened, whereas 18 of the 24 indicators improved.
The single deterioration, increasing P concentrations in the Puarenga Stream, suggests P-loads from nearby land or residents had increased. However, the actual P-load entering the lake was reduced by the alum treatment plant (located downstream of the monitoring station). For instance, waters leaving Lake Rotorua (via the Ohau Channel) possessed less nitrogen, less phosphorus, less E.coli and less algae/sediment, meaning they were clearer before entering Lake Rotoiti and the Kaituna River.
Since 2004, nearly 4% less N and 9% less P was exported every year from Lake Rotorua. Algal biomass also dropped by a remarkable 25% each year in the Ohau Channel. Importantly, these trends also mirrored those reported by DairyNZ and the University of Waikato back in 2012, which showed Lake Rotorua had experienced a 4% drop in N, 9% drop in P, 7% drop in algae and 3% improvement to water clarity since 2001 (report available here).
Together, these results indicate a marked improvement to the lake’s Trophic Level Index (a measure of its health) over the past decade.
From Dr Tom Stephens, DairyNZ
