Title: Lake water quality in the Ruamāhanga Whaitua Purpose: To assist the Ruamāhanga Whaitua Committee to understand the current water quality and ecological health of lakes in the whaitua Author: Tessa Bunny, Alton Perrie, Juliet Milne, Laura Keenan Date: July 2014 Contact the Ruamāhanga Whaitua Committee Masterton PO Box 41 [email protected] T 0800 496 734 F 06 378 2146 1 #1372936 Contents Key points ............................................................................................................................................. 2 1. Introduction ................................................................................................................. 3 2. Background .................................................................................................................. 3 3. What is the water quality of Lake Wairarapa? ............................................................. 4 4. Seasonal patterns in water quality .............................................................................. 4 5. How does the lake compare nationally? ...................................................................... 5 6. Phytoplankton blooms ................................................................................................ 5 7. What is influencing the water quality in Lake Wairarapa? .......................................... 5 7.1 Nutrient sources ...................................................................................................................... 5 8. What is the water quality of Lake Onoke? ................................................................... 6 9. What is influencing the water quality in Lake Onoke? ................................................. 7 10. Synthesis: water quality in Lakes Wairarapa and Onoke ............................................. 7 11. Lake Pounui: ecological condition ................................................................................ 8 12. What are the trends? ................................................................................................... 8 13. Knowledge gaps ........................................................................................................... 8 14. Glossary ....................................................................................................................... 9 15. For more information ................................................................................................ 10 16. References ................................................................................................................. 10 Appendix 1. How does GWRC monitor lake water quality? ................................................... 11 Lake Wairarapa water quality monitoring programme ...................................................................... 11 Lake Pounui assessment of submerged aquatic plant communities .................................................. 13 Lake Onoke water quality monitoring ................................................................................................ 13 Monitoring of Lake Onoke- the current state of water quality .......................................................... 14 The Lake SPI index used at Lake Pounui ............................................................................................. 14 1372936-V7 PAGE 1 OF 15 Key points • There are a variety of lakes in the Ruamāhanga Whaitua, which a range of biodiversity, recreational, aesthetic and food resource values – the main three being Lake Wairarapa, Lake Onoke and Lake Pounui. • Wairarapa Moana, (Lake Wairarapa, Lake Onoke and the wider wetland area) was highly modified by Lower Wairarapa Valley Development Scheme during the 1960s/70s. This has provided economic benefit, lessened floods, and increased agriculture land, but has had significant negative effects on the ecology of the lakes and loss of wetland area. • Lake Wairarapa has degraded water quality, with elevated levels of nutrients (particularly phosphorus), algal biomass and poor water clarity. The low water clarity and high phosphorus in Lake Wairarapa is primarily due to the shallow nature of the lake which makes the lake bed sediments prone to resuspension by wind and wave action. The lake can be classed as being in a ‘eutrophic’ to ‘supertrophic’ state (ie, high to very high nutrient levels). • Since the diversion of the Ruamahanga River out of Lake Wairarapa, the lake may also act as a ‘sink’ (reservoir for sediments) and there may be a historical build-up of sediments and nutrients stored on the lake bed (legacy nutrients). Algal blooms to do not appear to have been a significant issue in Lake Wairarapa but algal biomass is at times high and potentially toxic cyanobacteria species have been recorded at levels where they could pose a health risk to lake users. • There has been little significant change in the state of the Lake Wairarapa since monitoring began in 1994. • Lake Onoke is tidal barrier-bar type lake (also referred to as an intermittently closed and open lagoon (ICOL) and as long as the lake mouth remains open, it functions more like an estuary than a lake. There is a significant amount of flushing occurring due to the exchange of tidal salt water. • Lake Onoke has degraded water quality, with elevated levels of nutrients (particularly phosphorus), poor water clarity and, at times, high algal biomass. It is also classed as being in a ‘eutrophic’ to ‘supertrophic’ state. • The overall poor state of Lake Onoke and Lake Wairarapa is unsurprising given that they are both located in catchments dominated by pastoral landuse. Resuspension of lakebed sediments by wind appears to be a strong driver of water quality in both of these shallow lakes, particularly in Lake Wairarapa. Cleaner salt water inputs (tidal and/or back flow up into Lake Wairarapa) appear to improve water quality in both lakes (particularly Lake Onoke) through dilution of nutrients. Water quality in Lake Onoke is also strongly influenced by the prevailing flow and water quality in the Ruamahanga River. • Recent information on Lake Pounui is limited to a LakeSPI assessment (which looks at the communities of aquatic plants – a key indicator of lake health) undertaken in 2011. Based on this assessment, Lake Pounui was classed as having high ecological health which reflects the lake being surrounded in unmodified indigenous vegetation. Two invasive weed species are present in the lake ( Potamogeton crispus and Elodea canadensis ) but were considered to have a relatively limited impact on the native vegetation present. • Comparisons with available historical information indicate that aquatic plant communities in Lake Pounui have not changed significantly in the last 30–40 years. PAGE 2 OF 15 131372936-V7 1. Introduction This summary looks at the state of water quality and ecosystem health of the major lakes in the Ruamāhanga Whaitua – Lake Wairarapa, Lake Onoke and Lake Pounui. Significant factors include the historical loss of vegetation cover from farming which has led to loss of habitat. Flood protection measures have changed the river channel shape, diverted flows/natural intakes and this has resulted in less flushing, which has led to deterioration in water quality. 2. Background There are a variety of lakes in the Ruamāhanga Whaitua, which provide a range of biodiversity, recreational, aesthetic and food resource values. Lake Wairarapa is the largest and most well-known lake in the region. The lake is 7,850 ha which is almost equal in size to the Wellington Harbour. Other than some limited sampling in Lake Onoke, Lake Wairarapa is the only lake that GWRC routinely monitors for water quality. Lake Wairarapa is shallow, only 2.5 m at its deepest point. It is considered to be isothermal, which means the temperature does not change from the lake bottom to the lake surface. The lake is often turbid (cloudy) due to it being shallow and having windy conditions which stir up sediment from the lake bottom. Land use in the catchment is predominantly pastoral (~70%). Lake Onoke is approximately 622 ha in size and is the second largest lake in the Ruamāhanga Whaitua. The main inflow to the lake is the Ruamahanga River which drains the entire Wairarapa Valley – a large agricultural catchment (~64% of the upstream landuse is pastoral). Lake Onoke is a type of coastal lake called a ‘barrier-bar lake’ (also referred to as an intermittently closed and open lagoon (ICOL). It is tidal when the lake mouth (through a spit at its southern end) is open to the sea. It is around 5–6 m deep at its deepest point but the majority of the lake is shallow (<1 m). The lake depth varies depending on the tidal cycle (i.e. if the lake mouth is open, semi-blocked or blocked) and on the flow conditions in the Ruamahanga River. When the lake level is low, low tides can expose extensive mud flats in the centre of the lake. Lake levels are managed by GWRC’s Flood Protection Department and the lake mouth is typically opened within days to weeks of closure to minimise flood risk. The water quality in Lake Onoke is also affected by wind-activated suspension of lakebed sediment. Other key drivers of water quality include prevailing conditions in the Ruamahanga River and the tidal movement of salt water in and of the lake. The movement of salt water is significant because as long as the lake mouth is open the lake functions more like an estuary than a lake in that there is a significant amount of flushing occurring. When the mouth is closed (rare due