Lakes and wetlands: monitoring and reporting

Water quality measures

• Trophic states

  A lake’s trophic state tells us about the level of nutrients (such as the chemicals nitrogen and phosphorus) and algae in the lake water. It also tells us about the range of uses that a lake has, for example its suitability for swimming.

  The five trophic states determined by these indicators are:

  • Oligotrophic lakes are clear and blue, with very low levels of nutrients and algae. Lake Taupo is an oligotrophic lake.
    
    
  • Mesotrophic lakes have moderate levels of nutrients and algae. Lake Taharoa is a mesotrophic lake.
    
    
  • Eutrophic lakes are green and murky, with higher amounts of nutrients and algae. Lake Rotoroa (Hamilton Lake) and Lake Rotomanuka are eutrophic lakes.
    
    
  • Supertrophic lakes are fertile and saturated in phosphorus and nitrogen, often associated with poor water clarity. Excessive phytoplankton growth can occur in ideal conditions - when there's a calm, hot and sunny period of a few weeks.
    
    
  • Hypertrophic lakes are highly fertile and supersaturated in phosphorus and nitrogen. They have excessive phytoplankton growth which contributes to poor water clarity, poor suitability for recreational uses, and restricts the habitat for desirable fish. Lake Hakanoa and Lake Ngaroto are hypertrophic lakes.

   

  Trophic State	Secchi depth (m)	Total Phosphorus (mg/m3)	Total Nitrogen (mg/m3)	Chlorophyll a (mg/m3)
  Oligotrophic	> 7.0	< 10	< 200	< 2
  Mesotrophic	3.0 - 7.0	10 - 20	200 - 300	2 - 5
  Eutrophic	1.0 - 3.0	20 - 50	300 - 500	5 - 15
  Supertrophic	0.5 - 1.0	50 - 100	500 - 1500	15 - 30
  Hypertrophic	< 0.5	> 100	> 1500	> 30

  Table: Typical levels of total phosphorus, total nitrogen and chlorophyll a and Secchi depth in New Zealand lakes for different trophic states.
• Chlorophyll a

  Chlorophyll a is the green pigment in plants that is used for photosynthesis. Chlorophyll a is a good indicator of the total quantity of algae in a lake. Large amounts of algae in a lake can:

  • decrease the clarity of the water
  • alter the colour of the water (making it greener)
  • form surface scums
  • reduce dissolved oxygen
  • alter the pH of the water
  • produce unpleasant tastes and smells.
• Total phosphorous

  Total phosphorous measures the amount of phosphorous available for plant and algal growth. Phosphorus is a nutrient that can encourage the growth of nuisance aquatic plants such as algal blooms. High levels of phosphorus in water can come from either waste water or, more often, runoff from agricultural land
• Total nitrogen

  Total nitrogen measures the amount of nitrogen available for plant and algal growth. Nitrogen can encourage the growth of nuisance aquatic plants such as algal blooms. High levels of nitrogen in water can come from either waste water or, more often, runoff from agricultural land.
• Dissolved oxygen

  Dissolved oxygen is important for fish and other aquatic life to breathe. Water should be more than 80 percent saturated with dissolved oxygen for aquatic plants and animals to thrive in it.

  In deep lakes, where the waters don’t mix for several months over the summer, reduced dissolved oxygen in the stagnant bottom waters is of concern, especially for fish and aquatic animals. Decaying algal material that falls out of the surface water uses up dissolved oxygen in the bottom waters.

  In addition, plant nutrients such as nitrogen and phosphorus can be released from the lake bed sediments when the amount of dissolved oxygen in the bottom waters is reduced. When the lake waters mix in winter these nutrients become available for plant life in the surface waters. This can increase their growth causing algal blooms which reduces water clarity.
• E. coli bacteria

  Escherichia coli (E. coli) bacteria are used to indicate the human health risk from harmful microorganisms present in water – for example, from human or animal faeces.

  The summer median number of E. coli bacteria present in water samples should be less than 126 per 100 ml of water if it is to be used for recreation. The median is the middle number after results are ranked.

  The maximum number of E. coli bacteria present in any single water sample should be less than 550 per 100 ml of water.
• Secchi depth

  Secchi depth measures water clarity. Water clarity and underwater visibility is important for recreation such as swimming and water-skiing. It is also important from an aesthetic point of view – most people prefer to see clear water in our lakes.

  Clarity is measured using a Secchi disc attached to a tape measure. The disc is lowered into the water until it disappears; this depth is noted from the tape measure. The disc is lowered a little further and then slowly raised until it reappears, this depth is noted. The average of the two readings is the final Secchi depth visibility depth.

  In clear blue lakes, with low nutrients and low algae, such as Lake Taupo, visibility should be greater than 10 metres. In murky green lakes, with high nutrients and high algal growth, visibility is one to two metres or less.