Lakes and estuaries can be described by their nutrient status.
The trophic status of a lake or estuary refers to the primary productivity (amount of algae) produced in the water and the amount of nutrients (P and N) in the water. Oligotrophic waters usually have low primary productivity (high water quality and few algae) and are nutrient poor, while eutrophic waters have high primary productivity (low water quality and frequent algal blooms) due to excessive nutrients. Mesotrophic waters lie somewhere in between the two states. Eutrophication is an increase in the nutrients available in a waterbody which can subsequently increase primary productivity and degrade water quality, leading to a reduction in mahinga kai habitat and survival.
Potential impacts of eutrophication on water quality and mahinga kai
- Excessive plant and algae growth and decay - especially invasive weed species.
- Decreased dissolved oxygen (DO) levels - fish ‘breathe’ oxygen through their gills, therefore a decrease in available oxygen (anoxia) in the water column threatens their ability to respire, which may lead to death.
- Increased turbidity and decreased water clarity - water becomes cloudy and coloured green and brown, which reduces the ability of fish to see, prey, and detect predators.
- Seasonal release of nutrients stored in the lake bed sediment - contributes to the cycle of eutrophication.