Rivers and streams recover really quickly when the inputs of nutrients are stopped as nutrients will be flushed downstream. Nutrients will, however, cause problems in coastal areas (such as dead zones). There are also lag problems for nitrogen to enter freshwater. This is the time it takes once nitrogen is added on to the land and then make its way down through the soil and into aquifers, rivers, and streams. In some catchments this time period may be more than 100 years, so it may be a long time before we see the impacts of what we are doing now. So even if we stop direct inputs of nutrients on to the land and to rivers and streams, problems may still occur for many years to come.
The main pathway for nitrogen to enter freshwater is by leaching through the soil to groundwater – this process can take a long time in some soil types.
It is harder to remove nutrients from lakes. They cannot be flushed out like in rivers and will continue to cause problems even if inputs are stopped.
Firstly, for both rivers and lakes, the most important thing we need to do is stop the input of additional nutrients. There is no use trying to remove the problem if you keep adding it in.
Wetlands can take up existing nitrogen in lakes. Riparian planting can also do this to an extent. However, these methods will not remove nitrogen that is deposited on soil. Additionally, 90 per cent of New Zealand’s wetlands have been destroyed so new ones need to be created – and this costs a lot of money.
Other techniques can be used to remove nutrients in lakes, but are expensive and may have limited potential. For more details on techniques to remove nutrients from lakes, using the Rotorua lakes as an example, see this paper .
 Abell, J.M.; Hamilton, D.P. & Paterson, J. (2011). Reducing the external environmental costs of pastoral farming in New Zealand: experiences from the Te Arawa lakes, Rotorua. Australasian Journal of Environmental Management, 18, 3, 139-154.