Why freshwater management needs to include estuaries

By Waiology 11/11/2013


By Malcolm Green

Un-muddying the Waters : Waiology : Oct-Dec 2013There is a widespread view that cumulative effects – those effects that build up over time or that occur in combination with other effects – are the Achilles heel of the Resource Management Act.

Back in 2010, the Land and Water Forum, echoing previous legal commentary, argued that limits are the solution: “without limits it is hard to manage diffuse discharges… and impossible to deal with the cumulative effects on water bodies of water takes on the one hand and diffuse and direct discharges to water on the other”. In 2011, the National Policy Statement for Freshwater Management was gazetted, which requires that objectives be set for freshwater bodies that reflect a range of aspirations and values, and that limits for water quantity and quality be set to ensure that objectives are achieved. Most recently, the Government’s Freshwater Reform 2013 and Beyond (FRB2013) lays out the path for further reform, including managing within limits for water quality and quantity.

Estuaries are conspicuous by their absence from FRB2013, where “freshwater” is defined as “naturally occurring water on the Earth’s surface in bogs, wetlands, ponds, lakes, rivers and streams, and underground as groundwater in aquifers” and “freshwater body” is defined as “fresh water in a river, lake, stream, pond, wetland, or aquifer, or any part thereof, that is not located within the coastal marine area”. However, estuaries – and the coastal marine area for that matter – are the ultimate receiving waters for contaminants from the land that are conveyed by freshwater runoff, which raises an unpleasant spectre.

Hellyers tidal creek in the Upper Waitemata Harbour.
Hellyers tidal creek in the Upper Waitemata Harbour.

By not considering estuaries when limits for freshwater water quality are being determined, are we in danger of finding, years from now, that those limits are failing to deliver healthy estuaries?

Consider this. Setting contaminant load limits for first-order streams in catchment headwaters might seem to be an innocuous exercise: you set local limits to achieve local objectives – problem solved. However, the second-order stream further down the catchment collects and funnels the contaminants from all of its first-order tributaries, and if its assimilative capacity is exceeded by the sum of all of its tributary load limits thenwe have a problem.

Clearly, load limits to achieve objectives in first-order streams must also be cognisant of the objectives in the second-order stream that they feed into. Now, extrapolate this argument all the way down to the estuary, and you can see that the estuary is at the mercy of the entire of web of load limits being set for freshwater. So why aren’t estuaries included in the freshwater limit-setting process?

One answer might be that it is too hard to think about. However, there is a concept that could be helpful here. For any given catchment, there will be a “critical point”, which is the place (a particular stream reach, a lake, or the estuary) that has environmental objectives that require a stricter contaminant load limit to achieve than any other place in the catchment. If we can identify that critical point, and manage the catchment for the associated critical-point load limit, then we will not fail any objectives in the catchment.

It stands to reason that managing for the critical-point load limit will also lead to over-achievement of objectives at places in the catchment other than the critical point, which means that people will be paying for outcomes that are not necessarily local to them. It might take some public education to explain that.

If we manage for greater than the critical-point load limit, then we stand to fail to achieve at least the critical-point objective, and where the critical point is the estuary, the spectre raised previously will come true.

Are estuaries critical points? Possibly, for some estuaries and some contaminants, although I don’t really know. Here’s the thing, though: unless we include the whole catchment, including estuaries, when assessing limits, we will never find out, at least until it is too late. We need to be looking both up the catchment and down the catchment – right down to the estuary – to set limits.


Dr Malcolm Green is Principal Scientist, Coastal and Estuarine Physical Processes, NIWA.