Feature

Healthy Estuaries

There's more to a happy estuary than mud and mangroves.

Don Morrisey and Mal Green

Despite its popular image overseas as an unspoiled land, New Zealand has clearly changed a lot since humans arrived here.

One of the most obvious, and significant, impacts of humans on the landscape has been the clearance of huge areas of native vegetation for farming, habitation and extraction of timber. This has had a major effect on rivers, lakes, estuaries and the coast because they represent the receiving environment for the sediment that is eroded and transported from these disturbed areas by rainwater runoff.

Most of today's estuaries began to form around 15,000 years ago, as rising sea levels flooded river valleys until, about 6,000 years ago, sea level stabilised at its present height.

The fate of most estuaries since then has been to fill up with sediments. Marine sands typically accumulate near the mouth as sandbanks and delta deposits, under the influence of waves and tides.

Finer sediments, derived from erosion of soils in the river catchments, accumulate in more sheltered regions of the upper estuary.

After an initial, rapid phase of infilling, the process slows down and eventually, the estuary will fill up to the extent that the river flows across a depositional plain and disgorges its sediment load directly into the sea over a submarine delta.

In the final stages of infilling, channels are shoaled, turbidity increases throughout the estuary and sediment collects in backswamps, intertidal flats and marshes, as these features spread down the estuary.

This might suggest that if human activity increases the rate of delivery of sediments to estuaries, it is simply hastening an inevitable process and that, therefore, we should not be too concerned about it.

This is not necessarily so, for the following reason. We can conceive of a "healthy ageing" of an estuary, in which changes in the estuarine ecosystem keep pace with the changes in patterns of sedimentation that accompany the infilling of the estuary.

Conversely, "unhealthy" or "premature" ageing may be brought on by changes in the timing, rate or nature of sediment entering the estuary, such that the ecosystem may not be able to keep pace with changes in the physical environment.

Consequences of this might include degradation of water quality (in turn affecting primary producers), decreased biodiversity, dominance of invasive or otherwise undesirable species, premature disappearance of species and reduction of human amenity values.

Many, if not all, of these considerations involve human perceptions of what is desirable in an estuarine environment. Some of these perceptions relate directly to our own use of estuaries, so that we might, for example, wish to prevent the loss of commercially, recreationally or culturally important species of fish, areas used for recreation, or deterioration of the aesthetic quality of the estuary, such as decreased water clarity.

We do not, however, need to stop at these human interests. Accelerated rates of evolution of estuaries are an inevitable consequence of the growth of human populations and we are left with no choice but to manage this evolution since to do nothing is itself, in effect, a management decision.

With improved understanding of how estuarine ecosystems function, we can start to make decisions aimed at restoring more natural rates of change.

Two examples drawn from recent work by NIWA (the National Insitute of Water and Atmospheric Research) illustrate this point.

The first concerns predicting the effects of catchment development on sediment runoff to an estuary and estimating the associated risk of impacts on populations of animals inhabiting the intertidal flats.

The second concerns longer-term changes caused by enhanced sedimentation, in the form of the spread of mangroves within estuaries and the consequent loss of other habitats.

Predicting Risks from Development

Catchment development involves removal of vegetation cover and exposure of underlying soils. During the period of exposure there is a risk that rainfall will erode soils and wash them into waterways and, eventually, the receiving estuary. The amount of soil eroded and the time it takes to reach the estuary depend on the severity of the rainfall.

Once it reaches the estuary, the sediment may be dispersed, thus increasing turbidity throughout the estuary and affecting primary producers and filter-feeding animals such as many shellfish and worms.

Alternatively, it may be deposited rapidly, forming a "slug" that smothers the bed of the estuary and the animals and plants that live on and in it. Observation of the effects of natural slugs, and experimental studies, have shown that such rapid deposition can wipe out beds of shellfish, seagrass beds and other estuarine communities and cause changes lasting months or even years.

NIWA scientists have combined models of the erosion and transport of soils from the catchment of Okura Estuary, north of Auckland, during rainfall events and their dispersion and deposition in the estuary with experimental studies of the effects of slugs on animal communities in vulnerable parts of the estuary.

This has permitted an assessment of the relative risks of catastrophic sediment deposition during catchment development associated with various development scenarios.

This, in turn, has allowed the Auckland Regional Council, who commissioned the study, to assess the acceptability of the risks involved and to decide among scenarios.

Estuaries, Mangroves and People

Worldwide, mangroves have generally suffered from human activities, such as clearance for development and mortality from excessive sedimentation, freshwater runoff or grazing by livestock.

Gradually, however, their value as nursery areas for many species of fish and invertebrates, their role as primary producers and their contribution to coastal protection have become recognised. Mangroves are now protected habitats in many parts of the world.

Ironically, in some areas, including parts of New Zealand, enhanced rates of infilling of estuaries have led to the spread of mangroves. As intertidal flats grow upward and downshore, they provide suitable habitats that mangroves can colonise and the trees, in turn, provide a sheltered environment in which sedimentation is further enhanced.

The area of an estuary occupied by mangroves can change significantly over a period of decades. For example, in Whangapoua Harbour, on the Coromandel Peninsula, mangroves occupied about 12% of the area of the harbour in 1945. By 1993, they had expanded to cover roughly 25%.

Despite their undoubted ecological value, rapid expansion of mangroves presents a management problem because, like infilling itself, while the process may be natural and inevitable, the rate at which it is now occurring is often not.

One important consequence is that, valuable though they are, expansion of mangroves generally occurs at the expense of equally important habitats, such as sandflats and seagrass beds containing rich and diverse communities of animals and providing feeding areas for fish, birds and humans.

Human use of estuaries for activities such as sailing and fishing, is also restricted by the spread of mangroves. In response, community groups have formed in a number of northern New Zealand towns with the aim of managing mangroves and protecting local coastal amenity value.

In view of these conflicts between the march of the mangroves and the needs of other users of estuarine resources, including humans, there is a perceived need to manage the spread of mangroves to maintain a "desirable" balance of habitats.

The problem is that in New Zealand we currently have very limited information about mangrove ecology on which to base appropriate decisions.

Current work by NIWA, funded by the Public Good Science Fund and conducted in collaboration with Landcare Research, the University of Queensland and the Auckland Regional Council, is identifying areas of mangrove spread and comparing ecological values among areas of different ages.

Historical sequences of aerial photographs allow determination of the age of particular areas of mangroves. The study is identifying how the forest structure, ecological processes (such as the production and transfer of organic matter) and biological diversity change as these estuarine intertidal areas are colonised by mangroves and the forest grows and matures.

The ultimate aim of this work is to provide information that will allow environmental managers to make decisions about whether and how to control changes in mangrove distribution.

In the past, such decisions have swung from regarding mangroves as areas of wasteland suitable for clearance, "reclamation" and grazing of stock to regarding them as areas of high (but usually unquantified) intrinsic value to be unquestioningly preserved.

Much of this decision-making has been based on human perceptions of the value of mangrove habitats. The present study will provide a more rounded view of mangroves in relation to alternative types of habitat and other uses of estuarine resources.

For more information on NIWA's PGSF research programme on the effects of sediments on estuarine ecosystems, visit the programme's Web page at:
http://www.niwa.cri.nz/pgsf/esee/index.html

Malcolm Green is with NIWA in Hamilton.
Don Morrisey works at the National Institute of Water and Atmospheric Research Ltd in Hamilton.