Dynamic Processes in New Zealand Land-Water Ecotones

This paper reviews current knowledge of dynamic processes in New Zealand land-water ecotones drawing on published quantitative data wherever possible. Basic ecosystem processes in forested and natural unforested land-water ecotones are compared, and dynamic processes are discussed under the following headings: time scales of change; water movement; sediment trapping and transport; dissolved nutrient dynamics; dissolved oxygen; trophic interactions. Environmental "resetting" agents such as floods, fires and storms have been shown to be important regulators of change at the land-water interface. However, an element of stability is imparted by continuous allochthonous inputs from evergreen vegetation into the water at the interface, as well as an important contribution from terrestrial insects. Stable isotope studies have shown that such inputs are translated as a carbon source through aquatic food chains in some New Zealand streams. Dynamics of sediment movement and nutrients are governed by the complex patterns of water movement along the ecotone. Patterns of water movement at this interface are controlled by obstructions to flow providing eddies and "dead zones". Manning's coefficient of drag is a convenient measure of the degree of obstruction by aquatic vegetation, fallen branches etc. Nutrient dynamics along the land-water interface of lakes and streams are affected not only in moving surface water, but also in groundwater. New Zealand studies have, in recent years, concentrated on nitrogen pathways in ground waters at the interface where denitrification is shown to be an important N sink. This is controlled largely by dissolved oxygen and prevailing redox conditions. Implications for management of New Zealand land-water ecotones where ecosystem dynamics are governed by periodic physical disruptions such as floods, fires or dry-wet cycles are discussed.