Nutrient enrichment in mangrove ecosystems: a growing concern

Pip Nicholls
Anne-Maree Schwarz
Nicole Hancock

Fertilisation experiments are showing that some mangroves grow well with a little nitrogen.

Love them or loathe them, mangroves are an integral part of New Zealand’s estuarine ecosystems. Like all large plants, they significantly influence many key processes, such as nutrient cycling. They also play a vital role in coastal sediment stabilisation. In the tropics, mangroves are highly valued as habitats for many fish and invertebrate species; however, many of these mangrove ecosystems are under threat. Accordingly, overseas, millions of dollars are spent every year on mangrove conservation and restoration.

In New Zealand, as in other parts of the world, pressure from increasing human population puts the health of coastal ecosystems under threat. Impacts are generally associated with land development, agriculture, and horticulture. For example, increased nutrient inputs to fresh waters ultimately end up at the coast. While we have an understanding that there are correlations between mangrove growth and sediment mud content in some New Zealand estuaries, little is known about how eutrophication (increased nutrient concentrations) of the coastal zone might also affect mangrove ecosystems.

Overseas studies have shown clearly that the nutrients nitrogen and phosphorus affect mangrove growth. When nutrient concentrations increase, mangroves may grow faster. Increased nutrients can also potentially influence certain life stages, for example propagule (seed) production.

New Zealand is one of the few places in the world where the area covered by mangroves is expanding seaward. While increasing nutrients may be involved in this spread, we don’t know exactly how. This is one of the questions we hope to answer in a wide-scale experiment involving NIWA, the Smithsonian Environmental Research Center (USA), and the University of Queensland (Australia).

In this experiment we are investigating how changing nutrient levels affect physiology and growth rates of mangroves. The overall aim is to compare mangroves in tropical ecosystems (Belize and USA) and temperate ecosystems (Australia and New Zealand) and use the results to predict responses to environmental change.

Experimental work: New Zealand

In June 2001 NIWA set up the first field site in Waikopua Creek (Whitford, Auckland) where the substrate is fine sand/mud. In January 2003, a second site was established in the Whangapoua Estuary (Coromandel) where the substrate is relatively sandy.

A general pattern in mangrove forests is that taller, more robust trees tend to grow along the edges of channels, while farther back from the channel the trees are much smaller. Previous studies in other tropical/temperate areas have shown that the channel-edge trees and dwarf trees react differently to nitrogen and phosphorus.

To investigate this we fertilised 18 mangrove trees alongside a major drainage channel and 18 smaller trees 25 m away. We added fertiliser near the base of the trunk of each tree, using individual slow-release bags containing either nitrogen or phosphorus or no fertiliser (control). To measure the mangrove trees’ responses to fertiliser, we tagged and measured branches and measured tree height and diameter. Every six months fertiliser is re-applied and all the tagged branches are re-measured.

A response to nitrogen

In the first six months after the initial nutrient addition in Waikopua Creek there was no obvious difference in growth between treatments for any of the mangroves. However, over a longer period, the dwarf mangroves that were farther from the channel have started to show a marked response to additional nitrogen. Those trees are showing, on average, more branching and have longer branches than any of the other treatments, whether dwarf or fringe.

After three years we are still measuring more branch growth in the dwarf trees fertilised with nitrogen. These trees are distinguishable from the others, appearing very healthy because of their shiny robust leaves.

All the fringing mangrove trees on the channel edge showed steady but slow growth in comparison to the dwarf trees with nitrogen added. The patterns in growth were independent of treatment and do not indicate any effect on growth of the additional nitrogen or phosphorus.

The experiment at the sandier site in Whangapoua Estuary has been running for a shorter time. However, the pattern emerging is similar to that observed in Waikopua Creek.

What does this mean?

In Waikopua Creek, mangroves that are away from the channel, which tend to be smaller than those on the channel edge (fringe), are showing some evidence of being short of nitrogen under natural conditions because supplying extra nitrogen stimulated their growth.

Although these results are preliminary, there is evidence to suggest that some New Zealand mangrove forests, in areas prone to increased nitrogen inputs, may respond by growing faster and by increasing their potential to produce more reproductive propagules. If other physical conditions are suitable (that is, sediment, temperature, and salinity), this could have the potential to enhance rates of forest spread compared with areas with low nutrient inputs.

Altering one or more of the environmental conditions necessary for healthy mangroves will have positive or negative effects on mangrove growth and spread.

Long-term monitoring

We intend to continue fertilising and re-measuring mangrove trees in both experimental plots for the next two years. This will contribute to a clearer picture of the longer-term potential effects of eutrophication on New Zealand mangroves. Our results will be combined with those collected from the tropics (Belize, Florida, Panama) and other temperate areas (Australia) to help develop a global perspective on mangrove–nutrient relationships.

Pip Nicholls was formerly based at NIWA in Hamilton, where Anne-Maree Schwarz and Nicole Hancock are currently based.