KAWAI NUI MARSH

Soil, Sediment, and Water Interactions
To call Kawai Nui a marsh is to imply that soils are saturated with water all or much of the time, because a marsh typically consists of wet soils supporting growths of specially adapted herbaceous plants (in swamps, the dominant plants are woody trees). There may be open bodies of water present when the site is seasonally flooded, or these ponds may represent depressions too deep to be colonized by emergentA vegetation. The sequencing and patterning of open water versus vegetation cover is an important aspect of the ecology of a marsh for the reason that many of the animals that inhabit wetlands are dependent upon one or the other environment (e.g., most adult fishes usually require open water), or even utilize especially the boundary between open water and vegetation (e.g., waterfowl use open water to travel and stay safe from predators, while feeding and nesting in vegetation). In many places, larger bodies of open water would be regarded as distinct ponds or lakes, with vegetated, shallow water margins comprising the actual marshland.

In Kawai NuiB today, vast areas are covered by a variable depth of water, but bodies of open water comprise only a small portion of the total acreage. Most of the extant "lake" is hidden beneath floating vegetation. In some areas, the vegetation is of species that live and grow floating on the surface (e.g., water hyacinth; see Photo 1), but over most of Kawai Nui, the vegetation consists of plants rooted in organic soil, forming a mat of living and dead plant matter. This mat floats on water or on a sediment slurry.


Photo 1
In the middle of Kawai Nui is a lake of several acres extent, nearly covered by the floating plant known as water hyacinth (Eichhornia crassipes; here in flower).

The gross layers of the Kawai Nui basin are described under geology of the marsh. Links there lead to details gleaned by diligent studies of Joe Moye and his students at Hawaii Pacific University from two of the three layer sequences present: the upper peat, lagoon mud, and lower marine sediment layers. In this section, we discuss the physico-chemical interactions between the water of the marsh and essentially the soil layer. The soil consists of an upper, root-bound organic layer (an incipient peat) and underlying material that is peat in various stages of decay, merging into sedimentary or alluvial deposits. In some areas, the root-bound surface organic layer merges directly into saturated clay. Because of the high organic content of the peat and root-bound layers (soil O horizon), conditions develop that result in this layer physically separating from underlying mineral dominated material, and the intervening gap becoming filled with water or mud slurry that buoys up the organic layer. Where the mat separates and floats on water, where it floats on mud slurry, and where its layers do not appear to separate may be related to successional factors in the gradual transformation of this former lagoon to land. But there also may be seasonal and longer term influences related to groundwater inputs to the system.

We are very interested in the possible relationships between what might be interpreted as marsh successional stages and observable differences in the plant-soil-water interface. If relationships exists, understanding these would have implications on marsh restoration efforts.C However, mapping and plotting soil and vegetation changes over time for the entire 800+ acre marsh would be a difficult undertaking. In order to initially gain a basic appreciation of the phenomenon described, two representative "transect" trails were constructed, beginning in early 2002, out from the Kawai Nui shoreline at Na Pohaku o Hauwahine. Along each trail, a series of mat penetrating devices (Photo 2) was installed, separated at variable intervals averaging roughly 50 m. The selected


Photo 2
One of the PVC pipe devices constructed by Robert Moye to provide access through the vegetation mat at Kawai Nui Marsh for water level and water quality sampling. Each pipe is 4 inches in diameter and approximately 1 m (3 ft) long. Installed, the arms lay on the vegetation mat surface

transects were chosen to represent two kinds of areas thought to characterize most of this marsh: Transect "A" across an inland wet meadow and Transect "B" over a vegetation covered "lake." Installed, the devices were called wells, which they basically resemble.

Discussion continues:

Page 2 - Data Presentations
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FOOTNOTES:
A Emergent aquatic plants are those that grow above the water surface.
B In Hawaiian, ka wai nui means "the big water" alluding to a large expanse of open water, which this area presumably was when first discovered by the Polynesians.
C The U.S. Army Corps is developing "new" wetlands by diking and permanently flooding portions of the wet meadow at the inland end of Kawai Nui. Efforts at Na Pohaku at habitat restoration involve physically removing the peat/vegetation layer, exposing the water of the marsh.