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Water Quality

...The big picture...

While there is great public interest in water quality because of its real and perceived influences on the kinds of life present in a body of water, the usefulness of that body of water for human consumption and recreational contact, and the impact on downstream environments, especially nearshore waters in our island state, a marsh or wetland presents a difficult case with regard to all of these perceptions. The water quality in a marsh tends towards extremes. For the reason that wetland water quality norms are difficult to define, numerical water quality standards covering wetlands have yet to be established in state or federal regulations [However, see EPA's National Guidance Water Quality Standards for Wetlands (1990); for State of Hawaii, see Department of Health water quality regulations (August 2004) as a .pdf file].

A marsh is an aquatic environment in which water, sediment, and living matter are interacting in natural, but uniquely intimate ways in comparison with other aquatic environments such as streams, lakes, bays, and coastal waters. The normal circumstance is one of sluggish flow, with shallow water in contact with both sediment and a substantial biomass of microbes, vegetation, and animals. Water quality measurements usually reveal only part of the picture. In other aquatic environments, the biochemistry of the water column may be most of what is happening in the ecosystem—in a marsh, the surface water chemistry is at best a mere indication of what is going on in the mud and the living and dead plant matter.

...Dissolved oxygen...

Dissolved oxygen or DO—or more precisely a lack of dissolved oxygen—is another significant characteristic of a marsh. At first, this would seem not even possible. The water is generally shallow and full of green plants, the source of oxygen on a planet with an atmosphere that is 20% oxygen. But in a marsh, the sediment and plant biomass "overwhelm" the water realm: the sediment is high in organic matter from dead plants. Microbes feed on this

organic matter, using up available DO. Oxygen naturally diffuses only slowly into a water body from the atmosphere, so the shallowness is not much help if the water is slow moving, sluggish, or stagnant. Marsh plants are adapted to life rooted in anoxic sediment with their chlorophyll-bearing tissues up in the air and the light. Only a small portion of oxygen produced by marsh plants goes into the water and sediment—nearly all is released to the atmosphere, just as with upland plants. The dense vegetation, and especially where there is a covering layer of floating plants or peat as in Kawai Nui, these shade the water, preventing growth of algae and submerged plants that could contribute oxygen directly into the water.

With typically low or no oxygen (= anoxia), the chemistry of marsh water and especially marsh sediment is different than that in a lake or an upland soil. We know this from personal experience by the unique smells we associate with marsh lands. These smells are not methane (an odorless marsh gas produced in abundance in the organic-rich wet peats and muds), but reduced sulphur (that is, sulfides, which have a rotten-egg smell) and sulphur-bearing organic molecules. The oxygen content in the air, soil, and moving waters (even most pond and lake waters) is sufficient to oxidize sulfides to sulphates, which our noses barely detect.

The general characteristics of marsh chemistry described above give emphasis to the fact that water quality of Kawai Nui must be viewed in a context unique from that typically applied to other state waters. The form that layering of sediment, water, and plants takes in the marsh, and spatial differences in this layering, play a role in the properties we observe as water quality.


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This webpage edited and reposted Mar. 30, 2011. Copyright E. Guinther.