FROM DETRITUS TO SEA FOAM: THE FORMATION, REACTIVITY AND DISPOSITION OF NATURAL ORGANIC MATTER IN FRESH WATERS

Aquatic natural organic matter (NOM) in most freshwaters is primarily allochtonous and originates from the decomposition of plant litter within the catchment or recharge basin. Between detritus and aquatic ("dissolved") NOM, a series of poorly understood biogeochemical processes occurs, producing a complex suite of substances called NOM (humic substances are a subset). The properties of aquatic NOM depend critically upon the nature of the catchment flora and the geochemical/hydrological features of the catchment or recharge zone. We have investigated chemical characteristics of NOM in a large variety of natural water samples, aqueous plant-litter leachates and foam produced as an NOM-rich river enters the sea, in order to test hypotheses about the interrelationships among NOM samples. Consequently, we have an extensive library of NOM properties and reactivities. Inferences drawn from these data lead to an increasing understanding of the role of NOM in aquatic processes, including those involving anthropogenic chemicals. After a brief survey of parameters measured and the ranges noted, four aquatic systems will be discussed in more detail with respect to the nature of their associated NOM. FTIR and other evidence reveal similarities and differences between NOM from New Zealand's Inangahua River and leachate from red beech (nothofagus fusca), the principle tree in the catchment. Additional evidence illustrates fractionation processes as the river enters the Tasman Sea and deposits its NOM in part as foam on the adjacent shoreline. A nearby river will also be highlighted because of its very different catchment characteristics and NOM properties. Next, a groundwater NOM from a high altitude wetland in Colorado, USA will be discussed as an example of metal-rich systems. Finally, the Suwannee River, (USA), source of internationally recognized standard humic substances, will be discussed in terms of whole NOM characteristics as compared, in part, to standard fractions such as humic and fulvic acid.