The structure and function of freshwater ecosystems
are tightly linked to the watershed, or catchment, of
which they are a part (Hynes 1970, Likens 1984). As
water flows on its way to the sea, it moves through
freshwater systems in three spatial dimensions: longitudinal
(upstream–downstream), lateral (channel–
floodplain, or wetland–lake margin), and vertical (surface
water–groundwater). These dimensions represent
functional linkages among ecosystem compartments
over time (Ward 1989). Bodies of freshwater are ultimately
the recipients of materials generated from the
landscape, hence they are greatly influenced by terrestrial
processes, including human modifications of land
(Moyle and Leidy 1992).