Figure 1 illustrates a way of categorizing research studies by both research field and ecosystem type (the atmosphere is added as a source of inputs). A reading of the technical literature indicates that most ecosystem studies proceed horizontally in Figure 1. One important exception is the study of anadromous fish that migrate between the ocean and fresh waters. Another is the study of nutrient cycling, in which atmospheric inputs are followed through the gravitational phase of the hydrological cycle (i.e., vertically in Figure 1) to catchments, streams, lakes, and wetlands (e.g., Bormann and Likens, 1979; Urban and Eisenreich, 1988). Nutrient inputs from rivers to estuaries and oceans have been followed similarly. Given the great success and significance of such nutrient-cycling studies, which are very widely cited in the literature of ecosystem ecology and biogeochemistry, a strong argument can be made for broadening greatly such vertically oriented research on the linkages among aquatic ecosystems to include other aspects of ecosystem function, diverse examples of which are given in the following pages. There are, however, impediments to the pursuit of such studies.

It is difficult for any one person to acquire knowledge of more than one type of ecosystem. Few have expertise in both streams and lakes, or