W e attempted to determine the extent to which benthic diatoms contribute to water column primary productivity in shallow-water estuaries and to elucidate the primary mechanisms responsible for suspending the diatoms. A preliminarj study conducted i n M u g u Lagoon, Calqornia BE WS WE

Studies were conducted in four distinct geographic areas (biomes/sites) in northern United States to examine changes in key ecosystem parameter: benthic organic matter (BOM), transported organic matter (TOM), community production and respiration, leaf pack decomposition, and functional feeding—group composition along gradients of increasing stream size. Four stations ranging from headwaters (1st or 2nd order) to midsized rivers (5th to 7th order) were examined at each site using comparable methods. The results for each parameter are presented and discussed in light of the River Continuum Concept of Vannote et al. (1980). The postulated gradual change in a stream ecosystem's structure and function is supported by this study. However, regional and local deviations occur as a result of variations in the influence of: (1) watershed climate and geology, (2) riparian conditions, (3) tributaries, and (4) location—specific lithology and geomorphology. In particular, the continuum framework must be visualized as a sliding scale which is shifted upstream or downstream depending on macroenvironmental forces (1 and 2) or reset following the application of more localized "micro"—environmental influences (3 and 4). Analysis of interactions between BOM and TOM permitted evaluation of stream retentiveness for organic matter. Headwaters generally were most retentive and downstream reaches the least. Estimates of organic matter turnover times ranged between 0.2 and 14 yr, and commonly were 1—4 yr. Both turnover times and distances were determined primarily by the interaction between current velocity and stream retention. Biological processes played a secondary role. However, the streams varied considerably in their spiraling of organic matter due to differences in the interplay between retentiveness and biological activity. Differences in the relative importance of retention mechanisms along the continuum suggest that headwater stream ecosystems may be functionally more stable, at least to physical disturbances, than are the r intermediate river counterparts.

Four significant areas of thought, (1) the holistic approach, (2) the linkage between streams and their terrestrial setting, (3) material cycling in open systems, and (4) biotic interactions and integration of community ecology principles, have provided a basis for the further development of stream ecosystem theory. The River Continuum Concept (RCC) represents a synthesis of these ideas. Suggestions are made for clarifying, expanding, and refining the RCC to encompass broader spatial and temporal scales. Factors important in this regard include climate and geology, tributaries, location-specific lithology and geomorphology, and long-term changes imposed by man. It appears that most riverine ecosystems can be accommodated within this expanded conceptual framework and that the RCC continues to represent a useful paradigm for understanding and comparing the ecology of streams and rivers.

Natural suspended fine particulate organic matter (FPOM, was labeled with 14C and reinjected to estimate transport distances in the water column and retention times within the sediments of two Idaho streams. Transport of labeled FPOM particles declined exponentially with downstream distance. yielding mean transport distances of 800 and 580 m in Smiley Creek in 1989 and 1990 at a mean water velocity of 0.27 m ss' and mean depth of 0.34 m in both years, and a distance of 630 m in the upper Salmon River at a mean velocity of 0.29 m s ' and a depth of 0.14 m. These travel distances are equivalent to vertical deposition velocities of 0.7-l .6 mm s I, or -7-12% of the temperature-corrected quiescent-water fall velocities of the FPOM particles. The estimated deposition flux of -1.5 g (AFDM) m 1 d ' would turn over the standing stock of 0.8 g m 1 in surficial sediments twice daily.Sampling of benthic FPOM 24 h after release indicated that -99% of the 14C-labeled FPOM initially deposited in the 1,005-m study reach had been resuspended and exported; subsequent clearing was much slower. An advection-dispersion model satisfactorily simulated the deposition of 14C-labeled particles and their subsequent resuspension and export from the reach. Model-estimated retention times were 1.5-3 h for 99% of deposited sediments and 17 d for the remaining 1%.Thus, particles in surficial sediments exchange rapidly with the water column and migrate downstream several kilometers per day in alternating deposition and resuspension events. These results support the overall concept that conditions throughout a river system are strongly connected longitudinally and that OM introduced in headwater reaches can be transported large distances for later use or storage elsewhere in the river or for eventual export to estuaries. AcknowledgmentsThis work was performed under NSF grant BSR 88-178 19. Many people contributed to the successful labor-intensive field studies; they include Douglas Andrews, M. Arunachalam, James Check, Paul Dey, Peter Koetsier, Deron Lawrence, Michael McIntyre, Janet Mihuc, Tim Mihuc, Judy Minshall, Susannah Minshall, Dan Misner, Gregory Mladenka, Bruce Olenick, Christopher Robinson, George Watters, and the students and associates of the Community School in Ketchum, Idaho. Special recognition is due to C. Robinson for implementing field activities, G. Mladenka for developing dye and particle delivery systems, D. Lawrence for determining rhodamine, and M. Arunachalam and P. Dey for characterizing FPOM and determining respiratory loss of radioactive label. Don Edwards performed the labeling of the FPOM and radioanalyses of the transport samples. Benthic radioanalyses were done by IT Analytical Services. Kathy Lahala did the SEM photography. The manuscript was improved by the comments of the reviewers.

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