Michael D. Delong

The riverine productivity model: an heuristic view of carbon sources and organic processing in large river ecosystems

Our current views of the structure and function of large river ecosystems are based primarily on three influential and still valuable riverine models: (1) the river continuum concept, or RCC (e.g., Vannote et al. 1980); (2) the serial discontinuity concept (Ward and Stanford 1983), which integrated the effects of large dams and reservoirs on the RCC; and (3) the flood pulse concept in river-floodplain systems (Junk et al. 1989) and its relationship to the RCC (Sedell et al. 1989). With regard to food webs in large rivers, these models accentuated the importance of nutrients derived from either headwater streams or seasonal floodplain pulses and downplayed or virtually ignored the role of local instream primary production and riparian litterfall. We believe that the general portrayal of ecosystem function within large rivers needs substantial revision because previous models relied too heavily on data from either lower-order streams, floodplain rivers (thereby exluding large rivers with constricted channels), or main channel habitats with their dominant collector feeding guild (thus de-emphasizing nearshore areas where species in many feeding guilds congregate). We propose an alternative hypothesis the riverine productivity model (RPM) which stresses the varying importance to large river food webs of local autochthonous production and direct organic inputs from the riparian zone. The RPMs portrayal of ecosystem function differs most significantly from that of previous models for rivers characterized by constricted channels, such as the upper two-thirds of the Ohio River.

Isotopic analysis of three food web theories in constricted and floodplain regions of a large river

Abstract Analyses of stable isotope (δ13C and δ15N) and C:N ratios of food webs within a floodplain and a constricted-channel region of the Ohio River during October 1993 and July 1994 indicate that the increasingly influential flood pulse concept (FPC) does not, for either location, adequately address food web structure for this very large river. Furthermore, results of this study suggest that the riverine productivity model (RPM) is more appropriate than the widely known river continuum concept (RCC) for the constricted region of this river. These␣conclusions are based on stable isotope analyses of potential sources of organic matter (riparian C3 trees, riparian C4 grasses and agricultural crops, submerged macrophytes, benthic filamentous algae, benthic particulate organic matter, and transported organic matter containing detritus and phytoplankton) and various functional feeding groups of invertebrate and fish consumers. The FPC, which stresses the key contribution of organic matter, particularly terrestrial organic matter, originating from the floodplain to riverine food webs, was judged inappropriate for the floodplain region of the Ohio River for hydrodynamic and biotic reasons. The rising limb and peak period of discharge typically occur in November through March when temperatures are low (generally much less than 10°C) and greater than bank-full conditions are relatively unpredictable and short-lived. The major food potentially available to riverine organisms migrating into the floodplain would be decaying vegetation because autotrophic production is temperature and light limited and terrestrial insect production is minimal at that time. It is clear from our data that terrestrial C4 plants contribute little, if anything, to the consumer food web (based on δ13C values), and δ15N values for C3 plants, coarse benthic organic matter, and fine benthic organic matter were too depleted (∼7–12‰ lower than most invertebrate consumer values) for this organic matter to be supporting the food web. The RPM, which emphasizes the primary role of autotrophic production in large rivers, is the most viable of the remaining two ecosystem models for the constricted-channel region of the Ohio based on stable isotope linkage between sources and consumers of organic matter in the food web. The most important form of food web organic matter is apparently transported (suspended) fine (FTOM) and ultra-fine particulate organic matter. We propose that phytoplankton and detritus of an autochthonous origin in the seston would represent a more usable energy source for benthic (bivalve molluscs, hydropsychid caddisflies) and planktonic (microcrustaceans) suspension feeders than the more refractory allochthonous materials derived from upstream processing of terrestrial organic matter. Benthic grazers depend heavily on nonfilamentous benthic algae (based on gut analysis from a separate study), but filamentous benthic algae have no apparent connection to invertebrate consumers (based on δ13C values). Amphipod and crayfish show a strong relationship to aquatic macrophytes (possibly through detrital organic matter rather than living plant tissue). These observations contrast with the prediction of the RCC that food webs in large rivers are based principally on refractory FTOM and dissolved organic matter from upstream inefficiencies in organic-matter processing and the bacteria growing upon these suspended or dissolved detrital compounds. The conclusions drawn here for the Ohio River cannot yet be extended to other floodplain and constricted-channel rivers in temperate and tropical latitudes until more comparable data are available on relatively pristine and moderately regulated rivers.

Linking Ecosystem Services, Rehabilitation, and River Hydrogeomorphology

Assignment of values for natural ecological benefits and anthropocentric ecosystem services in riverine landscapes has been problematic because a firm scientific basis linking these to the rivers physical structure has been absent. We highlight some inherent problems in this process and suggest possible solutions on the basis of the hydrogeomorphic classification of rivers. We suggest this link can be useful in fair asset trading (mitigation and offsets), selection of sites for rehabilitation, cost-benefit decisions on incremental steps in restoring ecological functions, and general protection of rivers.

8 – Upper Mississippi River Basin

The Upper Mississippi River basin drains Lake Itasca in the bog and spruce swamps of northern Minnesota and flows south to join the Ohio River as a 10 th order alluvial river to form the largest river in North America. The progression of the river from the lake outlet to the great river creates an impressive range of physical, chemical, and biological diversity throughout the basin. The basin encompasses five terrestrial ecoregions, three biomes, and three physiographic provinces. Despite the variability created by climate and geology, commonalities are evident among rivers at the physiographic province and terrestrial ecoregion levels. Climatic conditions change considerably from the northern extreme of the basin to its southern boundary at the confluence with the Ohio River. This chapter presents the rivers acting as a representative within each region and that reflect both the common threads among rivers in the Upper Mississippi River basin and their unique attributes. The Upper Mississippi River is the only bioregionally outstanding river. This is partly due to the fact that there is only one species of fish, one species of crayfish, and one freshwater mussel, which are endemic to the region.

ONTOGENETIC AND TEMPORAL SHIFTS IN THE DIET OF THE AMPHIPOD GAMMARUS FASCIATUS, IN THE OHIO RIVER

-A field study was conducted to determine if ontogenetic or temporal shifts occur in the diet of the amphipod Gammarus fasciatus Say in the Ohio River. Amphipods were collected monthly from cobble and snag habitats in the Ohio River for I yr. Gut contents of collected amphipods were analyzed microscopically for the presence of detritus, filamentous algae, diatoms and animal matter. Gammarusfasciatus consumed each food type in different amounts, depending on amphipod size and month collected. Detritus was the most common food item found in amphipod guts (100% of microscope fields in guts examined), followed in order by filamentous algae, diatoms and animal matter (the ranges of each food type in the guts of G. fasciatus were: 0.036-0.287, 0.061-0.281, 0.002-0.072, respectively). Food use shifted ontogenetically: small G. fasciatus were limited to a diet consisting mainly of detritus, whereas larger animals consumed significant amounts of filamentous algae, diatoms and animal matter. There were also monthly differences in foraging, presumably due to differences in seasonal abundances of food types and other environmental factors. We suggest that the abundance of, and ability to use filamentous algae, diatoms and animal matter allow populations of G. fasciatus to persist and maintain significant year-round populations in the Ohio River.

Ecogeomorphology of Altered Riverine Landscapes: Implications for Biocomplexity Tenets

Existing theories are used to predict responses to anthropogenic changes in riverine ecosystems, but explicit hypotheses on ecosystem behavior in altered landscapes have not been incorporated into these models. This chapter describes the types of responses expected in altered riverine ecosystems as defined by the tenets of the RES. Disturbances changing attributes in one domain will alter attributes of others through secondary or tertiary mechanisms. This chapter addresses broad changes in the nature of riverine ecosystems through direct actions on attributes and provides examples of deviations from expected patterns. The tenets of the riverine ecosystem synthesis (RES) are presented by considering how they can be used to predict changes/responses from population to landscape levels. The tenets provide a framework for applying the RES in situations encountered in altered systems and for understanding how changes in the characteristics of an FPZ influence ecosystem structure and function independent of location along a longitudinal gradient of a riverine ecosystem. A goal of this chapter is to provide key policy makers and river managers with better tools for environmental management and rehabilitation of riverine landscapes.

Hierarchical Patch Dynamics in Riverine Landscapes

Understanding the nature of changes in biocomplexity from headwaters to a river mouth is an important path toward developing a conceptually cohesive model of riverine ecosystem structure and function. This chapter discusses another major component of model—aquatic applications of the Hierarchical patch dynamics (HPD) model.. The original, terrestrial-based HPD model integrates a general theory of spatial heterogeneity (patch dynamics) with hierarchy theory by expressing relationships among pattern, process, and scale in a landscape context. This chapter describes the meaning of patch. A patch is as a spatial unit differing from its reference background in nature and appearance, a depiction that could also be applied to temporal patches. The size of a patch is scale-, organismal-, and process-dependent and can vary greatly in temporal dimension and size (e.g., an individual rock to a river segment or a floodscape area). The HPD model is composed of five principal elements. First, ecological systems are viewed as ‘nested, discontinuous hierarchies of patch mosaics.’ Second, the dynamics of ecological systems are derived from a composite of intra- and inter-patch dynamics. Third, pattern and process are interlinked and scale-dependent. Various processes (e.g., nutrient spiraling) may create, modify, or eliminate patterns at certain spatial and temporal scales, while at the same time certain spatial and temporal patterns (e.g., differences in flow characteristics) can substantially alter ecological processes. Fourth, nonequilibrial conditions and stochastic processes play a dominant role in the so-called “ecosystem stability.” Fifth, a quasi-equilibrial, metastable state can develop at one hierarchical level through incorporation of multiple; nonequilibrial patches from the adjacent lower level.

Introduction to the Riverine Ecosystem Synthesis

This chapter provides an overview of the Riverine Ecosystem Synthesis (RES), which is an integrated model derived from aspects of other aquatic and terrestrial models proposed from 1980 to 2007, combined with the perspectives on functional process zones (FPZs) and other aspects of riverine biocomplexity. The RES pertains to the entire riverine landscape, which includes both the floodscape and the riverscape. This contrasts with many lotic models whose primary emphasis or support focuses on main channel systems within headwaters. This synthesis, which incorporates the ecosystem consequences of spatiotemporal variability across mostly longitudinal and lateral dimensions, has three broad components: a fundamental, physical model describing the hierarchical patchy arrangement of riverine landscapes within longitudinal and lateral dimensions based primarily on hydrogeomorphology and emphasizing a new geomorphic division (an FPZ) between the reach and the valley scale; ecological implications of the physical model in terms of an expandable set of 17 general to specific (testable) hypotheses, or model tenets, on biocomplexity, which is applicable in some form to both pristine and altered riverine landscapes; a framework for studying, managing, and rehabilitating riverine landscapes through the use of the hierarchical physical model and aquatic applications of the terrestrially derived hierarchical patch dynamics (HPD) model. The goal is to provide a framework for the development of a cohesive theory of riverine ecosystems over time rather than to produce a finished product. This chapter draws upon three primary components of river science that contribute to the study of riverine landscapes: lotic ecology, landscape ecology, and fluvial geomorphology.

Historical and Recent Perspectives on Riverine Concepts

This chapter gives a broad overview of the riverine ecosystem synthesis (RES) by exploring the nature and applications of all prominent riverine theories published even in the last few decades. Consequently, the focus and analysis of the review is only on those hypotheses, models, theories, and paradigms that address large-scale spatial patterns affecting the structure and function of riverine ecosystems and ecological regulation of communities at smaller spatiotemporal scales. At the larger spatial scale, this chapter concentrates the analysis on two (longitudinal and lateral) of the four recognized dimensions of rivers. It briefly covers briefly covers vertical dimension because less controversy seems to exist among stream ecologists about processes and patterns operating in this dimension. The fourth dimension, which involves temporal phenomena, is the longitudinal dimension that alludes to patterns and processes occurring along discharge and altitudinal gradients from headwaters downstream to the river mouth. And by the lateral dimension, this chapter refers to similarities and differences in communities from the main channel through slackwaters (riverscape) to the floodplains (floodscape). At smaller spatial scales, theories debating which biotic and/or abiotic factors regulate community structure and the importance of temporal phenomena are discussed. The review of selective aspects of other models is tailored to that synthesis and its specific contribution toward conceptual cohesiveness.

Practical Applications of the Riverine Ecosystem Synthesis in Management and Conservation Settings

The importance of river classification in conservation planning and management is widely recognized. The river characterization approach presented in this chapter as part of the riverine ecosystem synthesis (RES) provides an excellent framework to classify riverine landscapes at various scales. By focusing on the scale of FPZs, scientists, river managers, and people interested in river rehabilitation will be operating at a scale that is appropriate for entire river networks. The identification and mapping of FPZs can provide a framework for conservation planning and prioritization of management activities within river networks. Interdisciplinary research in river ecosystems is a relatively young endeavor and one that is fraught with problems—linking across scales and integrating different disciplinary approaches and conceptual tools. Frameworks are useful tools for achieving this because they help define the bounds for the selection and solution of problems. They also indicate the role of empirical assumptions, carry the structural assumptions, show how facts, hypotheses, models, and expectations are linked, and indicate the scope at which a generalization or model applies. The RES provides such an integrative framework for the study of entire riverine landscapes. A framework is neither a model nor a theory because models describe how things work and theories explain phenomena. In contrast, a conceptual framework helps to order phenomena and materials, thereby revealing patterns.