As often happens with scientific concepts or models, the more they are investigated, the more they are found wanting for all situations. But that is the scientific process: someone comes up with a model for how the world works – in this case, rivers – and it gets people thinking, talking, writing and researching, as well as criticising. Then someone comes up with a refinement of the previous model or an alternative one is proposed. The River Continuum Concept (RCC) seemed appropriate for particular types of rivers, especially temperate ones, that have lots of organic inputs from upland areas. The Flood Pulse Concept (FPC) set out to explain the seemingly conflicting observations from tropical rivers that have predictable and extensive flooding ‘pulses’. But both emphasized inputs derived from outside the immediate vicinity of the main channel – either upstream or the floodplain – as being the major drivers of production. A third model followed not long after, which emphasized the contributions of local, instream production to river functioning.
This third river function model – the Riverine Productivity Model (RPM) – came about largely because Jim Thorp and Mike Delong, US river biologists, did not believe that the contribution of the edge of large rivers to production, especially in the middle and lower reaches, had been given enough weight. They were also dissatisfied with how the RCC and FPC explained the structure of riverine food webs (Thorp and Delong 1994; Thorp et al. 1998). The RPM proposed that in some river sections, materials and energy are derived mainly through local production of phytoplankton (algae floating in the water column), benthic algae (algae growing on the bottom of rivers) and other aquatic plants, as well as directly from the riparian zone (the edge of rivers) via leaves and other sources of carbon, small particles or dissolved in the water. Thorp and Delong did not, however, reject the RCC or the FPC completely, suggesting instead that these models might be more or less relevant depending on river type or location within the river. Several studies have indicated that carbon in food webs can indeed be derived instream, as proposed by the RPM, but that it is more relevant for high gradient streams, those reaches downstream of dams and within the dams themselves (Thorp et al. 1998; Thorp and Delong 2002; Hoeinghaus et al. 2007).
The RPM initially lacked explicit reference to fish, apart from mentioning the ultimate source of their carbon. While not downplaying the significance of this process, the FPC explicitly included a number of processes involving fish. The Low Flow Recruitment Hypothesis (LFRH), proposed in 1999 by me, Alison King and John Koehn, to some extent filled this gap. It extended aspects of the RPM to fish, and considered spawning, recruitment and feeding of the young stages of fish (Humphries et al. 1999). From observations in temperate Australian rivers in regions with a Mediterranean climate, we noted that several species breed during the warm, low-flow periods in the year, and suggested that this is because these periods are more predictable than floods and because the production and concentrations of food for larval fish is greatest at this time. These observations have been supported by several studies in Australia and elsewhere (Humphries et al. 2002; King 2004a, b; Zeug and Winemiller 2008), although some have found that fish breed between the transition from floods to low flows (Elron et al. 2006). The Inshore Retention Concept (IRC), developed by Fritz Schiemer and colleagues in large, European rivers, in many ways analogous to the LFRH, proposed that the duration of retention of water in large rivers allows the development of zooplankton, as well as habitat for the young stages of fish, and is significantly affected by river regulation, channel alteration and disconnection from the floodplain (Schiemer et al. 2001a; Schiemer et al. 2001b; Schiemer et al. 2002).
As the concepts of river functioning developed during the last few decades, parallel ideas relating to geomorphology and habitat have also emerged that have helped to underpin and contextualise patterns and processes associated with riverine biota. The hierarchical framework for stream habitat classification (HFSHC) set out a template upon which fish (and other organisms) could be placed, classifying streams into segment, reach, pool/riffle and microhabitat subsystems, that has proved extremely useful in applied approaches to monitoring, managing and conserving river fishes (Frissell et al. 1986). The process domains concept (PDC) built on the HFSHC to emphasize the influence of geomorphology on disturbance patterns in rivers, and so ecosystem structure and function (Montgomery 1999). It argued that ‘process domains’ are “spatially identifiable areas characterized by distinct suites of geomorphic processes” (Montgomery 1999 p. 397) and are themselves nested within ‘lithotopo’ units: areas that have similar topographies and geologies. It effectively reinvigorated the concept of river zones with discrete functions governed geomorphologically; an idea shied away from by the proponents of the RCC, but also taken up by advocates of ‘functional process zones’ (Thoms et al. 2004). This approach has potential for testing hypotheses relating fish to habitat and in the assessment of river health, especially in large rivers (Walters et al. 2003; Boys and Thoms 2006), but in some ways could be considered somewhat of a backward step if the aim of river ecology is ultimately to unite the whole river system.
But the holy grail of one overarching model that describes how rivers function seemed as distant as ever, with at least three models in existence, none of which could explain all observations in all rivers and rivers sections. There was a crying need for synthesis. And synthesis is the topic of the next instalment of this brief history of ideas of river function.
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