In Part 1 of this brief history I talked about how ideas of zones in rivers – mainly related to fish abundance and distribution – were succeeded by those of the river continuum, emphasizing a more longitudinal, continuous movement of material, energy and species downstream. The River Continuum Concept, as it was known, sparked enormous interest and a lot of activity thereafter. But rivers come in all shapes and sizes and one size definitely does not fit all. Firstly, many rivers (most large ones in the world) are regulated to greater and lesser extents. So, people started to ask how regulation might affect river functioning. And then, those who worked in large, lowland rivers with extensive floodplains, questioned the generality of the RCC, and pondered on alternative sources of carbon in rivers and pathways of matter and energy.
The serial discontinuity concept (SDC), proposed by James Ward and Jack Stanford (Ward and Stanford 1983), took the idea of the river continuum and asked what would happen to the patterns and processes, as described in the RCC, if an instream dam or weir (those actually in the river channel, blocking the flow) was placed somewhere in the system. It contends that dams disconnect sections of the river upstream and downstream and displace resource gradients downstream to varying extents. In effect, the dam shifts everything downstream. The SDC does not say much about fish, but does make predictions about the nature of energy and materials that can be translated to fish relatively easily. This concept has been widely cited, but hypotheses coming from it, not well tested, which is somewhat surprising, given how common dams are in rivers. There is, however, general agreement that large instream dams (those actually in the river channel, blocking the flow) do have an influence on longitudinal patterns in fish assemblages. For example, Arajo et al. (2009) found that fish assemblages were affected more by damming and less by season in a large tropical Brazilian river; An and Choi (2003) recorded major changes in fish guild structure following the construction of a dam during a long-term study in Korea; and Reyjol et al. (2001) showed that in unregulated sections of a river in France, temperature determined the transition from the salmonid-dominated region to the cyprinid-dominated region, but that this changed with regulation and the presence of dams. So, the SDC certainly has merit but doesn’t seem to excite people as much as perhaps it should.
The next big idea was the flood pulse concept (FPC), proposed in 1989 by Wolfgang Junk, Peter Bayley and Richard Sparks, working in German and US rivers. It arose from a dissatisfaction with the RCC, based on observations largely derived from tropical floodplain rivers (Junk et al. 1989; Junk and Bayley 2008), although it has been more recently extended to temperate systems (Tockner et al. 2000) and to lakes (Wantzen et al. 2008) as well. The FPC emphasizes the floodplain as an important source of material and energy, inundation of the floodplain by a flood pulse as the catalyst for mobilising that material and energy; and movement of that material and energy from the floodplain into the main channel as the process that fuels food webs in floodplain rivers. The emphasis, therefore, is more on lateral connectivity than a longitudinal continuum. The flood pulse is termed a ‘batch process’, rather than a continuous process – occurring in discrete time periods. The FPC was the first real river model to put up front the significance of a conceptual framework for fish. It emphasized the importance of the floodplain for fish breeding and as a nursery habitat, while also relating fish production in the main channel to nutrients draining back from the floodplain as the flood receded. The main channel, according to the FPC was used only temporarily for migration, spawning, refuge during droughts or freezing and for hibernation. Many examples exist of fish whose life histories or peaks in feeding coincide with predictable flood pulses (e.g. Goulding 1980), Welcomme 1985). Indeed, the timing and duration of flooding has been shown to influence population dynamics (Finger and Stewart 1987; Stassen et al. 2009), growth of species associated with the aquatic/terrestrial transition zone (Gutreuter et al. 1999; Glemet and Rodriguez 2007) and/or production of young (Feyrer et al. 2006; Tonkin et al. 2008; King et al. 2009) in some species of riverine fish. For flatter, low-gradient rivers, it appears that the FPC can predict the source of carbon in food webs (Hoeinghaus et al. 2007).
The flood recruitment model (FRM) developed by John Harris and Peter Gehrke ( NSW Fisheries) for Australian systems, developed the FPC as it related to the recruitment of fish in Australia’s Murray-Darling Basin, arguing that flooding cued some species to spawn and perhaps use the floodplain, whereas others benefited through production of zooplankton derived from the floodplain, but which is washed into the main river channel (Harris and Gehrke 1994). Benefits derived from the flood pulse may be less obvious for some fishes, however. For example, species that normally living on floodplains or only occupying the edges of large rivers may be able to move among floodplain habitats during flooding, contributing to their reproductive success or overall survival (Tonkin et al. 2008). It is apparent, however, that the coupling of temperature and flooding are critical for floodplain use, which is often not the case in temperate systems (King et al. 2003).
These concerns about the coupling or otherwise of the key factors that influence use of the floodplain by fish, especially for breeding purposes, made some scientists question the generality of the FPC and FRM for some river systems. The development of other ideas, especially those related to production within the main channel of rivers, will be the focus of Part 3 of this history of river ecosystem ideas.
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