A brief history of river ecosystem function, part 1: fish zones to the river continuum

In an earlier post, I talked about humanity’s fascination for all things aquatic. And rivers, especially, hold a special place in many people’s imaginations and in their hearts. Rivers not only inspire romantic and religious types, however; they also inspire scientists, like me. Ever since philosophers like Aristotle began to seriously question the nature of the universe, people have wondered where rivers come from. Consider, for example, some of the most incredible (and often disastrous) explorations that have been made in order to find the source of such famous rivers as the Nile (have a read of Alan Moorehead’s wonderful books, The White Nile and The Blue Nile). But people have also asked how the animals and plants that live in rivers got there; how these organisms survive and reproduce and why in one part of a river you see completely different groups of species than in other parts. Despite this long fascination, it is only relatively recently that scientists have really begun to put the pieces of the puzzle together and to understand how rivers function. This post is part 1 of a brief history of how scientists think rivers work.

River ecology is a relatively young science. Until about 1970, rivers and streams were thought of as being a series of zones, largely defined by the dominant fish that lived in them.  In western Europe, there were thought to be four zones – the trout (Salmo), grayling (Thymallus), barbell (Barbus) and bream (Abramis) zones – and these moved successively downstream from the headwaters. In North America, Victor Shelford also proposed that particular fish species occurred in zones near Lake Michigan  and others, like Shelby Gerking in Indiana, had similar ideas. Scientists in other countries described patterns of zonation in river fishes, although they were, of course, specific to each region, which meant that generalisations were a little difficult. The zones typically included other co-occurring fish species, besides the dominant group, and were able to be predicted from stream width, slope and valley shape.

There was some argument in North America over whether fish were ‘replaced’ or ‘added’ as the river progressed downstream.  And in tropical systems, it was argued that fish species were added rather than replaced, unless a waterfall or other barrier to movement occurred.  Other species-based zones were also devised, based on leeches, amphipods (small crustaceans called ‘scuds’), crayfish, mayflies, stoneflies, and many other insect groups. The ‘fish-zone’ classification is still with us today to some extent in some parts of Europe, despite the fact that a review of many European studies concluded that there was little empirical support for discrete fish zones after all.

Illies (cited in Hynes 1970) noted that there were places along the river where assemblages of invertebrate species seemed to dramatically shift to other assemblages, and that these were reasonably well aligned with the fish-based zones. He proposed that these zones had ecological meaning and coined the terms rhithron (upstream, cool, high oxygen concentration, fast and turbulent flow, substratum of rock, stone or gravel, the fauna having narrow temperature tolerances and little plankton) and potamon (more downstream, warmer, oxygen concentration lower at times, flow slower, substratum mostly sand and mud, fauna having broader temperature tolerances and abundant plankton). There were other developments and refinements in terminology which helped to define various types of streams, from springs through to large, floodplain rivers.  I will talk about this terminology in a future post, because it is often a source of confusion for students, River ecologists – like all scientists – love jargon.

River scientists began, after a time, to question the ‘river-zone concept’, recognising its limitations; mainly because, while zones are by definition discrete, they don’t reflect the continuous nature of the movement of water, materials and energy in rivers. This dissatisfaction motivated an attempt to unite the various non-living (abiotic) and living (biotic) components and processes that make up river ecosystems into a continuous, interconnected whole. The River Continuum Concept (RCC) was the first such model, although it had some important precursors, and was to become a hugely influential (and cited) paper by Vannote, Minshall, Cummins, Sedell and Cushing in 1980. Robin Vannote and colleagues, working in North America, emphasized the upstream/downstream linkages and continuous gradients in rivers. They wanted to combine fundamental non-biological factors, like geomorphology (the physical structure of rivers and streams) and hydrology (the flow regime), with biological processes, like feeding and breeding. Their concept stressed the fact that streams – or at least the ones that Vannote and colleagues studied – had much of their organic matter (leaves, sticks, bark and plants) entering at the headwaters and that this organic matter was progressively processed as the river moved downstream, mostly by insects adapted to take advantage of increasingly smaller particles. Shredding and collecting insects dominated upstream, then grazing insects turned up in mid-reaches and were in turn replaced by collectors and zooplankton in the lower reaches.

The RCC says little about very large rivers or about fish and how they fit into the riverine continuum, besides that fish diversity increases as one moves downstream.  Many studies have investigated the RCC in relation to stream fishes, however, often asking if fish assemblage composition changes continuously or in jumps (like the  ‘fish-zone’ concept) from upstream to downstream. Some have found a continuum, whereas others have not. It has been suggested that for many river systems, factors other than the longitudinal transport and processing of organic matter have significant influences on fish assemblage composition. These include habitat diversity, salinity and the duration of the wet season and flooding in tropical regions. Matthews (1998) tested the zone vs. continuum dichotomy for the Strawberry-White River in Akansas and found that there was a “distinct upland region” in which species showed upstream and downstream limits, but that there were no other identifiably distinct zones and that in the lower regions of the river fish had broad distributions and tolerances.

In part 2 of this brief history of ideas related to river ecosystem function, I will describe later developments: some extending the RCC to rivers which have been regulated by dams, and others which found the RCC inadequate for describing what they saw in floodplain rivers, where sources of organic matter were quite different from the sorts of rivers studied by Vannote and colleagues.

General references:

Hynes HBN (1970) ‘The Ecology of Running Waters.’ University of Toronot Press, Toronto.

Matthews WJ (1998) ‘Patterns in Freshwater Fish Ecology.’ Kluwer Academic Publishers, Norewll, Massachusetts.

Welcomme RL (1985) ‘ River Fisheries.’ FAO, Rome.

Fish zone references:

Gerking SD (1945) The distribution of the fishes of Indiana. Investigations of Indiana Lakes and Streams 3, 1-137.

Huet M (1959) Profiles and biology of Western European streams as related to fish management. Transactions of the American Fisheries Society 88, 155-163.

Shelford VE (1911) Ecological Succession. I. Stream Fishes and the Method of Physiographic Analysis. Biological Bulletin 21, 9-35.

River Continuum Concept and some examples of tests of it:

Aarts BGW, Van Den Brink FWB and Nienhuis PH (2004) Habitat loss as the main cause of the slow recovery of fish faunas of regulated large rivers in Europe: The transversal floodplain gradient. River Research and Applications 20, 3-23.

Blachuta J and Witkoswki A (1990) The longitudinal changes of fish community, in the Nysa Klodzka River (Sudety Mountains) in relation to stream order. Polskie Archiwum Hydrobiologii 37, 325-342.

Dauwalter DC, Splinter DK, Fisher WL and Marston RA (2008) Biogeography, ecoregions, and geomorphology affect fish species composition in streams of eastern Oklahoma, USA. Environmental Biology of Fishes 82, 237-249.

Dettmers JM, Wahl DH, Soluk DA and Gutreuter S (2001) Life in the fast lane: Fish and foodweb structure in the main channel of large rivers. Journal of the North American Benthological Society 20, 255-265.

Mazzoni R, Fenerich-Verani N, Caramaschi ÃP and Iglesias-Rios R (2006) Stream-dwelling fish communities from an Atlantic rain forest drainage. Brazilian Archives of Biology and Technology 49, 249-256.

McNeely DL (1986) Longitudinal patterns in the fish assemblages of an Ozark stream. Southwestern Naturalist 31, 375-380.

Oberdorff T, Guilbert E and Lucchetta JC (1993) Patterns of fish species richness in the Seine River basin, France. Hydrobiologia 259, 157-167.

Vannote RL, Minshall GW, Cummins KW, Sedell JR and Cushing CE (1980) The river continuum concept. Canadian Journal of Fisheries and Aquatic Sciences 37, 130-137.

Walters DM, Leigh DS, Freeman MC, Freeman BJ and Pringle CM (2003) Geomorphology and fish assemblages in a Piedmont river basin, U.S.A. Freshwater Biology 48, 1950-1970.

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