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Stream Ecology: Concepts and Case Study of Macroinvertebrates in the Skeena River Watershed, British Columbia

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Stream Ecology: Concepts and Case Study of Macroinvertebrates in the Skeena River Watershed, British Columbia Stream ecology: concepts and case study of macroinvertebrates in the Skeena River Watershed, British Columbia by Alexander K. Fremier INTRODUCTION When hiking, it’s hard for me not to ponder the abundance of species and their apparent organization over the landscape. So, when swimming in a stream would one see similar patterns of species diversity? Species distributions are seemingly as well organized in flowing water as on land; yet, the processes controlling the patterns are quite unique to the aquatic environment. It is not hard to imagine that ecological patterns are dynamic in four dimensions, the usual three spatial ones and time. The spatial distribution of terrestrial plant species is limited by large physical processes such as temperature and precipitation as well as small factors, say proximity to a stream or topographic position. Time also plays a key role in distributions from seasonal to geologic time scales. This seems like a rather straight forward summary, however researchers have not always found linking pattern to process to be this simple. Physical processes driving species distributions in space and time tend to vary in importance from place to place and vary with the scale of inquiry; and, species respond to these factors in a range of ways. Stream systems provide a particularly complex web to untangle considering the dynamic ways in which it flows over and alters the landscape. Flow characteristics (such as velocity) change over small spatial scales, e.g. from the side of a channel to the main channel; they can change upstream to downstream and over many scales of time including seasonal flooding and glacial periods. Stream ecologists have aimed to describe these general patterns in various ways using a holistic approach infusing knowledge of hydrology and geomorphology to explain energy flows and ecological patterns. A few main concepts formed the structure with which investigators started to grapple with stream complexity, including the river continuum concept (Vannote et al. 1980), resource-spiraling concept (Elwood et al. 1983), serial discontinuity concept (Ward and Stanford 1983), flood-pulse concept (Junk et al. 1989), and hyporheic corridor concept (Stanford and Ward 1993). The scientific community continues to debate the merits and limitations of these concepts. More recent discussions have moved away Page 1 of 21 A.K. Fremier May 26, 2004 from a linear concept of stream systems and tend to emphasize scalar hierarchies and discontinuity concepts (Petts and Amoros 1996; Montgomery 1999; Rice et al. 2001). This paper aims to flesh out the insights of these early approaches and present them as sub-concepts in the overreaching framework that is more closely aligned with current scientific thought on issues in stream ecology. The latter section of the paper focuses on applying these concepts to the Skeena River watershed in British Columbia, Canada. CONCEPTS IN RIVER ECOLOGY River Continuum Concept The river continuum concept (RCC) posited that the physical variables in river systems, from headwaters to mouth, presented a continuous gradient of physical conditions that drive the biological strategies and river system dynamics (Vannote et al. 1980). This was largely argued from the standpoint that energy input, organic matter transport, storage, and use by macroinvertebrates functional feeding groups (i.e. how an organism retrieves its food), may be regulated largely by fluvial geomorphic processes. The continuum is described in the longitudinal direction using stream order where a first order stream has no tributaries and, depending on the system, the last order (usually > 6) runs into estuaries or the ocean directly. It states that the energy in the system for biological production at a given site is derived from three sources: local inputs or organic matter from terrestrial vegetation (allochthonous), primary production within the stream (autochthonous production), and transport of organic material from upstream. The proportion of these energy sources changes along the length of the stream (figure 1). The headwaters are dominated by terrestrial inputs or coarse particulate matter (e.g. leaf litter) with periphyton (algae attached to stream bed) also present. Middle reaches are mixed with coarse particulate matter, periphyton and vascular hydrophytes (aquatic plants) as well as the fine particulate matter lost from upstream communities. Further downstream receive less terrestrial inputs due to the increase area to perimeter ratio of the river channel. Lower reach energy sources are dominated by phyto- and zooplankton as well as the upstream fine particulate input. Periphyton and hydrophytes are not abundant at the lower reaches because high turbidity reduces penetration of sunlight. Plankton communities can persist in these waters due to the decreased slope and increased discharge. In Page 2 of 21 A.K. Fremier May 26, 2004 general, the RCC describes a gradient of energy inputs upstream to downstream of terrestrial inputs to in-stream production and associated downstream transport. Figure 1: The river continuum concept by Vannote et al. 1980. The proportion of invertebrate feed groups corresponds to changes in the physical factors in the longitudinal direction (Figure taken from USDA 2001). Page 3 of 21 A.K. Fremier May 26, 2004 Reflecting this gradient, macroinvertebrates populations are predicted to show a pattern relating to how they capture these energy sources. The right side of Figure 1 details how the proportion of feeding functional groups changes with stream order. The upper reaches are dominated by shredders and collectors, mid-reaches collectors and grazers, and low reaches by collectors. Collectors are species adapted to amassing fine particulate matter out of the water and therefore are predicted to increase downstream. Shedders in the upper reaches represent more of the macroinvertebrate species because of the increased allochthonous input, while grazers dominate the middle reaches due to the presence of in-stream production. Resource-Spiraling Concept A related concept in stream ecology is the resource-spiraling concept (RSC) (Elwood et al. 1983). It sets forth the concept that resources do not flow continuously downstream like the RCC suggests but are stored periodically in biological packages (e.g. organisms, detritus and waste products). Resources are released after decomposition and then can be recycled into new biological production. This does not entirely change the RCC but adds to the dialogue the spatial and temporal fluxes of resources. For example, Minshall et al. (Minshall et al. 1983) suggested that upper basin streams tend to conserve or store resources through high biological activity and physical barriers to organic matter. Conversely, large streams have lower rates of biological activity and retention. In addition, as discussed the next section, there is a lateral pattern to resource dynamics (Junk et al. 1989). Namely, flooding adds a significant supply of resources into the stream system by bringing about interactions with the floodplain. Resource spiraling in this context adds a larger spatial and temporal dynamic by proposing that resource inputs into the stream system are not limited to leaf litter and upstream particulate matter. The stream system includes the adjacent riparian lands. In this context, resource spiraling can incorporate resources that are stored in the surrounding floodplains over longer temporal scales. Serial-Discontinuity Concept The RCC and RSC assume little to no human involvement on the river system. The lack the structure to adequately deal with developed systems became a sticking point for opponents of the RCC (Johnson et al. 1995). They are argued that many, if not all, rivers have some level of human impact such as dam construction, channelization or riparian forest clearing, and therefore the RCC has little applied value on these rivers, the exact rivers that are trying to be restored. In Page 4 of 21 A.K. Fremier May 26, 2004 response, the serial-discontinuity concept (SDC) (Ward and Stanford 1983; Stanford and Ward 2001) was formulated and tried to incorporate impoundment into RCC. The SDC conjectured that even though dams were discontinuities in the river continuum, the RCC should still correctly predict the biophysical responses as a function of ‘discontinuity distance’ – upstream or downstream impact by dam. This concept, as well as the RCC, has been criticized by more recent studies because it lacks the construct to deal with the lateral and vertical dimensions of river systems. Recently, Stanford and Ward (Lamouroux et al. 1995) incorporated floodplains into the SDC and continue to maintain its efficacy as a predictive tool. Flood-Pulse Concept In contrast to the RCC, the flood-pulse concept (FPC) was derived on large floodplain rivers in temperate and tropical areas that caused investigators to consider the role of seasonal flooding in lotic systems (Junk et al. 1989). The RCC could be acceptably modified to account for brief and unpredictable floods in low-order streams; however, researchers recognized that as the size of a floodplain increased, the frequency of floods decreased, yet their predictability increased. This suggests that larger streams have a more stable flood regime. This predictability, they hypothesized, of the stream/floodplain exchange should result in adaptation of biota distinct from those species in streams dominated by stable lotic or lentic habitats. Therefore, the FPC states that the most important
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