Ecological Linkages Between Headwaters and Downstream Ecosystems: Transport of Organic Matter, Invertebrates, and Wood Down Headwater Channels¹

Abstract

Headwater streams make up a large proportion of the total length and watershed area of fluvial networks, and are partially characterized by the large volume of organic matter (large wood, detritus, and dissolved organic matter) and invertebrate inputs from the riparian forest, relative to stream size. Much of those inputs are exported to downstream reaches through time where they potentially subsidize river communities. The relative rates, timing, and conversion processes that carry inputs from small streams to downstream reaches are reasonably well quantified. For example, larger particles are converted to smaller particles, which are more easily exported. Also, dissolved organic matter and surface biofilms are converted to larger particles which can be more easily intercepted by consumers. However, the quality of these materials as it affects biological activity downstream is not well known, nor is the extent to which timing permits biological use of those particles. These ecological unknowns need to be resolved. Further, land uses may disrupt and diminish material transport to downstream reaches by removing sources (e.g., forest harvest), by affecting transport and decomposition processes (e.g., flow regulation, irrigation, changes in biotic communities), and by altering mechanisms of storage within headwaters (e.g., channelization). We present conceptual models of energy and nutrient fluxes that outline small stream processes and pathways important to downstream communities, and we identify informational gaps that, if filled, could significantly advance the understanding of linkages between headwater streams and larger rivers. The models, based on empirical evidence and best professional judgment, suggest that navigable waters are significantly influenced by headwater streams through hydrological and ecological connectivities, and land use can dramatically influence these natural connectivities, impacting downstream riverine ecosystems.

Benthic and hyporheic macroinvertebrate distribution within the heads and tails of riffles during baseflow conditions

“…The presence of fine sediments can inhibit access to trophic resources, with some species in other studies reported to be confined to areas of clean gravel (Wood & Armitage, 1999;Rabeni et al, 2005). The distribution of organic matter is also frequently patchy (Maridet et al, 1997;Wipfli et al, 2007), but typically higher volumes are recorded at riffle heads due to their proximity to upstream pools which are often a store of organic material (Mermillod-Blondin et al, 2000). Some of this material will be transported as suspended particulate organic matter and carried into the interstitial spaces of the sediments at the head of riffles (Pusch, 1996;Brunke & Gosner, 1999), and may explain the higher abundances of shredders recorded in these areas in this study.…”

Effect of experimental and seasonal warming on litter decomposition in a temperate stream

“…These woodland streams constitute the majority of watercourses in hydrological networks in temperate regions (Allan and Castillo 2007), which makes them important participants in the global carbon cycle (Battin et al 2008). Also, the rate at which litter decomposes in these small streams, together with their headwater position in the river continuum (Vannote et al 1980), determines the amount and form of carbon (e.g., particulate organic carbon, dissolved organic carbon) that is transported downstream, shaping food webs both locally and in larger streams and rivers (Wipfli et al 2007). It is therefore important to study the effect of increases in water temperature on litter decomposition if we want to predict the contribution of freshwaters to the carbon cycle, and anticipate changes in aquatic food-webs and stream functioning.…”

Biodiversity values of small headwater streams in contrasting land uses in the Auckland region

“…In recent years, headwater streams have received increasing attention, with several North American, Mediterranean and Australian studies highlighting their local-and landscape-level importance (Meyer & Wallace 2001;Freeman et al 2007;Meyer et al 2007;Wipfli et al 2007;Maasri et al 2008;Clarke et al 2008). Headwater streams may harbour unique species of aquatic invertebrates because they differ physically and chemically from higher-order streams and because they can act as refugia, protecting invertebrates from competitors, predators and floods (Meyer et al 2007).…”