Invertebrate colonisation of leaf packs of different palatability in an upland river.

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Invertebrate colonisation of leaf packs of different

palatability in an upland river.

Introduction

Freshwater invertebrates are an extremely diverse group.  Communities often consist of hundreds of species (Allan, 1975 and Benk et al, 1984).  The group is so diverse due to the unstable nature of the benthic environment.  Most of the invertebrates live on the stream bed or in leaf packs (conglomerations of organic matter fixed within the stream) due to the difficulties in free swimming in the high flow environment of a river.  A large proportion of the stream invertebrates are insects (others include crustaceans and molluscs) and have been studied in great detail.  

Energy enters river systems in two ways, either through in-stream photosynthesis or via imported organic matter from riparian vegetation.  In many streams the level of light which reaches the photosynthetic algae is to low to allow it produce enough to support the system.  So in most cases the major source of energy is imported.  Most of this energy enters the stream in autumn in the form of fallen leaves.  The leaves tend to aggregate in areas of low flow and are known as leaf packs.  These leaf packs are then processed in situ by the stream community.  The leaves that enter the stream are nutrient poor as the tree re-absorbs most of the useful sugars and amino acids.  When the leaves enter the stream they begin a process called ‘conditioning’.  At first leaching occurs, over a period of a couple of days most of the remaining nutrients are dissolved out of the leaf.  Then fungi and bacteria begin to colonise the leaves and start to break down the lignin and cellulose of which they are made.  After a couple of weeks the leaves become soft enough for aquatic invertebrates to begin to feed on them.  Not all leaves breakdown at the same rate.  For example Alder (Alnus glutinosa) leaves break down quicker than Oak (Quercus robur) leaves.  This breakdown continuum means that a stream contains nutrients throughout the year, not just in autumn when the major input occurs.

The invertebrates which live in streams are mainly omnivorous, however they do show large variation in the way which they utilise there resources.  Cummins in 1973 placed the invertebrates into categories which he called functional feeding groups.  These groups classified the invertebrates by their feeding methods and behaviours, rather than by their taxonomy.  Cummins classified them into six groups: shredders, filter feeders, grazers, predators, parasites and macrophyte piercers.  These groups are not distinct and many species perform more than one role.  However they are still the best way of linking species to their food resource.

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This study will compare the colonisation and breakdown rates of different leaf types (including one artificial) in a river.  The aim of this study is to test if leaf packs are more important to stream invertebrates as a food source than as a habitat.  Although studies similar to this one have been carried out (Rowe & Richardson, 2001; Dobson & Hildrew, 1992) non have taken place in the river Sett which is in excellent condition due to its location in the High Peak National park in Derbyshire and flows from its source through a moor.  Also few studies have ...

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