Chapter 1: General Introduction

Chapter 1: General introduction Chapter 1: General Introduction 1.1 Introduction The biogeochemical cycling of key elements within ecosystems is comprised of a complex array of abiotic and biotic processes. The stoichiometry or the mass-balance of elements is a measure of their total mass present within an ecosystem. Ecological stoichiometry theory recognizes that the ratio of ecosystem elements controls the rates of the biotic ecological processes which recycle these elements (Sterner & Elser 2002, Cross et al. 2005). However, the rates at which these ecological processes occur can also be influenced by many other abiotic parameters and processes. At the basal level of aquatic systems, autotrophic and heterotrophic communities are responsible for some of the main biotic processes that control the cycling of the key ecosystem elements, carbon (C), nitrogen (N) and phosphorus (P) (Sterner & Elser 2002, Cross et al. 2005). Photosynthesis is the dominant process that produces carbon, while autotrophic respiration or the heterotrophic decomposition of organic matter are the main pathways for carbon utilisation in aquatic systems. As both of these processes require N and P, the concentrations at which they are available to consumers will control the rate at which these elements are retained and recycled within the system (Sterner & Elser 2002, Cross et al. 2005). In lotic systems, the cycling of key elements is also controlled through abiotic ecosystem processes and their interaction with biotic processes (Brookes et al. 2005). The main abiotic factors that can affect the biotic process of autotrophic production in streams are light (Bunn et al. 1999, Bott et al. 2006), substrate (Cardinale et al. 2002, Atkinson et al. 2008), and hydrology (Uehlinger 2000, Acuoa et al. 2004). In contrast, rates of heterotrophic decomposition and respiration are influenced by the quantity and chemical composition or quality of organic matter (Aldridge et al. 2009, Hladyz et al. 2009, Gibson & O’Reilly 2012). The quantity of organic matter present within streams is controlled through the balance of import, retention, breakdown and export processes (Webster et al. 1999, Lepori et al. 2005). Riparian vegetation has been identified as a major source of allochthonous organic matter 1 Chapter 1: General introduction contributing leaves, bark and woody fractions that are transported laterally to streams (Webster & Meyer 1997, Reid et al. 2008, França et al. 2009). Stream reaches can also receive allochthonous organic matter from upstream areas via material that is transported longitudinally when surface water is present (Gurtz et al. 1988, Boulton & Lake 1992). Organic matter that enters stream reaches through longitudinal or lateral links will be retained through abiotic processes associated with channel geomorphology and hydrology (Lepori et al. 2005, Quinn et al. 2007). The rate of organic matter breakdown through abiotic (i.e. physical fragmentation) and biotic (i.e. heterotrophic decomposition) processes will also control the quantity of organic matter retained in reaches (Webster et al. 1999). These processes can in turn be related to the quality of the retained organic matter, which is largely determined from its source (Findlay & Sinsabaugh 1999). High quality low-molecular weight organic matter is characterised as having low lignin:cellulose (LCI) ratios and is generally from autochthonous sources, while low quality high-molecular weight organic matter has high lignin:cellulose ratios and originates from allochthonous sources (Melillo et al. 1992, Gessner & Chauvet 1994, Royer & Minshall 2001). The quantity and quality of organic matter can play an important role in providing habitat and energy for invertebrates, fish and heterotrophic communities (Erhman & Lamberti 1992, Jones 1997), which subsequently affect the retention and cycling of nutrients within reaches (Aldridge et al. 2009, Gibson & O’Reilly 2012). Channel geomorphology is controlled through a combination of local and catchment factors including riparian vegetation, hydrological regime, climate and geology (Gregory et al. 1991, Stoffel & Wilford 2012). In upland streams, channel geomorphology is generally considered to be complex with steep gradients and distinct riffle, run and pool areas. Channels are usually constrained by bedrock that can be exposed in large areas. In comparison to lowland streams, the channels are narrower and are not connected to wide floodplains, so the sources of organic matter to channels are predominantly from adjacent and surrounding vegetation (Gregory et al. 1991, Campbell et al. 1992). Upland streams are also characterised by having lower discharges and higher surface water velocities in comparison to lowland streams (Beschta & Platts 1986). In some catchments, the hydrological regime in upland streams is temporally variable ranging from floods to cease-to-flow events depending on the timing and intensity of rainfall (Boulton 2 Chapter 1: General introduction & Lake 1992, Acuoa et al. 2004). The high-surface water velocities are important for the import, retention, and export of nutrients and organic matter within reaches during periods of connectivity (Boulton & Lake 1992, Harms & Grimm 2010, Stewart & Ryder 2012 - see Appendix 1). During cease-to-flow events, the complex geomorphology of reaches can cause surface water to fragment into disconnected pools, with stream energetics in pools relying on retained organic material (Ylla et al. 2010). Knowledge of the abiotic and biotic processes that control the biogeochemical cycling of key elements is important for understanding the functioning of stream ecosystems. These abiotic and biotic processes are strongly linked to organic matter dynamics (Cross et al. 2005), which can interact with channel geomorphology and hydrological parameters (Tzoraki et al. 2007). Therefore, exploring the interaction among organic matter and nutrient dynamics, geomorphology and hydrology will offer further understanding of the drivers of stream function in intermittent upland streams. 1.2 Anthropogenic effects to upland streams Within agricultural landscapes, land-use management practices have led to the widespread removal of riparian vegetation along upland streams (Robertson et al. 1999, Allan 2004). In combination with other anthropogenic influences including the application of fertiliser, the removal of large woody debris from channels, planting of exotic species of vegetation, and the alteration to flow regimes, these impacts have cumulatively led to major impacts on biogeochemical processes within these systems (Sabater et al. 2000, von Schiller et al. 2007, Hadwen et al. 2010). Over the last twenty years, there has been an improved effort to restore biogeochemical processes and/or stream functions through a range of biophysical restoration techniques (Lake et al. 2007, Bernhardt & Palmer 2011). These management interventions commonly focus on the replanting of streambank and riparian vegetation at reach-scales, as this is the spatial scale at which effective on-ground restoration activities can be implemented (Mika et al. 2010, Bernhardt & Palmer 2011). However, there has been little research into whether these efforts to restore the structural complexity of riparian habitats have been 3 Chapter 1: General introduction successful in restoring biogeochemical processes in adjacent streams (Mika et al. 2010, Bernhardt & Palmer 2011). 1.3 Riparian vegetation Riparian zones represent the interface between terrestrial and aquatic ecosystems. Spatially, the riparian zone encompasses the area that runs laterally from the main stream channel to the wetted area defined during flood events, and horizontally from the soil subsurface to the height of the streamside vegetation (Gregory et al. 1991). The vegetation present in the riparian zone can be compositionally and structurally diverse ranging from herbs, to shrubs and larger trees. The abundance and composition of riparian vegetation is governed by both spatial and temporal processes including; the local climate, soil and topographic conditions, and the recovery from disturbances such as floods, fire, storms/wind, and plant diseases (Gregory et al. 1991, Stoffel & Wilford 2012). Riparian vegetation plays several important functions in stream biogeochemical processes. In particular, vegetation is an important source of allochthonous organic matter and nutrients to aquatic ecosystems (Fisher & Likens 1973, Cummins 1974). Coarse, fine and dissolved fractions of organic matter are an essential food resource and habitat for aquatic heterotrophic and autotrophic communities, and for higher trophic levels including invertebrates and fish (Fisher & Likens 1973, Cummins 1974, Erhman & Lamberti 1992). The rate of organic matter processing and utilisation by aquatic biota can facilitate the longitudinal cycling of nutrients and the stoichiometric balance of nutrients within reaches (Newbold et al. 1981, Cross et al. 2005). In particular, aquatic heterotrophic communities including bacteria and fungi actively take up nutrients (predominantly N and P) from the water column in order to decompose organic matter and acquire C for energy and growth (Bärlocher et al. 2011, Bärlocher et al. 2012 - see Appendix 1). Organic matter that has a high C:N:P (also known as LCI – lignin:cellulose index) will lead to an increase in uptake of N and P from the water column by heterotrophs (Aldridge et al. 2009, Gibson & O’Reilly 2012). The C:nutrient stoichiometry (C:N:P) of organic matter is dependent

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