Ecosystem Structure and Function in a Hydrogeomorphically Complex Riverine Landscape
Dynamic patterns and processes underlie the physical templates that house ecological communities. These patterns and process operate over various scales to create spatial and temporal variability within the landscape, resulting in heterogeneous systems that support diverse communities. The physical template is important to biota because it is the foundation on which communities establish and persist in the landscape. The template governs composition of communities as well as ecosystem processes that support the structure and function of communities in space and through time.
In riverine landscapes the physical template is created by the interaction of hydrology, geomorphology and ecology operating over multiple spatial and temporal scales. Understanding the hydrogeomorphic character underpinning ecosystems in riverine landscapes, and the mechanisms that elicit biological responses, is essential to understanding the processes by which freshwater ecosystems maintain patterns of biodiversity and ecosystem function.
The aim of this thesis was to investigate how ecosystem structure and function are influenced by the hydrogeomorphic character of the riverine landscape. A literature review was first undertaken to identify work currently available to inform on the hierarchical way in which riverine ecosystems operate. Review of the literature highlighted the need for further work to investigate habitat-biota relationships through an interdisciplinary lens that incorporates hydrology, geomorphology and ecology. Added to this interdisciplinary approach is the application of scale, whereby the spatial and temporal context of variables and ecological response are considered.
From review of the literature, four objectives were developed to address current deficiencies identified in the literature: 1) describe the hydrogeomorphic character of the physical template of the Upper Mississippi river-floodplain across multiple spatial and temporal scales; 2) investigate the influence this character has on fish community structure; 3) determine if the same hydrogeomorphic variables responsible for describing the physical template also influences fish community structure and food-web dynamics; and 4) describe the response of fish communities and food webs to temporal variability over a connection event.
This work demonstrated that hydrogeomorphic variables from multiple spatial and temporal scales contribute to the heterogeneous nature of patches of the Upper Mississippi river floodplain. Variables included patch-scale descriptors of entry morphology and depth, short-term hydrology variables describing the duration and magnitude of connection events 2-years prior to sampling, long-term duration and magnitude of connections and the nature of rises and falls over the historical record, and finally, landscape scale variables of where a patch was located in a pool and its proximity to neighbouring patches. Five hydrogeomorphically distinct habitat patches were identified, including four different types of backwater habitats not previously described in the Upper Mississippi river-floodplain, and a floodplain lake group.
Three hydrogeomorphically distinct off-channel habitats, namely floodplain lakes and two types of backwaters supported distinct fish communities in the 2012 sampling season. In general, differences were driven by black crappie (Pomoxis nigromaculatus) dominating floodplain lakes, Group 2 backwaters having a high abundance of gizzard shad (Dorosoma cepedianum) and bluegill (Lepomis macrochirus) dominating Group 5 backwaters.
Fish community structure and food web structure changed temporally over the connection event sampled in 2013 in response to decreasing connectivity and associated changes in conditions, although the change in food web structure was strongest. Structural changes to fish assemblages were influenced by the functional response of breeding events among common species such as bluegill (Lepomis macrochirus), black crappie (Pomoxis nigromaculatus) and largemouth bass (Micropterus salmoides). Functional changes in food web structure were seen through shifts in trophic assemblages (represented by the interaction of species present and their trophic position in each community), a decrease in mean trophic position, food chain length and average consumer δ15N, and broadening of the consumer carbon base as connectivity decreased. Consumers also expanded their carbon food source earlier in some patch types than other (i.e. floodplain lakes compared to backwaters). A spatial pattern was also reflected in food webs with trophic assemblages varying between patch types with rheophilic species dominating channels and a greater diversity of macroinvertebrate taxa in off-channel patches. Mean trophic position was lower in floodplain lakes, intermediate in backwaters and highest in channels. Fish community structure did not vary spatially.
Hydrogeomorphic variables responsible for differentiating the physical character of the Upper Mississippi river-floodplain were not the same set of variables influencing fish communities and food-web structure, although there was some overlap. The mixture and importance of variables associated with fish assemblages in space were different among patch types (i.e. floodplain lakes and sub-groups of backwaters) and included patch-scale characteristics and long-term hydrology, and in the case of backwaters, which pool they were located in. The mixture and influence of variables associated with changes in fish assemblage and trophic assemblage through the connection event also changed over time.
In conclusion, an interdisciplinary, multi-scale approach is critical for describing the hydrogeomorphic character of riverine landscapes. Characterisation of the physical template should be done prior to biological sampling so patches can be targeted based on their physical character in order to draw out key habitat-community relationships. Added to this should be the tendency to move way from broad classifications of habitats (e.g. floodplain lake vs backwater) since such classifications could be too simplistic and may miss biota-relevant patterns in the landscape. Instead, individual characteristics defining the physical nature of a habitat need to be linked to biological responses across scales. This need is reinforced by the finding that the suite of variables responsible for describing the physical template is not necessarily the same as the suite found to be important to biota.
A hierarchical framework and interdisciplinary approach enable a holistic view of the spatiotemporal heterogeneity of a system and provides a firm basis for investigating how the physical template influences and helps to maintain ecosystem structure and function through space and time. Understanding the mechanisms that elicit these responses in riverine landscapes is essential in our quest to improve and maintain the biostructure of communities and thus functioning ecosystems.