Oral Presentation Freshwater Sciences 2023

SPATIAL AND TEMPORAL CHANGES IN ECOSYSTEM STRUCTURE AND FUNCTION THROUGH A CONNECTION EVENT IN THE UPPER MISSISSIPPI RIVER FLOODPLAIN (#137)

Munique C Reid 1 , Michael A Reid 2 , Sarah Mika 1 , Ross Thompson 3
  1. School of Environmental and Rural Science, University of New England, Armidale, NSW, Australia
  2. Geography and Planning, University of New England, Armidale, NSW, Australia
  3. Institute for Applied Ecology, University of Canberra, Canberra, ACT, Australia

Community response to the dynamic patch mosaic of river-floodplain landscapes can be both structural and functional in space and through time. Understanding the mechanisms that elicit these responses is essential in our quest to improve and maintain functioning ecosystems. To contribute to this understanding, we tracked structural and functional changes in fish communities (via fish surveys) and food webs (using stable isotope analysis) in distinct patches (channel, backwater and floodplain lake habitats) through a connection event in the Upper Mississippi River. Environmental variables from multiple spatial and temporal scales were also linked to community responses.

Fish community structure and food web structure changed over the connection event, although the changes in fish community structure were weaker. Structural changes to fish assemblages were strongly influenced by the functional response of breeding events among common species. Functional changes in food webs were seen through shifts in trophic assemblages (represented by the interaction of species present and their trophic position in each community) over the event and decreases in mean trophic position, food chain length and average consumer δ15N with decreasing connectivity. Consumers broadened their carbon base over the connection event and this pattern also manifested spatially, with consumers expanding their food sources earlier in the disconnection phase in floodplain lakes compared to backwater patches. Food web structure also varied spatially over the connection event but surprisingly fish community structure did not. Overall mean trophic position was lower in floodplain lakes, intermediate in backwaters and highest in channels.

We suggest that community structure is maintained by means of functional adjustments, seen here through changes in trophic assemblage brought about by increases in primary consumers, fish breeding and broadening of carbon range by consumers. This capacity to maintain structure, both spatially and temporally via functional adjustment, supports resilience of communities to natural disturbance.