Floods connect tropical river-floodplain food webs but shrink fish community isotopic trophic niches

Lateral connectivity between the floodplain and river channel is
hypothesised to expand fish community isotopic trophic niches and increase
overlap of river-floodplain food webs.  To evaluate how flood
magnitude influences trophic dynamics in a large tropical free-flowing
river (Roper River, Australia), we measured community isotopic niches in
low and high magnitude flood years in wetland and river
habitats.  Contrary to our hypothesis, isotopic niche area of the
river fish community contracted following the high magnitude flood year,
when compared to a low magnitude flood year. Wetland fishes maintained
more similar niche space regardless of flood conditions, but showed the
same pattern of contracting niche area following a large magnitude
flood.  Niche overlap was lowest for river low magnitude flood
and wetland high magnitude flood and highest for river and wetland high
magnitude flood.  Our results suggest that river-floodplain fish
communities exploit a wider isotopic range of food sources during low
flood years, including enriched sources of potentially marine origin,
while sharing abundant less diverse resources in high magnitude flood
years. As water resources in tropical rivers are developed, our findings
highlight the importance of conserving multiple dimensions of connectivity
in riverine landscapes to maintain intact river-floodplain food webs.