Widespread loss of the seagrass Syringodium isoetifolium after a major flood event in Moreton Bay, Australia: Implications for benthic processes

• Major flood event results in significant changes to benthic macroflora composition.
• S. isoetifolium loss is likely due to combined effects of low salinity and light.
• Changes in benthic macroflora composition result in a shift in benthic metabolism.

Seagrass ecosystems play a significant role in coastal biogeochemical processes, but are under threat from both natural and anthropogenic disturbances. In this study, we investigated the change in benthic macroflora community composition in a 259 ha area after a major flood event in a shallow coastal embayment (Moreton Bay, Australia). Almost 50% of the seagrass meadow was lost from the study area with seagrass loss restricted to Syringodium isoetifolium, which declined in distribution by almost 100%. Other seagrass species including Zostera muelleri appeared unaffected; however macroalgal species Caulerpa taxifolia and Udotea argentea also disappeared from the study area. Review of previous research suggests that difference in species tolerance to low salinity levels in combination with light deprivation is the most likely cause of the observed change. Compilation of benthic metabolism data indicates this change potentially resulted in a switch from net benthic heterotrophy to net benthic autotrophy within the study area, with associated implications for carbon and nutrient cycling through the seagrass meadow. This study highlights the need for monitoring seagrass meadows to a dominant species/community level and demonstrates the importance each community can play in the broader ecosystem.