Henderson, C.J., Olds, A.D., Goodridge Gaines, L.A. et al. Flood effects on estuarine fish are mediated by seascape composition and context. Mar Biol 171, 138 (2024). https://doi.org/10.1007/s00227-024-04459-6
Estuary Ecosystem
Estuaries are important ecosystems that contain a diverse number of habitats. From sandy plains to salt marshes and mangroves, these systems provide important services to many fish species. These can include spawning grounds, nurseries, and feeding areas.
However, intense flooding can reduce local fish populations by altering the physical environment and changing the water chemistry of an estuary. These impacts are normally temporary with fish returning within a few days or weeks. But human development of estuaries can magnify flooding impacts by removing important barriers such as mangroves or wetlands that mitigate flooding. There is concern that continued urbanization coupled with flooding impacts will reduce fish populations and diversity in estuaries. Fisheries would then suffer, losing out on catches, impacting the local economy.
In this study, scientists studied a once in 100 year flooding event in Queensland, Australia to investigate how fish communities react to flooding. These reactions may differ depending on the habitat they live in and if urbanization is present. They hypothesized it would take longer for fish communities to recover from a flood when in the presence of urbanization compared to a natural environment.
Diversity and Impacts
Six different estuary habitats were studied including:
- Bare sediments
- Log snags (dead trees)
- Mangrove forests
- Rocky structures
- Salt marshes
- Seagrass meadows
All six habitats post-flood had reduced fish populations six months after. Their proximity to urban development versus the mouth of the estuary influenced the intensity of these impacts. Seagrass meadows were actually the only habitat that were negatively impacted by urban development. Rather, the mouth of the estuary had a stronger, more positive influence on the other habitats that reduced flooding impacts.
Seagrass meadows were more than likely the only habitat impacted by urbanization because of the lack of hard structures present. Log snags, mangrove forests, rocky structures and salt marshes all have some sort of hard structure in them that can break up fast moving currents and/or provide shelter. Seagrass meadows are relatively flat sandy plains with flexible grasses that swing in the current. When near urbanization, concrete walls along shorelines offer no shelter and their hard surface increases current speed, making it difficult for fish to stay in the habitat. Bare sediments experience similar influences, but the lack of plant growth already makes it a less productive habitat that fish would stay in.
On the other hand, the influence the mouth of the estuary had on flood impacts actually makes sense. Strong currents and wave action in this region make the area naturally resistant to increased water flow from flooding. These properties also help dilute fresh water more quickly, maintaining water chemistry that fish can tolerate. If fish were forced to leave the estuary, the mouth would be the first area to see them return. Therefore increasing its diversity before other habitats further in.
Conservation and Protection
The mouth of an estuary is an important refuge from flood impacts based on this study. We need to maintain the natural buffers of these regions as humans continue to develop. Not only will this help fish populations seek refuge from flooding, but maintain them for important fisheries.
I am a recent MSc graduate in marine biology from Bangor University, where I studied population dynamics of elasmobranchs off the coast of Wales. My interests lie in ecological data analysis to understand environmental processes and identify natural patterns. However, nothing beats being in the field and interacting directly with the marine life.