Floating along marsh edges: The impact of invasive water hyacinth (Eichornia crassipes) on estuarine species assemblages and predation risk

Abstract

Estuaries are dynamic environments where the safety of habitat refuges can be altered by a variety of natural and anthropogenic disturbances. After tropical storms or other high water run off events, water hyacinth (Eichhornia crassipes), an invasive floating freshwater plant, can enter estuarine habitats where it can colonize marsh edges and may impact the abundance and distribution of estuarine species. In freshwater environments, hyacinth reduces water quality, increases habitat complexity, and alters freshwater aquatic invertebrate abundance and diversity. Yet, little is known about how this plant impacts estuarine species as it floats from freshwater tributaries to oceanic environments. To examine how water hyacinth impacts estuarine marsh fauna, we examined the effects of water hyacinth on nekton communities and fish predation risk using trapping and tethering techniques along a marsh edge that had been recently colonized by water hyacinth. We placed funnel traps under water hyacinth and in open water habitats to examine how water hyacinth impacted macrofaunal abundance and diversity. We also examined how water hyacinth impacted predation risk by tethering killifish at increasing distances from the marsh edge in open water and water hyacinth habitats. We found that shrimp, blue crabs, and killifish were generally more abundant under water hyacinth, but there was no difference in species richness or diversity between the two habitats. In tethering studies, killifish survival was generally similar between water hyacinth and open water likely due to the increased abundance of blue crabs underneath water hyacinth. This result suggests water hyacinth does not alter predation risk overall, but may alter the distribution of blue crabs and piscivorous fish predators. We did observe short term increases in killifish survival underneath small migrating hyacinth patches indicating patch size may further affect blue crab colonization and predation. Our results indicate that water hyacinth alters the safety of marsh edge habitats by increasing blue crab abundance, ultimately increasing their encounter rates with fish prey. Climate driven increases in tropical storms and coastal restoration efforts that alter salinity gradients will likely increase the abundance of water hyacinth in estuarine marshes offering important post-disturbance habitat structure to fish and invertebrates and increasing predation rates of blue crabs along marsh edges.