Free-floating Invasive Fern Affects Freshwater Marsh Ecosystem Structure: Changes to Water Quality And Chemistry, Aquatic Vegetation, Fish, and Invertebrates

Abstract

Aquatic invertebrates are important to the function freshwater marshes. They are vital to the transfer of energy from primary producers and organic matter to higher trophic levels. The structure of macroinvertebrate communities is directly associated with aquatic macrophyte diversity and abundance. Submerged macrophytes produce oxygen, are a food source, and provide physical habitat, which allows numerous aquatic organisms to exist. Introduction of the invasive free-floating aquatic fern giant salvinia, Salvinia molesta Mitchell, poses a risk to freshwater ecosystems through limited light penetration, decreased submerged macrophyte abundance, altered water quality and, changes in macroinvertebrate community structure and energy transfer. The objective of this dissertation was to understand the impacts of giant salvinia on freshwater structure, specifically the impacts to environmental conditions, native submerged macrophytes, macroinvertebrates and communities, including effect on aerial colonization. Field surveys revealed that giant salvinia affected environmental conditions through decreased light availability, dissolved oxygen, and habitat complexity, and increased concentration of orthophosphate and ammonium. These alterations changed macroinvertebrate community structure, relative to native submerged macrophytes, resulting in decreased macroinvertebrate richness and abundance. Total energy of the macroinvertebrate communities in giant salvinia infestations was greatly reduced, potentially resulting in a loss of transferable energy to adjacent trophic levels. A greenhouse study found that nutrient additions to the water accelerated the negative impact of giant salvinia on environmental conditions and submerged macrophyte biomass. While biomass decreased, the submerged macrophyte was able to tolerate the low light conditions under a full giant salvinia mat for three weeks. Finally, an outdoor mesocosm study showed that a full mat of giant salvinia disrupted the aquatic insect life cycle by limiting aerial colonization of aquatic insects, leading to communities with lower abundance and richness relative to native macrophyte. The inability to complete their lifecycle means individuals cannot replenish themselves, leading to diminished species pool and reduced energy potential. This research exhibited new, unstudied impacts from giant salvinia to ecosystem structure in freshwater marshes. Together these findings demonstrate larger disruptions to freshwater marshes from giant salvinia than previously reported, including large disruptions to the flow and transfer of energy within the aquatic ecosystem.