Aquatic invasive species impacts in Hawaiian soft sediment habitats

The impacts of invasive species on coral reefs in Hawaii have been reported for over 35 years. In contrast, there are immense gaps in understanding these impacts on other coastal habitats, such as lagoons and wetland ponds, despite a diversity of psammophytic, canopy-forming habitats including the seagrasses Ruppia maritima and Halophila hawaiiana. This dissertation investigated two invasive species associated with sediment-dominated habitats, a bryopsidalean alga and tilapia.;The invasion of the alga Avrainvillea amadelpha into Halophila hawaiiana seagrass meadows was assessed in a four-year study. Independent experiments elucidated how competitive these two co-occurring species are under different physical disturbances and temporal scales. The meadow suppression hypothesis was tested; it predicts that healthy seagrass meadows have evolved mechanisms to inhibit bryopsidalean algae meadow development. Data showed that although the algal meadows contained about 95% more biomass than the seagrass meadows, Halophila was the superior competitor, supporting the meadow suppression hypothesis .;A survey across the Main Hawaiian Islands revealed that Ruppia maritima was scarce despite extensive suitable habitat in waterbird sanctuaries. Empirical work supported the hypothesis that tilapia catastrophically eliminated the seagrass from habitats where the fish invaded. Excessive consumption by tilapia resulted in a phase shift from seagrass habitat to microphytobenthos. The capacity for tilapia to opportunistically shift their diet to microphytobenthos is a significant trait in this invasion process. This is the first report of invasive fish negatively impacting a seagrass, as well as, the first experimental evidence of an introduced fish negatively affecting biodiversity in Hawaiian coastal habitats.;Baseline surveys near the first report of Avrainvillea amadelpha found a previously unknown deep-water alga, belonging to the Udoteaceae, forming vast meadows that extended to 90 m. Avrainvillea amadelpha was found to 70 m co-occurring with the unknown alga. Based on the position of the unnamed alga in a ribosomal DNA phylogeny of the Udoteaceae and a morphological study, the unnamed green alga is proposed as a new genus and species.;The results presented in this dissertation demonstrate for the first time that, similar to corals reefs in Hawaii, vegetated soft bottom habitats are also contending with invasive species.