Benthic community composition and ecosystem function in southern Kachemak Bay, Alaska

Abstract

The benthic environment in southern Kachemak Bay, Alaska provides critical ecosystem services, processing organic matter from the land and the sea, and returning nutrients to the food web. In Kachemak Bay, concerns have arisen that marine agriculture (mariculture) operations in the form of salmon hatcheries and oyster farms may have negative impacts on the local environment. This thesis investigates the seafloor sediment environment to assess these potential impacts under salmon hatchery net pens in Tutka Bay Lagoon, and under an oyster farm in Jakolof Bay. We found that beneath the net pens, high waste inputs of fish food, feces and dead fish contribute to negative impacts on the local environment. Very few sediment organisms were found, and most are likely excluded from that habitat by lack of oxygen that arises due to inputs of excess organic matter. Furthermore, the hatchery site showed high oxygen and carbon fluxes associated with microbial and physical processes, because macrofaunal organisms were not present. Time series data show how the lagoon frequently experiences conditions that are outside of the regionally expected values and indicate that waste inputs from hatchery operations, together with the isolated nature of the lagoon, contribute to low oxygen conditions persisting in the deeper portion of the lagoon. Overall, the Tutka Bay Lagoon salmon hatchery has an adverse impact on the local environment. However, oyster farms sites did not show any negative impacts on this specific environment, being similar in terms of their community and ecosystem to non­ mariculture sites, which are good measures of ecosystem status.