Evaluating The Role Of Exotic Dreissenid Mussels In The Disappearance Of The Benthic Amphipod Diporeia Spp. In The Laurentian Great Lakes

Abstract

The burrowing amphipod Diporeia spp. has historically been the dominant macroinvertebrate and a high quality prey item for fish in benthic habitats of deep freshwater lakes of North America. In the mid-1990s, Diporeia populations began to decline in many of the Laurentian Great Lakes as exotic zebra and quagga mussels (Dreissena spp.) expanded after introduction from eastern Europe via transoceanic shipping. By 2010, this Great Lakes ecosystem health indicator had disappeared from vast areas of benthic habitat < 100 m depth in Lakes Erie, Ontario, Michigan, and Huron. Two common hypotheses for the decline of Diporeia are 1) dreissenids compete with the amphipod for food ("food limitation" hypothesis) or 2) dreissenids have carried some disease or toxin harmful to the amphipod ("pathogen" hypothesis). This study evaluated both hypotheses utilizing field and experimental approaches. The field component compared Diporeia 's response to dreissenid expansion in Lake Ontario to that in the Finger Lakes of central New York. Diporeia populations of Lake Ontario generally declined before quagga mussels expanded to depths >30 m, suggesting that remote ecosystem disruption by shallow mussels played a role. Diporeia populations in the Finger Lakes coexist with dreissenid mussels and represent a key anomaly to the pattern of decline seen in the Great Lakes. The experimental component of this study evaluated the pathogen and food limitation hypotheses by exposing Diporeia to dreissenids in controlled laboratory conditions. Short-term (28-day) exposures using different combinations of Diporeia and quagga mussel populations did not indicate that quagga mussel populations from Great Lakes or Finger Lakes carry pathogens or toxins harmful to Diporeia. Longer-term (90-day) exposures suggest that the direct presence of dreissenids can actually benefit Diporeia leading to weight gain and higher fecundity relative to no-mussel controls. Diporeia may utilize biodeposits or periphyton associated with quagga mussels. A feeding trial with terrestrial organic matter (partially decomposed leaf litter), an alternative food source available in Cayuga Lake, also led to weight gain for Diporeia although survival was low. Fatty acid tracers, particularly the monounsaturate palmitoleic acid (16:1w7), were useful for assessing the dietary role of diatoms for Diporeia. The findings of my dissertation suggest that dreissenids may not have initiated the Diporeia decline. However, the continued expansion of dreissenid mussels in lakes Michigan, Huron, and Ontario is driving ecosystem disruptions including the disappearance of the spring diatom bloom that have major ramifications for the survival of remnant profundal populations of Diporeia.