Paper No. 7
Presentation Time: 3:50 PM
DISTRIBUTIONAL CONTROLS AND BURROW CHARACTERISTICS OF THE MAYFLY HEXAGENIA IN KENTUCKY LAKE: IMPLICATIONS FOR THE STRATIGRAPHIC RECORD
The potential of biogenic structures to elucidate depositional environments in ancient lacustrine systems has not been realized, in spite of recent interest in the depositional processes and histories of lakes. This reflects the fact that relatively little is known about factors controlling the distribution of macrobenthic animals in modern lakes. Widespread and locally abundant occurrence of the burrowing mayfly Hexagenia offers an opportunity to evaluate factors controlling its distribution as well as the morphology and function of its burrow. Because mayflies appeared in the Late Carboniferous, and burrowing mayflies in the Early Cretaceous, paleoecological information about Hexagenia could be applied in interpretation of Cretaceous and Cenozoic lacustrine sequences. We investigated the distributional controls, behavior, and rate of sediment overturn of burrowing mayfly Hexagenia using SCUBA, PONAR grabs, and experimental aquariums in order to provide a uniformitarian basis for interpreting ancient lacustrine sediments. Burrowing Hexagenia modify lake sediments through mixing and oxygen pumping. They often create apartment-style burrow systems with multiple chambers vertically stacked, thus maximizing the amount of oxygenated water flowing through each burrow. Hexagenia burrows are U-shaped and bilaterally symmetrical. Morphology varies little between burrows, burrow diameter is constant within a burrow, and ratio of burrow length to burrow width is invariant. Burrows consistently are oriented perpendicular to the sediment-water interface (SWI), even on near-vertical channel-margins. The preferred perpendicular orientation of the burrows relative to the SWI can be used to determine the steepness of ancient lake floors. Modern Hexagenia has high dissolved oxygen (DO) requirements (> 1 ppm); if this were so in the past, abundance of Hexagenia burrows may be a proxy for high DO. Kentucky Lake provides an ideal setting to examine how Hexagenia population densities vary in relation to hydrodynamic regime and depositional processes. Population densities are highest along the margins of the lake near depositional sinks, and vary significantly across east/west and north/south transects of the lake apparently in response to changes in sediment consistency, compaction, and composition.