Crawford Lake Consumers: Water Column and Palynological Studies

Abstract

Despite their important role in lake ecosystems, the fossil record of consumers has been underutilized compared to the remains of algae and plants in paleoenvironmental studies. Cladocerans, chironomids, and testate amoebae were found in palynological preparations of sediments throughout Crawford Lake (a unique meromictic lake in Ontario, Canada), but rotifer lorica and cysts of aloricate ciliates were only preserved in seasonally laminated sediments in the monimolimnion of this lake, demonstrating the exceptional preservation potential in this portion of the lake water column. Relatively diverse assemblages of consumer palynomorphs were associated with anthropogenic impact on this lake, and the annual chronological resolution afforded by varves allowed these to be related to historic events in the small watershed, the most notable being the operation of a lumber mill on the south shore of the lake, and to archeological and pollen evidence of several phases of agricultural settlement between the 13th and 15th centuries. Lower diversity of consumer palynomorphs between the Iroquoian and Euro-Canadian settlement phases (i.e., late 15th through early 19th centuries) mainly reflects the sharp decline in most rotifer taxa and the cladoceran Bosmina longirostris, but the persistence of the rotifers Keratella hiemalis and Kellicottia longispina is evidence that the lake ecosystem did not return to pre-human impact conditions after abandonment of the Iroquoian settlement. Understanding how the trophic level of consumers responded to natural and anthropogenic stressors relied heavily on rarely preserved rotifer lorica, but the observation that the cladoceran B. longirostris tended to thrive relative to the typically more common Daphnia at times of cultural eutrophication may have broader application in palynological studies of lakes. Contrary to long-standing assumption, the exceptional preservation of organic-walled microfossils in undisturbed seasonal laminae in the deep basin of Crawford Lake cannot be explained by anoxia. Observations of seasonal migration of zooplankton to and from the mixolimnion in conjunction with instrumental measurements of dissolved oxygen, temperature, and conductivity in the water column almost monthly from October 2019 through September 2020 demonstrated that this meromictic lake is uncharacteristically well-oxygenated below the chemocline. Instead, exceptional preservation is attributed to the lack of bioturbation and the suppression of bacterial decomposition in the cold, nearly brackish, highly alkaline bottom waters devoid of benthos larger than ostracods able to migrate into the deep basin via interstitial waters. The annual resolution possible in sediments deposited in a typically well-oxygenated setting is an attractive feature in the search for a potential GSSP to define the Anthropocene Epoch using plutonium from fallout of thermonuclear testing as a primary marker.