Donna R. Francis

Quantitative paleotemperature estimates from d 18 O of chironomid head capsules preserved in arctic lake sediments

A paleoenvironmental perspective of temperature change is paramount to understanding the significance of recent warming in the Arctic. Late Quaternary sediments from many arctic lakes provide environmental archives with decadal resolution, but reconstructions are hampered by the relative insensitivity of many traditional proxies to temperature. Here, we show that the δ18O of head capsules of chironomid larvae are equilibrated with the δ18O of lakewaters in which they live. In suitable lakes, lakewater δ18O is controlled by the δ18O of local precipitation, which is strongly correlated to mean annual air temperature (MAT). From this correlation, chironomid δ18O can be used to examine past changes in MAT. We illustrate the potential of this novel approach to paleothermometry with examples from two arctic lakes that reveal strong regional paleoclimatic gradients in the early Holocene.

Recent changes in a remote Arctic lake are unique within the past 200,000 years

The Arctic is currently undergoing dramatic environmental transformations, but it remains largely unknown how these changes compare with long-term natural variability. Here we present a lake sediment sequence from the Canadian Arctic that records warm periods of the past 200,000 years, including the 20th century. This record provides a perspective on recent changes in the Arctic and predates by approximately 80,000 years the oldest stratigraphically intact ice core recovered from the Greenland Ice Sheet. The early Holocene and the warmest part of the Last Interglacial (Marine Isotope Stage or MIS 5e) were the only periods of the past 200,000 years with summer temperatures comparable to or exceeding todays at this site. Paleoecological and geochemical data indicate that the past three interglacial periods were characterized by similar trajectories in temperature, lake biology, and lakewater pH, all of which tracked orbitally-driven solar insolation. In recent decades, however, the study site has deviated from this recurring natural pattern and has entered an environmental regime that is unique within the past 200 millennia.

A record of hypolimnetic oxygen conditions in a temperate multi-depression lake from chemical evidence and chronomid remains

A multiproxy paleolimnological study of Douglas Lake, Michigan, was undertaken to elucidate the history of productivity and oxygen depletion in three basins of this multi-depression lake. Indicators investigated in three dated cores included chlorophyll a, Fe and Mn stratigraphy, and fossil chironomid assemblages. The coring sites were chosen to correspond to modern studies of oxygen depletion rates, and to determine if conclusions reached in these studies were supported by paleolimnological evidence. Stratigraphies of chlorophyll a, Fe and Mn indicate that two of the basins, South Fishtail Bay and Fairy Island, have been eutrophic and anoxic for a long period of time, predating European settlement. The third basin, Grapevine Point, has been consistently less productive, and had less severe oxygen depletion. Results of the chironomid analysis agree with these conclusions, including a change from mesotrophic to eutrophic indicator taxa in the Grapevine Point basin. All three cores show evidence of increasing trophic state in the most recent sediments, supporting some of the conclusions reached in the modern studies. It is also demonstrated that deforestation of the watershed had profound effects on littoral chironomid assemblages. Paleolimnological investigations also demonstrated the individual nature of the separate basins in Douglas Lake.

Response of small New England ponds to historic land use

This palaeolimnological study addresses whether the timing, magnitude and nature of lake-ecosystem changes closely track changes in land-use intensity and forest cover in the watershed, and the extent to which lakes retuni to pre-disturbance states following the substantial long-term declinie in human activity that is typical for much of the rural eastern United States. Land-use intensity in the watersheds increased rapidly witlh European settleminent and forest clearance, peaked in the mid-nineteenth century when 60-80% of the land was cleared for agriculture, and then declined to the present as natural reforestation resulted in 65-90% forest cover. Land-use intensity in the three watersheds studied ranged from limited logging to extensive clearance for pasturing and to total clearance for pasture and tillage. In contrast to many studies in which human activity continues to increase throughout the settlement period, all three watersheds now support mature, growing forest and are in their most natural condition in the last 200 years. Dated cores from three ponds were analysed for pollen, fossil chironomids, percenit organic matter and sedimentation changes to compare lake and vegetation change among sites. Increased sedimenit-accumulation rates and small increases in productivity occurred in all lakes during the settlement period. Both tremain higher thani pre-settlement levels, indicating that despite lengthy periods without disturbance and a return to completely forested conditions, the systems and sediment records have not returned to pre-disturbance states. In contrast with results from many other palaeolimnological studies, the magnitude of lake response was slight, probably due to the low intensity of nineteenth-century agriculture and the small watershed sizes.

Chironomids record terrestrial temperature changes throughout Arctic interglacials of the past 200,000 yr

Quaternary interglacial periods provide glimpses of a warmer Arctic and useful perspectives on possible future conditions, but records of Arctic terrestrial conditions over multiple interglacial periods are rare. Here, we take advantage of a site in the Canadian Arctic where lacustrine sediments representing the past three interglacial periods are preserved in an extant lake. We use subfossil insects (chironomids) preserved in this exceptional sedimentary archive to derive temperature reconstructions through the Holocene up to A.D. 2005, through the Last Interglacial sensu stricto (marine isotope stage or MIS 5e), and a portion of the penultimate interglacial (MIS 7). Chironomid-inferred temperatures are warmest for the early Holocene and MIS 5e, two periods with enhanced Northern Hemisphere insolation forcing relative to today. Twentieth-century warming at this site apparently caused the recent extirpation of cold stenothermous chironomid taxa. Assemblages from MIS 5e have close analogs in modern training set data as determined by squared-chord distance, and MIS 5e species assemblages are very similar to Holocene assemblages at this site. MIS 7 sediments record summer temperatures similar to those of the mid- to late Holocene, followed by a descent into glacial conditions. Even MIS 7 chironomid assemblages, dating back ∼200,000 yr, have close modern analogs. These lake sediments also provide direct evidence for a period of regional deglaciation between MIS 5e and the Holocene (most likely MIS 5a). To our knowledge, the data presented here represent the longest paleotemperature record thus far generated using chironomids. The existence of close modern analogs for ancient chironomid assemblages at Lake CF8 suggests that this method can provide useful paleotemperature estimates extending back hundreds of millennia.

Effect of Substrate on Colonization of Experimental Ponds by Chironomidae (Diptera)

ABSTRACT The influence of substrate type on larval chironomid community structure was studied in a controlled experiment utilizing small artificial ponds. Wading pools containing three types of sterilized sediments of different particle size and organic content were colonized naturally over two eight-week periods. Total abundance and species composition varied with substrate type. Clay treatment ponds had the lowest abundance and were dominated by Chironomus spp. Gravel and sand-silt treatment ponds were dominated by Kiefferulus dux. Results agree with field studies and indicate that substrate is important in structuring chironomid assemblages in lakes and ponds. Data obtained using multiplate samplers did not agree well with data from benthic samples, indicating that artificial substrates do not always provide a representative sample of the benthic habitat.

Additional Data on Mercury Accumulation in Northern Michigan River Otters

ABSTRACT This study documented mercury contamination in river otter (Lutra canadensis) in the Keweenaw Bay area, northern Michigan. Total mercury concentrations in liver and kidney from 11 otters were determined by atomic absorption spectroscopy and ranged from 0.79 to 4.07 μg g−1 wet weight. There were no significant differences in mercury concentrations between males and females, or between tissue types. A significant difference did exist between mercury levels in mature animals and those in immature animals (<1 year old). Data indicate that otters in areas with no known point sources are contaminated with mercury.

Bryozoan statoblasts in the recent sediments of Douglas Lake, Michigan

Statoblast valves produced by the freshwater bryozoan Plumatella nitens were recovered in three sediment cores from Douglas Lake, Michigan. Douglas Lake is a multi-depression lake of glacial origin. The region was heavily logged from 1880 to 1920. Sediment cores were taken from three of the seven depressions, and dated using 210Pb isotopes and pollen. Sedimentation rates were very low in the Grapevine Point core as compared to the other two cores. Concentrations of statoblasts ranged from three to 140 ml-1 of wet sediment. Profiles of statoblast concentrations and accumulation rates indicate a sharp decline in Plumatella populations corresponding to the time the Douglas Lake watershed was being clear-cut. It appears that logging and the resulting increase in erosion had adverse effects on bryozoan populations, and possibly on the entire littoral zone of the lake. High sediment loads to the lake could have caused mortality in Plumatella by interfering with the feeding of these animals, or by decreasing numbers of macrophytes which are frequently used as substrate. Populations of Plumatella have not recovered to pre-disturbance levels.

Mercury Distribution in the Biota of a Great Lakes Estuary: Old Woman Creek, Ohio

A study of mercury contamination in a wetland ecosystem was undertaken to assess degree of contamination in various compartments of the food web, and determine the most important routes for mercury to enter fish species. Old Woman Creek is a freshwater estuary on the southern shore of Lake Erie and has been recognized as an important nursery area for fish. Although there is no point source of mercury to the wetland, there is evidence of biomagnification of mercury in the food web. Mercury concentrations in water were not detectable (< 20 ng/L) by methods used in this study. The sediments had a mean concentration of 0.04 /μg total Hg/g wet weight. Zooplankton, benthos, and macro-phytes had less than 0.05 μg total Hg/g wet weight, but mean concentrations in fish tissue ranged from 0.001 to 0.636 μg total Hg/g wet weight. There was a positive correlation between mercury concentration and both length and weight in channel catfish Ictalurus punctatus and common carp Cyprinus carpio. In general, channel catfish accumulated higher levels of mercury than common carp, suggesting that the pelagic food web may be more efficient in delivery of mercury (to channel catfish) than the benthic pathway (to common carp). The highest value for fish occurred in a bowfin Amia calva. Tissue from one great blue heron Ardea herodias had the highest concentration at 1.62 μg total Hg/g wet weight. In this system, as in others, top predators tended to accumulate the highest mercury levels and were therefore most susceptible to mercury poisoning, even at sublethal levels.

Distribution of Midge Remains (Diptera: Chironomidae) in Surficial Lake Sediments in New England

Abstract A survey of larval midge remains from surficial sediments in 37 New England lakes was undertaken in order to relate midge distributions to environmental factors. The lakes are located along a transect from northern New Hampshire to southern Connecticut. The midges proved to be a very diverse group of insects in these freshwater habitats. A total of 65 chironomid taxa were recovered. Canonical correspondence analyses indicated that the environmental variables which best explain the distribution of chironomid taxa were mean July air temperature, percent sediment organic matter, pH, and lake depth. This knowledge about the relationship between midge distribution and mean July air temperature can be applied to midge assemblages preserved in older lacustrine sequences to improve our understanding of past environmental conditions in the region.