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2007
Ekdahl, EJ, Teranes JL, Wittkop CA, Stoermer EF, Reavie ED, Smol JP.  2007.  Diatom assemblage response to Iroquoian and Euro-Canadian eutrophication of Crawford Lake, Ontario, Canada. Journal of Paleolimnology. 37:233-246.   10.1007/s10933-006-9016-7   AbstractWebsite

Diatom and geochemical data from Crawford Lake, Ontario, have been used to document limnological responses to periods of cultural disturbance resulting from native Iroquoian occupation of the watershed (1268-1486 AD) and Euro-Canadian agriculture and deforestation (1867 AD-present). Here, we further develop the high-resolution nature of the Crawford Lake sediment record to examine the physical, chemical and biological aspects of limnological response to human disturbances in the lake catchment area with exceptional detail. We report detailed diatom abundance and flux data for individual taxa from Crawford Lake, and further describe the relationship between assemblage composition and environmental conditions using canonical correspondence analysis (CCA). Diatom assemblage data are used to calculate diatom inferred-total phosphorus (DI-TP) concentrations for the past similar to 1,000 years. We also examine the diatom community response during and after periods of disturbance by Iroquoian and Euro-Canadian populations, and compare this response to existing geochemical proxies of lake production and new elemental geochemical indicators of catchment area erosion. In particular, we explore the differing limnological response to the two distinct periods of cultural eutrophication and examine the limnological processes that occurred during the period of low (or no) human activity (1487-1866 AD), when geochemical indicators of lake production recovered to pre-disturbance conditions, but diatom assemblages notably did not. Our results illustrate the highly susceptible nature of diatom communities to periods of anthropogenic disturbance, and emphasize that ecological indicators (such as diatom assemblages) should be included with other proxies (such as nutrient concentrations and physical characteristics) when assessing disturbance and recovery in lake systems.

2004
Ekdahl, EJ, Teranes JL, Guilderson TP, Turton CL, McAndrews JH, Wittkop CA, Stoermer EF.  2004.  Prehistorical record of cultural eutrophication from Crawford Lake, Canada. Geology. 32:745-748.   10.1130/g20496.1   AbstractWebsite

Cultural eutrophication-the process by which human activities increase nutrient input rates to aquatic ecosystems and thereby cause undesirable changes in surface-water quality-is generally thought to have begun with the start of the industrial era. The prehistoric dimension of human impacts on aquatic ecosystems remains relatively undescribed, particularly in North America. Here we present fossil plankton data (diatoms and rotifers), organic and inorganic carbon accumulations, and carbon isotope ratios from a 1000 yr sediment core record from Crawford Lake, Ontario, Canada. The data document increased nutrient input to Crawford Lake caused by Iroquoian horticultural activity from A.D. 1268 to 1486 and show how this increased nutrient input elevated lake productivity, caused bottom-water anoxia, and irreversibly altered diatom community structure within just a few years. Iroquoian settlement in the region declined in the fifteenth century, yet diatom communities and lake circulation never recovered to the predisturbance state. A second phase of cultural eutrophication starting in A.D. 1867, initiated by Canadian agricultural disturbance, increased lake productivity but had comparatively less impact on diatom assemblages and carbon-storage pathways than the initial Iroquoian disturbance. This study deepens our understanding of the impact of cultural eutrophication on lake systems, highlights the lasting influence of initial environmental perturbation, and contributes to the debate on the ecological impacts of density and agricultural practices of native North American inhabitants.

2000
Teranes, JL, Bernasconi SM.  2000.  The record of nitrate utilization and productivity limitation provided by δ15N values in lake organic matter - A study of sediment trap and core sediments from Baldeggersee, Switzerland. Limnology and Oceanography. 45:801-813. AbstractWebsite

The response of nitrogen isotopic composition of organic matter to changes in nitrate utilization, N-limitation and lake productivity was studied in Baldeggersee, a small eutrophic lake in central Switzerland. Nitrogen isotope ratios were measured on organic matter accumulation in sediment traps, collected daily from March 1995 through October 1996, and on individually sampled light and dark annual laminae from a 108-yr sediment core sequence (1885-1993). Nitrogen accumulation in the sediment traps averaged 0.04 g N m(-2) d(-1). delta(15)N values of the sediment trap material increased from 11 parts per thousand to 13 parts per thousand (atmospheric N-2) as primary productivity decreased surface water [NO3-] from 1.4 to 0.7 mg N L-1 during the seasonal stratified periods. Very small amounts of isotopically enriched organic matter (delta(15)N similar to 15-20 parts per thousand) of heterotrophic and/or detrital origin accumulated in the winter months. Nitrogen accumulation in the core sediments average 4 g N m(-2) per year. delta(15)N values of the sediment core material increased up-core (from delta(15)N similar to 6 parts per thousand to delta(15)N similar to 11 parts per thousand); several abrupt positive isotope shifts (>2 parts per thousand) occur in the upper Dart of the record. Comparison between sediment delta(15)N values and surface water [NO3-] measurements for the period of 1976-1993 reveals that the abrupt positive delta(15)N shifts occurred in years where unusually large phytoplankton blooms depleted surface waters nitrate to concentrations of <0.7 mg N L-1. A 3 parts per thousand negative delta(15)N shift, observed between 1973-1975 at the time of maximum anoxic conditions and meromixis, can be attributed to phytoplankton incorporation of ammonia, which was present in concentrations of up to 0.7 mg L-1 in the epilimnion. Preservation of isotopic shifts in the sediment core which are clearly related to water column processes, and the similarity of core top delta(15)N values to the weighted average N-isotopic composition of sediment trap material indicate that the sediment record reliably reflects the surface-generated delta(15)N signal. Our results from sediment trap and uppermost core samples provide the first conclusive evidence that N-isotopes in lacustrine organic matter record the increasing isotopic enrichment of surface water NO3- due to its utilization by phytoplankton. Artificial aeration of the Baldeggersee bottom water since 1982 has effectively reduced water column anoxia. Thus, water column denitrification, which could considerably increase the isotopic composition of residual nitrate, does not occur in the present lake. Even so, present day nitrogen isotopic values in the sediment core data are higher than most all previous periods suggesting that water column denitrification has never been a dominant influence on sediment delta(15)N values. Instead, the up-core 6 parts per thousand increase in delta(15)N values better corresponds to the documented history of external N-loading from agricultural runoff in the watershed over the last 100 years. These nitrogen sources have characteristically high delta(15)N values (10-20 parts per thousand) and could lead to progressive N-15-enrichment of the Baldeggersee dissolved inorganic nitrogen pool.

1997
Lotter, AF, Sturm M, Teranes JL, Wehrli B.  1997.  Varve formation since 1885 and high-resolution varve analyses in hypertrophic Baldeggersee (Switzerland). Aquatic Sciences. 59:304-325.   10.1007/bf02522361   AbstractWebsite

According to their microstratigraphical composition the laminated sediments of Baldeggersee are true varves. Two varve time-series from freeze cores taken in the deepest part (66 m) of Baldeggersee have been analysed for annual and seasonal layers. The varve time-series covers the period of 1885 to 1993. The two freeze-cores from Baldeggersee can be accurately correlated by means of distinct lay ers (e.g. marker varves, turbidites), lithological units, and varve measurements. The Baldeggersee varve chronology has been checked by independent high-resolution Cs-137 dating. Mobility of cesium has not been detected in Baldeggersee. A highly positive correlation between phosphorus concentration in the lake water and median grain size of calcite crystals has been observed and allows, together with the results of geochemical analyses, to hindcast the trophic state in Baldeggersee for periods where no limnological data are available. The main features of the Baldeggersee varve time-series can be summarized as follows: before 1885 there are packets of 5-10 varves interrupted by massive, homogeneous marl beds, indicating oscillating hypolimnetic oxygen levels until the lake ultimately became anoxic in 1885. Between 1885 and 1905 varves are thin, with a high carbonate content. Between 1905 and 1910 the thickness of varves increased steadily, and an increase in calcite grain size suggests a major step in eutrophication. Median grain size values increase again at the end of the 1940s, whereas the thickness of the seasonal lavers decreased between the early 1940s and the early 1960s. At the onset of the 1960s, during the time of highest epilimnetic phosphorus concentrations in Baldeggersee, the total phosphorus concentrations in the sediments as well as the varve thickness and the total accumulation rates all increased. At the beginning of the 1980s the median grain size decreased substantially as a result of lower phosphorus concentrations in Baldeggersee.