The Medieval Climate Anomaly and Little Ice Age in Chesapeake Bay and the North Atlantic Ocean

A new 2400-year paleoclimate reconstruction from Chesapeake Bay (CB) (eastern US) was compared to other paleoclimate records in the North Atlantic region to evaluate climate variability during the Medieval Climate Anomaly (MCA) and Little Ice Age (LIA). Using Mg/Ca ratios from ostracodes and oxygen isotopes from benthic foraminifera as proxies for temperature and precipitation-driven estuarine hydrography, results show that warmest temperatures in CB reached 16-17 °C between 600 and 950 CE (Common Era), centuries before the classic European Medieval Warm Period (950-1100 CE) and peak warming in the Nordic Seas (1000-1400 CE). A series of centennial warm/cool cycles began about 1000 CE with temperature minima of ~ 8 to 9 °C about 1150, 1350, and 1650-1800 CE, and intervening warm periods (14-15 °C) centered at 1200, 1400, 1500 and 1600 CE. Precipitation variability in the eastern US included multiple dry intervals from 600 to 1200 CE, which contrasts with wet medieval conditions in the Caribbean. The eastern US experienced a wet LIA between 1650 and 1800 CE when the Caribbean was relatively dry. Comparison of the CB record with other records shows that the MCA and LIA were characterized by regionally asynchronous warming and complex spatial patterns of precipitation, possibly related to ocean-atmosphere processes.     

Lake districts of north‐western Russia: identification of subregions based on analyses of hydrologic data

1. There are more than 400 000 lakes in the north‐western part of European Russia. Data from 50 gauging stations (located on lakes with water surface area ≤ 400 km²), within the region, was used to identify regions that exhibit common hydrologic patterns. 2. Analyses of common hydrologic patterns permitted the division of the overall area into four regions within which lakes exhibit common hydrologic and limnologic features.     

Relationships between DOC concentration and epilithon stable isotopes in boreal lakes

1. Littoral biota in boreal lakes are known to assimilate epilithon. Being able to predict the stable isotopic composition of these alga will help to identify those systems in which δ¹³C and δ¹⁵N analysis can be used in foodweb investigations of allochthony and biomagnification.
2. In a survey of 15 boreal lakes, the concentration of dissolved organic carbon (DOC) explained 76% of the variation in epilithon δ¹³C, and 86% of the variation in epilithon δ¹⁵N.
3. Because both δ¹³C and δ¹⁵N values were depressed and similar to terrestrial values in humic (high DOC) lakes, it will be more difficult to successfully employ stable isotopic techniques for estimating allochthony in such systems. Lower δ¹⁵N values in humic lakes also indicate that trophic positions estimated by stable isotopes are not directly comparable to those of similar biota inhabiting clearwater lakes, unless autochthonous baseline corrections are made.     

Resource competition among saline-lake diatoms under varying N/P ratio, salinity and anion composition

1. A series of resource competition experiments were performed with four diatom taxa isolated from saline lakes in the Northern Great Plains of North America. Three variables were tested in a factorial design: N/P ratio (6 : 1, 12 : 1, 24 : 1, 48 : 1), salinity (5, 8, 11‰), and anion composition (sulphate versus bicarbonate-dominated medium).
2. Coexistence of Chaetoceros elmorei , Cyclotella quillensis , and Cymbella pusilla occurred at the two lowest N/P ratios. These three taxa have similar N requirements and thus competitive exclusion by the taxon with the lowest N requirement may have required more time.
3. Anomoeoneis costata , which had the highest nitrogen requirements and slowest growth rate of these four taxa, declined in all cases. Cyclotella biovolume concentration consistently declined at the two highest N/P ratios, indicating a higher P requirement, which is consistent with its distribution pattern.
4. In general, we found opposite trends with respect to all variables for Chaetoceros , a taxon that mainly inhabits sulphate systems, versus Cymbella , a bicarbonate-associated taxon. Our results suggest that Cymbella may be found mainly in bicarbonate systems because of a higher C requirement, whereas Chaetoceros may be less abundant in these systems, in part, because of competitive interactions with bicarbonate-associated taxa.     

Associations of diatoms in the surficial sediments of lakes and peat pools in Atlantic Canada

The distribution of diatom frustules in the surficial sediments of lakes and ombrotrophic peat pools in Atlantic Canada are examined especially in relation to pH and bog lake succession. Sedimentary diatom associations for peat pools, strongly acidic lakes, and weakly acidic lakes are characterized. These associations and the derived diatom-pH calibration functions will facilitate improved paleoecological interpretation of natural ontogenetic pathways and assessments of recent anthropogenic impacts.     

The relationship between surface sediment diatom assemblages and pH in 33 Galloway lakes: Some regression models for reconstructing pH and their application to sediment cores

The species composition of surface sediment diatom assemblages in 33 Galloway lakes, pH range c. 4.5 to 7.4, is related in a statistically significant manner to water acidity (or to factors closely associated with pH). Predictive models of summer mean pH, using simple regression equations of Index B (Renberg & Hellberg, 1982) with both Scandinavian and Galloway data sets, and multiple regression equations using diatom pH preference groups and individual species, are described and applied to fossil diatoms in six sediment cores. Although multiple regression of individual species gives the highest correlation coefficient (r²=0.87) in the modern data set this method is least appropriate for reconstructing pH values from fossil material where predictor species are often absent. Of the four methods examined here it is suggested that multiple regression of diatom preference groups is probably the most suitable for pH reconstructions from sediment cores.     

Empirical relationships between profundal macroinvertebrate biomass and environmental variables in boreal lakes of Alberta, Canada

1. We empirically modelled profundal macroinvertebrate biomass (PMB) from collections of macroinvertebrates and a suite of trophic, water chemistry and morphometric variables from 26 lakes located within the Boreal Mixedwood and Boreal Subarctic ecoregions of Alberta, Canada.
2. Hypolimnetic dissolved oxygen (DO) concentrations (mg L⁻¹) explained 37% of the variance in PMB ((g m⁻²)^0.1) as a quadratic function, whereas water temperature (°C) explained 23% of variance in a linear relationship.
3. In contrast with other studies based on oligotrophic systems, indirect indicators of food availability (i.e. chlorophyll a and total phosphorus concentrations and Secchi depth) were not significant predictors of PMB in this study.
4. The predictive power of our models may be improved by obtaining more frequent (seasonal) estimates of PMB and hypolimnetic DO concentration and by quantifying biotic factors such as competition, predation, life history phenologies and recruitment success.     

白洋淀最低生态水位研究

由于自然条件的变化、工农业的不合理开发、人口的剧增以及水库和涵闸水利设施的盲目建设等,引发了白洋淀一系列生态环境问题,主要表现为干淀、水污染、生态环境退化、泥沙淤积、湖泊萎缩以及航运中断等。恢复和重建已遭破坏的白洋淀湿地生态系统,并使其至少维持在最低生态水位上是至关重要的问题。在分析白洋淀生态系统功能的基础上,利用水量面积法、最低年平均水位法、年保证率设定法和功能法等4种方法对其最低生态水位进行分析和计算,计算结果分别为7.32 m、7.33m、7.2 8m和7.2 7m。通过合理性分析,认为取它们的平均值即7.30 m作为白洋淀最低生态水位是合理的,符合白洋淀实际情况。