Source of Ovarian Preovulatory Progesterone

AN increase in plasma progesterone has been detected before ovulation in several species. There is, however, no definitive information as to which ovarian compartments produce this progesterone. Previous results with the golden hamster have demonstrated a preovulatory progesterone surge after the critical period for gonadotrophin release on the afternoon of pro-oestrus (day 4)^1,2. This is paralleled by the development of 3β-hydroxysteroid dehydrogenase (3βHSD) activity in the theca interna and granulosa of pre-ovulatory follicles³, suggesting they could be a source of preovulatory progesterone. As the interstitium also contains 3βHSD activity, it could be an additional source of progesterone³. Hamster corpora lutea, however, have negligible 3βHSD activity during pro-oestrus³ and are not capable of producing progesterone when isolated and incubated in vitro (Leavitt et al., unpublished work).     

Distribution and regulation of urea in lakes of central North America

1. Urea accounts for ～50% of global nitrogen (N)‐based fertiliser; however, little is known of the factors regulating its distribution and abundance in freshwaters. Improved understanding of urea biogeochemistry is essential because its use as fertiliser is expected to double by 2050 and because pollution with urea can promote outbreaks of toxic cyanobacteria in phosphorus (P)‐rich lakes in regions with intensive agricultural or urban development. 2. Biweekly measurements of urea concentration and diverse limnological variables (water chemistry, hydrology, algae, zooplankton) were taken during two summers (2008, 2009) in a chain of seven productive lakes within a 52 000‐km² catchment in central Canada to quantify environmental and anthropogenic correlates of temporal and spatial patterns of urea occurrence. 3. Mean (±SD) urea concentrations varied between 29 ± 14 and 132 ± 65 μg N L^?1, generally increased from headwater to downstream sites and represented 10–50% of bioavailable N (as sum of , and urea). Principal components analysis demonstrated that urea concentrations were elevated in agriculturally impacted lakes with abundant dissolved organic and inorganic nutrients (N, P, C) and low O₂ concentrations, but were not correlated consistently with plankton abundance or community composition. Urea concentrations were more than twofold greater in lakes receiving N from cities than in agriculturally affected basins, despite low summer concentrations of urea in tertiary‐treated urban effluent (c. 50% of lake values). Multiple regression models evaluated using Akaike Information Criterion showed that mean water‐column O₂ concentration was the single best predictor of in situ urea concentrations (r² = 0.91, P = 0.002), but that urea concentrations were also correlated significantly with changes in longitudinal position and Secchi depth and with concentrations of , non‐urea dissolved organic N (DON) and dissolved inorganic carbon. 4. Additional seasonal surveys of up to 69 closed‐basin lakes within a 100 000‐km² region during 2004 and 2008 revealed that urea was abundant in 100% of measured sites and exhibited concentrations (81 ± 48 μg N L^?1) similar to those observed in lakes with surface drainage (58 ± 38 μg N L^?1). Further, non‐urea DON accounted for 50–99% of the total dissolved N pool in both open‐ and closed‐basin lakes. 5. When combined with an extensive literature review and previous mass‐budget analyses of the study lakes, these findings allowed the development of a first‐generation model of the mechanisms regulating urea content of P‐rich lakes of central North America. In this model, water‐column concentrations of urea are predicted to be regulated mainly by algal decomposition in anoxic environments (sediments, hypolimnion), followed by redistribution into surface waters. Consequently, anthropogenic activities can increase the urea content of lakes by stimulating primary production, sedimentation and deepwater anoxia and by increasing influx of undegraded urea from agricultural and urban sources.     

Combining palaeolimnological and limnological approaches in assessing lake ecosystem response to nutrient reduction

1. Palaeolimnological data and limnological time‐series data are highly complementary. Sediment records extend time‐scales, integrate subannual variability and expand the range of sites that can be studied, but they suffer from taphonomic biases and occasionally from uncertain chronology. Observational time‐series data, on the other hand, are highly resolved but are very limited in extent both in space and time. 2. Palaeolimnological and observational data‐sets need to be combined in oligotrophication research to establish (i) the past and present status of lakes needed to identify reference conditions; (ii) changes in ecosystem state; (iii) responses to nutrient reduction; and (iv) the potential role of other factors (e.g. additional stressors, climate change) that might confound predictions of future state.     

Disappearance of the Incest Taboo: A Cross-Cultural Test of General Evolutionary Hypotheses

Employing a cross-cultural sample of 121 societies, this research tests and supports three hypotheses offered by Yehudi Cohen (1978) concerning the relationship between the general evolution of society and the extension of the incest taboo. Cohen generally proposes that the number of relatives included in the incest taboo will be reduced as societies become technologically and socially more complex. More specifically, Cohen identifies the development of trade institutions as eliminating the need for extended incest regulations. Furthermore, Cohen proposes that as the incest taboo contracts and becomes less important, violations of this taboo are treated less severely.     

Modelling carbon isotope fractionation in tree rings based on effective evapotranspiration and soil water status

Environmental influences on carbon isotope fractionation in tree rings require further elucidation in order to use this parameter as a biological marker of climatic variations. δ¹³C values in tree-ring cellulose of beech (Fagus sylvatica L.) were analysed for the period from 1950 to 1990. A bioclimatic model of water balance was used to give the actual evapotranspiration as well as the soil water content on a daily basis. δ¹³C shows a significant decrease from –24·5‰ to –25‰ over this period. Internal CO₂ concentration changes from 200 to 220 ppm in relation with the rise of atmospheric CO₂. Beside a slight non-significant inter-individual variation, a large year-to-year variation exists. The relative extractable soil water of July, combined with the value of δ¹³C for the previous year, predicts as much as 70% of this variance. Air temperature or precipitation accounted for less variation. δ¹³C is strongly correlated with basal area increment, but appeared a more reliable indicator of water status at the stand level.     

Interdecadal declines in flood frequency increase primary production in lakes of a northern river delta

Human activities and climate change have greatly altered flooding regimes in many of the world's river deltas, but the impact of such changes remains poorly quantified on decadal to multidecadal timescales. This study identified the response of delta lake primary production (measured as the concentration of sedimentary pigments) to variations in flood frequency using spatial surveys and paleolimnological analyses of lakes in the Peace‐Athabasca Delta (PAD), Canada. Surveys of 61 lakes spanning a range of hydrological conditions showed that those lakes that received flood waters less frequently were associated with elevated algal production (surface sedimentary pigments) and, in some lakes, increased growth of emergent macrophytes and epiphytic diatoms. Paleolimnological analyses of five lakes corroborated the contemporary spatial survey results by showing that production of pigments from most algal groups increased during recent periods of lower flood frequency in the 20th century as determined from increases in cellulose‐inferred lake‐water oxygen isotope composition and plant macrofossils, but remained stable in a ‘reference’ basin. In general, past periods of elevated algal production coincided with the increased abundance of submerged macrophytes or emergent vegetation that provide habitat for attached algae. These results suggest that interdecadal declines in river discharge arising from increased aridity, hydrologic regulation or consumptive water use will cause long‐term increases in primary production and alter ecosystem processes (carbon sequestration, biological diversity) in aquatic delta ecosystems similar to the PAD where lakes become nutrient‐rich in the absence of flooding.     

Hierarchical regulation of pelagic invertebrates in lakes of the northern Great Plains: a novel model for interdecadal effects of future climate change on lakes

Endorheic lakes of the northern Great Plains encompass a wide range of environmental parameters (e.g., salinity, pH, DOC, Ca, nutrients, depth) that vary 1000‐fold among sites and through the past 2000 years due to variation in basin hydrology and evaporative forcing. However, while many environmental parameters are known to individually influence zooplankton diversity and taxonomic composition, relatively little is known of the hierarchical relationships among potential controls or of how regulatory mechanisms may change in response to climate variation on diverse scales. To address these issues, we surveyed 70 lakes within a 100 000 km² prairie region to simulate the magnitude of environmental change expected to occur over 100–1000 years and to quantify the unique and interactive effects of diverse environmental parameters in regulating pelagic invertebrate community structure at that scale. Multivariate analyses showed that salinity was the principal correlate of changes in invertebrate composition among lakes, with a sequential loss of taxa between salinities of 4 and 50 g total dissolved solids L^?1 until one to two species predominated in highly saline systems. In contrast, changes in the concentrations of Ca²⁺ and other mineral nutrients exerted secondary controls of invertebrate assemblages independent of salinity, whereas lake depth provided a tertiary regulatory mechanism structuring species composition. In contrast to these large‐scale hierarchical patterns, seasonal surveys (May, July, September) of a subset of 21 lakes in each of 2003–2005 revealed that annual meteorological variation had no measurable effect on pelagic invertebrates, despite large differences in temperature, precipitation, and evaporation arising from regional droughts. Together these findings show that pelagic invertebrate communities in saline lakes are resilient to interannual variability in climate, but suggest that lakes of the northern Great Plains may provide a sensitive model to forecast centennial effects of future climate change.