Competition for Light and Nutrients in Layered Communities of Aquatic Plants

Dominance of free-floating plants poses a threat to biodiversity in many freshwater ecosystems. Here we propose a theoretical framework to understand this dominance, by modeling the competition for light and nutrients in a layered community of floating and submerged plants. The model shows that at high supply of light and nutrients, floating plants always dominate due to their primacy for light, even when submerged plants have lower minimal resource requirements. The model also shows that floating-plant dominance cannot be an alternative stable state in light-limited environments but only in nutrient-limited environments, depending on the plants’ resource consumption traits. Compared to unlayered communities, the asymmetry in competition for light—coincident with symmetry in competition for nutrients—leads to fundamentally different results: competition outcomes can no longer be predicted from species traits such as minimal resource requirements ( rule) and resource consumption. Also, the same two species can, depending on the environment, coexist or be alternative stable states. When applied to two common plant species in temperate regions, both the model and field data suggest that floating-plant dominance is unlikely to be an alternative stable state

Dutch iliac stent trial:Long-term results in patients randomized for primary or selective stent placement

Purpose: To determine long-term results of the prospective Dutch Iliac Stent Trial. Materials and Methods: The study protocol was approved by local institutional review boards. All patients gave written informed consent. Two hundred seventy-nine patients (201 men, 78 women; mean age, 58 years) with iliac artery disease were randomly assigned to undergo primary stent placement (143 patients) or percutaneous transluminal angioplasty (PTA) with selective stent placement in cases in which the residual mean pressure gradient was greater than 10 nun Hg across the treated site (136 patients). Before and at 3, 12, and 24 months and 5-8 years after treatment, all patients underwent assessment, which included duplex ultrasonography (US), ankle-brachial index (ABI) measurement, Fontaine classification of symptoms, and completion of the Rand 36-Item Health survey for quality-of-life assessment. Treatment was considered successful for symptoms if symptoms increased at least one Fontaine grade, for ABI if ABI increased more than 0.10, for patency if peak systolic velocity ratio at duplex US was less than 2.5, and for quality of life if the RAND 36-Item Health Survey score increased more than 15 points. Effects of both treatments on symptoms, quality of life, patency, and ABI were compared by using survival analyses. Results: Patients who underwent PTA and selective stent placement had better improvement of symptoms (hazard ratio [HR], 0.8; 95% confidence limits [CLs]: 0.6, 1.0) than did patients treated with primary stent placement, whereas ABI (HR, 0.9; 95% CLs: 0.7, 1.3), iliac patency (HR, 1.3; 95% CLs: 0.8, 2.1), and score for quality of life for nine survey dimensions did not support a difference between treatment groups. Conclusion: Patients treated with PTA and selective stent placement in the iliac artery had a better outcome for symptomatic success compared with patients treated with primary stent placement, whereas data about iliac patency, ABI, and quality of life did not support a difference between groups. (c) RSNA, 200

Nitrogen fixation does not axiomatically lead to phosphorus limitation in aquatic ecosystems

A long-standing debate in ecology deals with the role of nitrogen and phosphorus in management and restoration of aquatic ecosystems. It has been argued that nutrient reduction strategies to combat blooms of phytoplankton or floating plants should solely focus on phosphorus (P). The underlying argument is that reducing nitrogen (N) inputs is ineffective because N 2 -fixing species will compensate for N deficits, thus perpetuating P limitation of primary production. A mechanistic understanding of this principle is, however, incomplete. Here, we use resource competition theory, a complex dynamic ecosystem model and a 32-year field data set on eutrophic, floating-plant dominated ecosystems to show that the growth of non-N 2 -fixing species can become N limited under high P and low N inputs, even in the presence of N 2 fixing species. N 2 -fixers typically require higher P concentrations than non-N 2 -fixers to persist. Hence, the N 2 fixers cannot deplete the P concentration enough for the non-N 2 -fixing community to become P limited because they would be outcompeted. These findings provide a testable mechanistic basis for the need to consider the reduction of both N and P inputs to most effectively restore nutrient over-enriched aquatic ecosystems

A large sample study of spin relaxation and magnetometric sensitivity of paraffin-coated Cs vapor cells

We have manufactured more than 250 nominally identical paraffin-coated Cs vapor cells (30 mm diameter bulbs) for multi-channel atomic magnetometer applications. We describe our dedicated cell characterization apparatus. For each cell we have determined the intrinsic longitudinal, \sGamma{01}, and transverse, \sGamma{02}, relaxation rates. Our best cell shows \sGamma{01}/2\pi\approx 0.5 Hz, and \sGamma{02}/2\pi\approx 2 Hz. We find a strong correlation of both relaxation rates which we explain in terms of reservoir and spin exchange relaxation. For each cell we have determined the optimal combination of rf and laser powers which yield the highest sensitivity to magnetic field changes. Out of all produced cells, 90% are found to have magnetometric sensitivities in the range of 9 to 30 fTHz. Noise analysis shows that the magnetometers operated with such cells have a sensitivity close to the fundamental photon shot noise limit

Potential energy landscape-based extended van der Waals equation

The inherent structures ({\it IS}) are the local minima of the potential energy surface or landscape, $U({\bf r})$, of an {\it N} atom system. Stillinger has given an exact {\it IS} formulation of thermodynamics. Here the implications for the equation of state are investigated. It is shown that the van der Waals ({\it vdW}) equation, with density-dependent $a$ and $b$ coefficients, holds on the high-temperature plateau of the averaged {\it IS} energy. However, an additional ``landscape'' contribution to the pressure is found at lower $T$. The resulting extended {\it vdW} equation, unlike the original, is capable of yielding a water-like density anomaly, flat isotherms in the coexistence region {\it vs} {\it vdW} loops, and several other desirable features. The plateau energy, the width of the distribution of {\it IS}, and the ``top of the landscape'' temperature are simulated over a broad reduced density range, $2.0 \ge \rho \ge 0.20$, in the Lennard-Jones fluid. Fits to the data yield an explicit equation of state, which is argued to be useful at high density; it nevertheless reproduces the known values of $a$ and $b$ at the critical point

Population dynamics in compressible flows

Organisms often grow, migrate and compete in liquid environments, as well as on solid surfaces. However, relatively little is known about what happens when competing species are mixed and compressed by fluid turbulence. In these lectures we review our recent work on population dynamics and population genetics in compressible velocity fields of one and two dimensions. We discuss why compressible turbulence is relevant for population dynamics in the ocean and we consider cases both where the velocity field is turbulent and when it is static. Furthermore, we investigate populations in terms of a continuos density field and when the populations are treated via discrete particles. In the last case we focus on the competition and fixation of one species compared to anotherComment: 16 pages, talk delivered at the Geilo Winter School 201

Social seed networks and climate change adaptation in East Africa

Our study investigates the routes of information flow through which smallholder exchange seed and information. We then examine how these exchanges among farmer networks can be improved. In order to examine the characteristics of these networks, our study analyzed household-level survey data collected by Bioversity International from sites in Kenya, Uganda, and Tanzania. Current seed sources and climate adaptation strategies were determined by creating summary statistics from the survey data. UCINET network analyses were conducted to acquire a more specific understanding of the current social networks among farmers. Seed networks were categorized by country, gender, and crop and then analyzed to determine where the distribution of seed and information can be improved and utilized