Phytoplankton succession in Lake Valencia,Venezuela

Phytoplankton counts and supporting physical and chemical data were taken on Lake Valencia, Venezuela, over a five-year interval. The data are used to test the validity of a successional paradigm for class-level taxa. According to the paradigm, formulated from previous studies of Lake Lanao, Philippines, and from data on temperate lakes, the order of taxa from early to late succession is: diatoms, chlorophytes, blue-green algae, dinoflagellates. A successional episode is considered to begin when stability of a water column is restored after deep mixing. As the episode progresses, there is a steady decrease in concentration of the limiting macronutrient (in this case, N). In a test of the validity of the paradigm for Lake Valencia, dates of exceptional population increase or decrease were obtained for each taxon. Since nitrate concentration declines steadily as succession progresses, the entry of a given taxon into the successional sequence is indicated quantitatively by the mean nitrate concentration on dates of exceptional increase in population density, and exit from the successional sequence is indicated by mean nitrate concentration on dates of exceptional population declines. The successional position of each major taxon, bounded by its entry and exit in the sequence, can be mapped on the complete spectrum of nitrate concentrations observed in the lake. For Lake Valencia, the nitrate mapping procedure agrees exactly with the predictions based on the successional paradigm. Conformance of Lake Valencia phytoplankton with predictions made a priori suggests that there is a generalized pattern in the phytoplankton succession of the mixed layers of temperate and tropical lakes.     

Composition and seasonality of the zooplankton community of Lake Valencia, Venezuela

The composition and temporal abundance patterns of planktonicherbivores in Lake Valencia, Venezuela, were studied over a5-year period. The herbivore community was typically composedof only 9 or 10 species, of which four (Notodiapromus deeveyorus,Mesocyclops decipiens, Brachionus calyciflorus, Ceriodaphniacornuta) accounted for more than 90% of community biornass (67µg l^–1 mean dry mass). Annual average biomass variedby a factor of three over the 5-year study and this is comparableto interannual variation observed in lakes of the temperatezone. Lake Valencia has a pronounced seasonal cycle based onan annual mixing period and an annual stratification period.Herbivores were usually more abundant during the mixing seasonthan the stratification season. Proximate control of herbivoreabundance patterns could be ascribed largely to predation byChaoborus. Predation pressure was diminished during the circulationseason because the Chaoborus population was reduced by two mortalitymechanisms: catastrophic losses due to chemical changes at overturn,and increased predation by fish coincident with loss of thehypolimnetic refuge. Chaoborus fed on all herbivore speciesin Lake Valencia, but electivities for specific herbivores variedgreatly. The early developmental stages of both copepod speciesexperienced low predation pressure; Notodiaptomus nauphi escapedpredation entirely. Adult copepods, on the other hand, werehighly vulnerable to predation. A statistical analysis of temporalpatterns of abundance and production for the major herbivorespecies suggests that closely related species responded differentlyto growth control mechanisms; the two species with similar responses(M.decipiens and B.calyciflorus) also had coincident abundancepatterns. Despite statistical differences in responses to growthcontrol mechanisms, there was remarkable dietary similarityamong the species. There was no obvious dietary specializationand no statistical evidence of strong selectivity for or aversionto particular algal species. Zooplankton species compositionis very simple in Lake Valencia in contrast to the high speciesrichness of many other kinds of tropical communities. Herbivorecommunity abundance is highly seasonal in Lake Valencia; theamplitudes of abundance fluctuation exceed those of other largetropical lakes and are comparable to those in lakes of the temperatezone. Herbivore production in Lake Valencia, and in tropicallakes in general, is not necessarily higher than that in temperatelakes, but production per unit biomass probably is higher.     

Anthropogenically induced fish diversity reduction in Lake Valencia Basin, Venezuela

Lake Valencia in north central Venezuela, one of the largest natural inland bodies of water north of the equator, is undergoing a very rapid deterioration process as a consequence of anthropogenically induced changes. At the Ichthyology Laboratory of Instituto de Zoología Tropical, we have been studying the fishes of the northern tributaries of Lake Valencia since the early 1990's. As a result of collections made in seven northern tributaries of the basin from 1990 to 1993, we have identified 15 species of fish in 6 families. Comparison of our results with those of several authors who have collected in the area since the 1920's, indicate a 59.5%decrease in fish diversity in a 30 year span. The Lake Valencia region has undergone a rapid, considerable industrial and urban growth in recent years. This rapid increment in population size has produced a vast contamination of the lake and its tributaries by activities associated with a growing population. This study will contribute to the understanding of the biodiversity of the region by recording changes in the characteristics of the ichthyofauna in a highly modified environment.     

Differential habitat utilization by sexes of Sarotherodon mossambicus in Lake Valencia, Venezuela: significance for fitness

Adult male and female Sarotherodon mossambicus in Lake Valencia, Venezuala select different littoral habitats. Males prefer areas with soft bottoms and high turbidity, while females favour areas with hard bottoms and low turbidity. Soft bottoms provide suitable sediment for male breeding site construction and high turbidity helps protect territorial males from visionally orientated predators, but the food available in this habitat is of poor quality and males feed relatively little. The hard bottom littoral provides food of superior quality and females feed in these areas. Condition factors are highest for subadult males, intermediate for adult females and lowest for adult males. Differential habitat utilization by sexes of S. mossambicus in Lake Valencia appears to contribute to individual fitness by providing males and females with specific requirements for reproduction.     

Time to pregnancy--a model and its application

Biological fertility is poorly measured by the number of children born in industrially advanced societies. The time elapsing from when a couple decides to have a child to clinically recognizable pregnancy is a useful alternative. Time to pregnancy can be collected in broad categories in large samples. A model for condensing important information from such data is presented, which fits several large samples of reported waiting times. It is shown that multiparous women conceive more quickly than primiparous women.     

A Poisson model of sequence comparison and its application to coronavirus phylogeny

In this paper, we propose two metrics to compare DNA and protein sequences based on a Poisson model of word occurrences. Instead of comparing the frequencies of all fixed-length words in two sequences, we consider (1) the probability of ‘generating’ one sequence under the Poisson model estimated from the other; (2) their different expression levels of words. Phylogenetic trees of 25 viruses including SARS-CoVs are constructed to illustrate our approach.     

Transport of phosphorus,nitrogen, and carbon by the Apure River,Venezuela

The concentrations and transport of carbon, nitrogen, and phosphorus were studied in the Apure River, Venezuela, over a period of 21 months. The drainage basin, which is in western Venezuela, encompasses 167,000 kM², most of which has low relief and supports tropical savanna vegetation. Average runoff for the period of study was 361 mm/year. Discharge-weighted mean concentrations were 188 ug/l for total P, 957 ug/l for total N, and 9.8 mg/l for organic C. Annual transport was 0.68 kg· ha^-1· year^-1 for total P, 3.45 kg · ha^-1· year^-1 for total N, and 35.4 kg · ha^-1 · year^-1 for organic C. Particulate matter accounted for 68% of P, 54% of N, and 37% of C transport. The yield of carbon from the Apure watershed agrees well with empirical predictions, based mostly on the temperate zone, for watersheds of similar size and water yield.Seasonal patterns in chemistry are tied strongly to the hydrologic cycle. When the wet season begins, rising water flushes organic matter from side channels and produces a sharp increase in particulate C and N. Particulate P, which is associated more with mineral material, also increases during rising water. All dissolved constituents except inorganic C also increase over the rising-water phase. As the river inundates the floodplain, the concentration of nitrate declines, whereas the concentrations of dissolved organic C and N continue to rise. At high water the floodplain appears to store sediments that are later remobilized. During low water, all fractions except dissolved inorganic C tend to be at minimum concentration.Soluble reactive P, total dissolved P, dissolved inorganic C, and dissolved organic C were successfully modelled as hyperbolic functions of discharge. No significant relationships were found between concentration and discharge for any particulate fraction because the flushing and storage mechanisms affecting these fractions occurred within specific hydrologic phases, rather than as a smooth function of discharge. No significant relationships were found for any nitrogen fraction. For nitrate, and thus for total dissolved N, of which nitrate is a major component, poor conformance to standard models is explained by association of key mechanisms (e.g. uptake) with specific hydrologic phases. Particulate components and nitrate in this sense violate the continuity assumptions implicit in the standard models.     

A numerical method to model excitable cells

We have extended a fast, stable, and accurate method for the numerical solution of cable equations to include changes in geometry and membrane properties in order to model a single excitable cell realistically. In addition, by including the provision that the radius may be a function of distance along an axis, we have achieved a general and powerful method for simulating a cell with any number of branched processes, any or all of which may be nonuniform in diameter, and with no restriction on the branching pattern.     

A model for one-dimensional morphoelasticity and its application to fibroblast-populated collagen lattices

The mechanical behaviour of solid biological tissues has long been described using models based on classical continuum mechanics. However, the classical continuum theories of elasticity and viscoelasticity cannot easily capture the continual remodelling and associated structural changes in biological tissues. Furthermore, models drawn from plasticity theory are difficult to apply and interpret in this context, where there is no equivalent of a yield stress or flow rule. In this work, we describe a novel one-dimensional mathematical model of tissue remodelling based on the multiplicative decomposition of the deformation gradient. We express the mechanical effects of remodelling as an evolution equation for the effective strain, a measure of the difference between the current state and a hypothetical mechanically relaxed state of the tissue. This morphoelastic model combines the simplicity and interpretability of classical viscoelastic models with the versatility of plasticity theory. A novel feature of our model is that while most models describe growth as a continuous quantity, here we begin with discrete cells and develop a continuum representation of lattice remodelling based on an appropriate limit of the behaviour of discrete cells. To demonstrate the utility of our approach, we use this framework to capture qualitative aspects of the continual remodelling observed in fibroblast-populated collagen lattices, in particular its contraction and its subsequent sudden re-expansion when remodelling is interrupted.     

A model for slow axonal transport and its application to neurofilamentous neuropathies

A model for slow axonal transport is developed in which the essential features are reversible binding of cytoskeletal elements and of soluble cytosolic proteins to each other and to motile elements such as actin microfilaments. Computer simulation of the equations of the model demonstrate that the model can account for many of the features of the SCa and SCb waves observed in pulse experiments. The model also provides a unified explanation for the increase and decrease of neurofilament transport rates observed in various toxicant-induced neuropathies.     

A subject-specific pelvic bone model and its application to cemented acetabular replacements

A subject-specific three-dimensional finite element (FE) pelvic bone model has been developed and applied to the study of bone–cement interfacial response in cemented acetabular replacements. The pelvic bone model was developed from CT scan images of a cadaveric pelvis and validated against the experiment data obtained from the same specimen at a simulated single-legged stance. The model was then implanted with a cemented acetabular cup at selected positions to simulate some typical implant conditions due to the misplacement of the cup as well as a standard cup condition. For comparison purposes, a simplified FE model with homogeneous trabecular bone material properties was also generated and similar implant conditions were examined.The results from the homogeneous model are found to underestimate significantly both the peak von Mises stress and the area of the highly stressed region in the cement near the bone–cement interface, compared with those from the subject-specific model. Non-uniform cement thickness and non-standard cup orientation seem to elevate the highly stressed region as well as the peak stress near the bone–cement interface.     

A numerical simulation of the flow in a T-type bifurcation and its application to an end to side fistula

Some recent surgical procedures such as arteriovenous fistula, or coronary and cerebral by-passes were observed to yield counterdirectional branching blood flows. There is a lack of knowledge about the role of the counterdirectional flow ratio, the angle of anastomosis and the shear stresses in the process of thrombogenesis that leads to shunt occlusion. The program developed to simulate these hemodynamic conditions uses an efficient numerical scheme for the solution of the 2-D Navier-Stokes equations and can be easily adapted to flows in other geometrical configurations.     

太湖沉积物微生物生物量及其与碳、氮、磷的相关性

对太湖沉积物中微生物生物量碳(MBc)、氮(MBN)、磷(MBp),以及沉积物总有机碳(TOC)、总氮(TN)、总磷(TP)进行测定并进行相关性分析,揭示太湖沉积物微生物对太湖沉积物营养盐的响应及反馈特征.结果表明:沉积物微生物生物量(MB)在湖体沿岸地区大于湖心区,平均值为184.66 mg·kg-1,MBc在西部沿岸区以及竺山湾和梅梁湾区域较高,平均值为127.57 mg·kg-1;MBN在梅梁湾、贡湖部分区域以及靠近梅梁湾和贡湖的湖心区域和东部沿岸区较高,平均值为19.25 mg·kg-1;MBp在东部沿岸区及其附近的湖心区最高,平均值为19.09 mg·kg-1;沉积物TOC高值区(≥2.30 g· kg-1)主要集中在竺山湾、西部沿岸区、梅梁湾、贡湖地区,平均值为1.59 g·kg-1;沉积物TN高值区(≥0.30g· kg-1)主要集中在贡湖、梅梁湾、竺山湾部分地区以及西部沿岸区,平均值为0.21 g·kg-1;沉积物TP高值区(≥1.20g·kg-1)主要集中在东部沿岸区以及湖心部分区域,平均值为0.55 g·kg-1;太湖沉积物TOC/TN在7～19,平均值为8.97,表明太湖沉积物中的有机质具有明显的双重来源,其中陆源有机质主要集中在西部沿岸区;太湖沉积物MB与沉积物TOC和TN呈显著正相关,与沉积物TP相关性不显著;沉积物MBc/MBN与沉积物TOC/TN显著相关.太湖沉积物微生物主要受沉积物TOC、TN影响,且沉积物TOC/TN的变化显著影响微生物群落结构.     

A constitutive model for two-dimensional soft tissues and its application to experimental data

A constitutive relation proposed by Shoemaker (Ph.D. dissertation, 1984) to model the mechanical behavior of membraneous or two-dimensional soft tissues is described. Experiments by Schneider (Ph.D. dissertation, 1982) on human skin and Lee et al. (Am. J. Physiol., 249, H222-H230, 1985) on canine pericardium, and the application of the constitutive model to biaxial stress-strain data from these experiments, are discussed. Some experimental data and predictions of the model obtained by curvefitting are presented for comparison. Values of material parameters are also presented. It is concluded that the constitutive model is well able to fit results of individual tests, and that its generality (judged by consistency of parameters from test to test of the same specimen), though not complete, does compare favorably with some other results presented in the literature.     

A model for random degradation of two-stranded polymers and its application to deoxyribonucleic acid

The molecular weight distribution and weight-average molecular weight are derived from statistical theory for an idealized two-stranded polymer undergoing random bond scission. For a first-order scission process it is predicted that a plot of the reciprocal square root of the molecular weight versus time should be linear over a wide range of degradation, thus providing a new criterion for the two-stranded structure. It is also shown that a single strand may give this form of molecular weight decay when bond scission results from two rate-determining steps. Hence independent data on the kinetics of bond scission are needed to distinguish between one- and two-stranded structures on the basis of kinetics alone. The predictions are borne out by existing data on the degradation of deoxyribonucleic acid ( DNA) under various conditions involving widely different extents of damage to the hydrogenbonded structure. The slope of the proposed kinetic plot depends on the molecular weight of the starting material and the rate constant for bond scission and is found to agree precisely with independent measures of these quantities for the case of enzymic degradation. A fit of the data for acidic degradation to the theory demonstrates its validity where only a small fraction of the hydrogen bonds are intact. The case of thermal degradation studied by Doty et al. is of particular interest in that the data follow the predicted two-strand kinetics under conditions where the strands had been thought to be separated. A possible two-step mechanism for a single strand appears unlikely on the basis of existing kinetic data; however, such data are at present insufficient to rule out the latter structure, and it is concluded that further data on the kinetics of bond scission are needed to establish the structure of DNA under the conditions of these experiments.