Recovery of soil microbial diversity and functions along a tropical montane forest disturbance gradient

Logging and forest conversion are occurring at alarming rates in tropical forests. These disturbances alter soil microbial community structure and functions. While direct links between changes in soil properties, such as pH and microbial community structure are well established, the indirect effects of logging and forest conversion on soil microbial community structure and functions are poorly understood. We used a space-for-time substitution to investigate the changes in soil microbial diversity and functions across a forest recovery gradient in the tropical montane forests of northern Borneo. We used surface (top 5 cm) soil to assess soil physicochemical and microbial (next-generation DNA sequencing) properties, and standardized litterbags (Tea Bag Index) to assess litter decomposition and stabilization. Our results show that bacterial and fungal diversity increases with recovery time and reaches pre-disturbance levels between 60- and 80-years post-disturbance. Litter decomposition rate constants increased linearly with increasing bacterial and fungal diversity. Litter stabilization also increased linearly with fungal diversity, but was highest at intermediate levels of bacterial diversity. Our results provide insights on the effects of forest logging and conversion on soils and highlight the tight coupling between soil microbial diversity and soil functions in tropical montane forests

Are there internal Kelvin waves in Lake Tanganyika?

It is generally believed that the Earth's rotation has negligible impact on the water circulation in basins which are very narrow or located near the Equator. However, herein evidence is presented of the influence of the Earth's rotation on the hydrodynamics of Lake Tanganyika, which is both very narrow (width/length approximate to 0.08) and located near the Equator. Numerical simulations exhibit small upwellings at the western shores as a result of the thermocline oscillations induced by the southeasterly winds of the dry season. These structures tend to propagate cyclonically around the lake similar to internal Kelvin waves. Numerical experiments in which f is varied concludes that internal Kelvin waves are present in Lake Tanganyika. It is also evidenced from this study that the internal Kelvin waves cannot be anticipated based on classic scaling arguments

Analysis of wind-induced thermocline oscillations of Lake Tanganyika

An analysis is presented of the wind-induced thermocline oscillations of Lake Tanganyika, East Africa. The region undergoes a four month dry season and the wet season for the rest of the year. The dry season is characterised by nearly constant high southeasterly winds, while for the rest of the year mild wind blows generally from the northeast. Observations show that the dry season high winds cause tilting of the thermocline, being higher/lower than normal at the southern/northern ends of the lake. The thermocline tries to restabilise itself after the cessation of dry season winds and oscillates for the rest of the year. A non-linear reduced-gravity model is used to study the thermocline oscillations of the lake. The numerical simulations satisfactorily represent the oscillations, their period and amplitude. Different forcing conditions (thermocline depth, wind stress and stability) are used in the model and their effect on the period and amplitude of the oscillations are studied. The amplitude of oscillations ranges from 15 to 45 m, while their period varies from 3 to 4 weeks according to the variation in the model parameters. Wavelet transform is used to study the evolution of periods of oscillations with depth in the time series of observations and along the length of the lake using model simulations. Wavelet spectra presents several dominant modes including the semidiurnal, diurnal, synoptic, intraseasonal variability, besides the modes representing the wind-induced thermocline oscillations

Origin of intraseasonal variability in Lake Tanganyika

[1] Intraseasonal thermocline oscillations in Lake Tanganyika are analysed using observations near Mpulungu and simple analytical/numerical models, in order to understand their origin. The region around the lake is characterised by strong and persistent southeast winds during the four months dry season, lasting from May to August. The associated wind-stress causes the tilting of the thermocline which oscillates for the whole year. The wavelet transform spectra of temperature at 30 m depth of the lake near Mpulungu indicates the presence of various scales of motion, localised in frequency and time. The dominant modes of thermocline oscillations are intraseasonal variability with 3-4 weeks periods. Similar results are obtained from a reduced-gravity model with various wind forcing, including the observed forcing, and a simple analytical solution. In addition, the model results indicates that the dominant mode of oscillation exhibits one node only. From the study, it is inferred that the free modes of oscillations of the lake are in resonance with wind pulses

Tanganyika Lake, modeling the eco-hydrodynamics

peer reviewedThe model consists of a four-component ecosystem model, coupled to a hydrodynamic model. The hydrodynamic model considers the Lake as two homogeneous layers of different density lying above each other, representing the warm epilimnion (surface mixed layer) and cold dense hypolimnion (lower layer) separated by a thermocline (Naithani et al., 2002, 2003

A simple model of the eco-hydrodynamics of the epilimnion of Lake Tanganyika

1. The ecosystem response of Lake Tanganyika was studied using a four-component, nutrient-phytoplankton-zooplankton-detritus, phosphorus-based ecosystem model coupled to a nonlinear, reduced-gravity, circulation model. The ecosystem model, an improved version of the earlier eco-hydrodynamics model developed for Lake Tanganyika, was used to estimate the annual primary production of Lake Tanganyika and its spatial and temporal variability. The simulations were driven with the National Centres for Environmental Protection (NCEP) records for winds and solar radiation forcing. 2. The simulated annual cycles of the four ecosystem variables and the daily net primary production were compared with the observations. The comparison showed that simulations reproduced realistically the general features of the annual cycles of epilimnial phosphate, net primary production and plankton dynamics. 3. The climatic simulations for the years 1970-2006 yielded a daily averaged integrated upper layer net production ranging from 0.11 to 1.78 g C m(-2) day(-1) and daily averaged chlorophyll-a (chl-a) from 0.16 to 4.3 mg m(-3). Although the nutrient concentrations in the epilimnion during the strong wind years were high, the net production was low, which is partly because of the greater vertical mixing, produced by strong winds, exposing the phytoplankton to low light conditions in deeper waters. The simulated annual net production and chl-a agreed quite well with observed production available in the literature. 4. We envisage using this model to predict the future scenarios of primary productivity in the lake

Possible effects of global climate change on the ecosystemof Lake Tanganyika

peer reviewedAny change in the air temperature, wind speed, precipitation, and incoming solar radiation induced by increasing greenhouse gasses and climate change will directly influence lakes and other water bodies. The influence can cause changes in the physical (water temperature, stratification, transparency), chemical (nutrient loading, oxygen) and biological (structure and functioning of the ecosystem) components of the Lake. In this work an influence of the likely effects of the climate change on the above three components of Lake Tanganyika are studied by means of a simple ecological model. Simulations for the years 2002-2009 have been performed using the wind and solar radiation data from the National Centres for Environmental Protection (NCEP) reanalysis. Various possible climatic scenarios are studied by changing the surface layer depth, its temperature and the wind stress. Any change in any of the above physical forcing parameters modifies the timing and intensity of the dry season peaks of the biogeochemical parameters. It is seen that the gross production increases as temperature of the surface layer increases and its depth decreases. High temperature and low wind stress, reduces the biomass. The effects of a slight increase in lake water temperature on the Lake Tanganyika ecosystem might be mitigated by increased windiness, if the latter was sufficient to induce greater mixing. © 2011 Springer Science+Business Media B.V

Influence of cyclonic perturbations on surface winds around Dumont d'Urville, East Antarctica, using wavelet transform

Wavelet analyses of the surface winds at Dumont d'Urville, an East Antarctic station, have been performed for the months April, June, September, and December 1993, representing autumn, winter, spring, and summer, respectively. The 3 hourly 10 min averaged meteorological station data have been used. The site is characterized by intense katabatic winds that arrive from the southeast. During the year 1993 the maximum occurrence of wind was from 160 degrees direction sector. The wavelet transform spectra of horizontal wind show prominent periodicities in the interval between 2 and 20 days. The spectral energies in the diurnal scale are comparatively very low even in summer. A comparison with wavelet spectra of pressure for synoptic scale (2-10 days) confirms the influence of synoptic disturbances that are frequent in coastal regions of the Antarctic continent. The same analysis carried out on the wind components, parallel and transverse to the main katabatic flow direction, reveals that these periodicities are mainly due to the katabatic flow behavior. Although the katabatic wind flow is the dominant force characterizing the day-to-day weather conditions, the influence of transient cyclones offshore cannot be ignored; in situations of considerable synoptic activity the intensity and duration of katabatic wind flow are prolonged

CHOLTIC - Epidémies de choléra au lac Tanganyika induites par les changements climatiques?

Financé par BELSPO, le projet de recherche CHOLTIC a été initié afin d'investiguer les conditions environnementales favorisant l'émergence des épidémies de choléra et leur persistance dans la région du lac Tanganyika. Un suivi environnemental standardisé comprendra des aspects liés à la météorologie, la limnologie in situ et par télédétection, l'hydrodynamique, le phytoplancton, le zooplancton, l'épidémiologie, la bactériologie et la génétique. L'environnement humain sera aussi considéré prenant en compte les déplacements des pêcheurs et des commerçants en relation avec la pêche afin d'en tenir compte pour tester différentes hypothèses d'origine de l'infection et de la propagation de la bactérie causant le choléra