Comparing flow regime,channel hydraulics,and biological communities to infer flow–ecology relationships in the Mara River of Kenya and Tanzania

Abstract

Equatorial rivers of East Africa exhibit unusually complex seasonal and inter-annual flow regimes, and aquatic and adjacent terrestrial organisms have adapted to cope with this flow variability. This study examined the annual flow regime over the past 40 years for three gauging stations on the Mara River in Kenya and Tanzania, which is of international importance because it is the only perennial river traversing the Mara-Serengeti ecoregion. Select environmental flow components were quantified and converted to ecologically relevant hydraulic variables. Vegetation, macroinvertebrates, and fish were collected and identified at target study sites during low and high flows. The results were compared with available knowledge of the life histories and flow sensitivities of the riverine communities to infer flow–ecology relationships. Management implications are discussed, including the need to preserve a dynamic environmental flow regime to protect ecosystems in the region. The results for the Mara may serve as a useful model for river basins of the wider equatorial East Africa region.

Editor Z.W. Kundzewicz; Guest editor M. Acreman     

Planetary nebulae in the Galaxy

It is shown that the planetary nebulae can be divided into three types according to the values of the mass of shell and a central star. The criteria are given using which one can determine the mass type of the nebula. The distance scale of each mass-type planetary nebulae is given. The distribution of planetary nebulae in the Galaxy, their formation rate, scale-height and other physical and kinematic characteristics are investigated. A catalogue of planetary nebulae emitting in the radio range is given.     

Concentration and Diffusion of Nutrients in the Interpore–Bottom Water System of the Ob River Estuary

Oceanology - On cruise 76 in 2019, the R/V Akademik Mstislav Keldysh completed the Gulf of Ob–Kara Sea slope transect. Samples of bottom water and two sediment layers 1 cm thick were obtained...     

Hydrodynamic processes in young binaries as a source of cyclic variations in circumstellar extinction

Hydrodynamic models of a young binary accreting matter from the remnants of a protostellar cloud have been calculated by the SPH method. Periodic variations in column density in projection onto the primary component are shown to take place at low inclinations of the binary plane to the line of sight. These can result in periodic extinction variations accompanied by brightness variations in the primary. Generally, there can be three periodic components. The first component has a period equal to the orbital one and is attributable to the streams of matter penetrating into the inner regions of the binary. The second component has a period that is a factor of 5–8 longer than the orbital one and is related to the density waves generated in a circumbinary (CB) disk. Finally, the third, longest period is attributable to the precession of the inner CB disk regions. The relationship between the amplitudes of these cycles depends on the model parameters as well as on the inclination and orientation of the binary in space. We show that at a dust-togas ratio of 1: 100 and amass extinction coefficient of 250 cm² g^?1, the amplitude of the V-band brightness variations in the primary component can reach 1 ^m at a mass accretion rate onto the binary components of 10.8^?8 M _⊙ yr^?1 and a 10° inclination of the binary plane to the line of sight. We discuss possible applications of the model to young, pre-main-sequence stars.     

Buried paleo-channels on the New Jersey continental margin: channel porosity structures from electromagnetic surveying

We report on a marine electromagnetic (EM) survey across two portions of the New Jersey continental margin that have been previously shown to contain buried paleo-channels. The EM method used provides bulk porosity estimates to depths of around 20 m below the seafloor and is thus able to place porosity constraints on the nature of the channel infill and the contrast in physical properties across the channel boundaries. Our data show that a key condition for the channels to have an electrical signature is that they incise an underlying regional unconformity, R, thought to represent a subaerially eroded surface, exposed during the late Wisconsinan glaciation. Channels that cut R are seen through increases in apparent porosity. Another seismically imaged channel sequence, which lies within the outer-shelf sediment wedge sequence above R, does not have an electrical signature, indicating that the sediments above and below the channel boundaries have similar physical properties.     

The limits of validity of the Frequency Ratio Method: The particular case of γ Doradus stars

This work provides a general vision of the limits of validity of the Frequency Ratio Method applied to the g‐mode pulsators in asymptotic regime, the γ Doradus stars. In particular, the work is mainly focused on the role of rotation which is found one of the most important source of uncertainty of the method. The particular case of the moderately rotating γ Doradus star HD48501 is discussed. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Off disk-center potential field calculations using vector magnetograms

We investigate a potential field calculation for off disk-center vector magnetograms that uses all the three components of the measured field. There is neither any need for interpolation of grid points between the image plane and the heliographic plane nor for an extension or a truncation to a heliographic rectangle. Hence, the method provides the maximum information content from the photospheric field as well as the most consistent potential field independent of the viewing angle. The introduction of polarimetric noise produces a less tolerant extrapolation procedure than using the line-of-sight extrapolation, but the resultant standard deviation is still small enough for the practical utility of this method.National Research Council Resident Research Associate on leave from Indian Institute of Astrophysics, Bangalore 560034, India.     

Authors' Reply

Abstract— Jull et al. propose an alternative interpretation of our depth vs. ¹⁴C data measured on a peat core from the central Tunguska impact site (Rasmussen et al., 1999). We find that the proposed alternative is untenable.