A model of phytoplankton production in the lower River Rhine verified by observed changes in silicate concentration

The production of phytoplankton in the three main branches andsedimentation areas of the River Rhine in the Netherlands wasanalyzed using a simulation model describing the carbon andsilicate metabolism. This model is based on data derived froma sampling programme in which river water was followed duringdownstream transport. A ‘plug-flow model’ was developed,including sky irradiance and light attenuation in the water,and integrating photosynthetic rates determined in the laboratory.On the basis of the silicate content of diatom-dominated phytoplanktonand silicate regeneration in the river bottom, changes in silicateconcentrations were simulated and found to match observed changesin dissolved silicate. Low sìlicate concentrations wereshown to restrict the maximum population density of diatoms.Depth- and time-integrated rates of photosynthesis were shownto permit multiplication of the phytoplankton at a rate of upto one doubling day^–1 In the primary production periodApril-August 1988. values of 0.48–6.33 g C m^–2 day^–1,close to the few values reported for highly eutrophic riversand lakes, were observed. Model runs, including phytoplanktonproduction and losses, such as respiration, sedimentation andplanktonic grazing, were carried out to simulate the downstreamdevelopment of phytoplankton biomass. These simulations confirmthe view that a substantial part of the phytoplankton biomassand production is grazed or settles in the river delta despiteresidence times of only 52–97 h.     

Antiparallel beta-sheets in the crystal structure of the heptapeptide Met-Glu-His-Phe-Arg-Trp-Gly (ACTH 4-10)

The conformation of the molecules in ACTH 4-10 has been determined as part of a study of the conformations of the biologically active N-terminal fragments of the adrenocorticotropic hormone (ACTH). ACTH 4-10 crystallizes in two different superstructures. The substructure considered in the present work, is monoclinic, space group C2, a = 25.75(1) A, b = 27.78(1) A, c = 20.35(1) A, beta = 114.0(1) degrees, Z = 8 molecules ACTH 4-10 plus 22 weight per cent solvent. The crystals contain antiparallel beta-sheets, the orientations of the side groups are not found, because of disorder. The present crystal structure and those of other linear oligopeptides emphasize that antiparallel beta-sheets are energetically favourable. It is very unlikely, however, that the ACTH 4-10 crystals contain the molecules in their biologically active form.     

Ecological cues, gestation length, and birth timing in African buffalo (Syncerus caffer)

We examined annual variation in the timing of conception andparturition in the African buffalo (Syncerus caffer) and thesynchrony of birth timing with resource cues, using 8 yearsof monthly birth, rainfall, and vegetation data, measured asNormalized Difference Vegetation Index (NDVI). Monthly birthshad the strongest significant correlations with NDVI and rainfalllevels 12 and 13 months in the past, respectively. In addition,the synchrony of current year births corresponds most stronglyto the synchrony of the previous year's NDVI distribution. Becausethe gestation period of buffalo has been estimated to be around11 months, these findings suggest that improved protein levels,occurring approximately a month after the first green flushof the wet season, are either a trigger for conception or conceptionhas evolved to be synchronous with correlated environmentalcues that ensure females enter a period of peak body conditionaround the time of conception and/or parturition. With a gestationperiod of approximately 340 days, parturition occurs to takeadvantage of the period when forage has its highest proteincontent. A comparative analysis of gestation periods withinthe subfamily Bovinae indicates that African buffalo have aprotracted gestation for their body size, which we suggest isan adaptation to their seasonal environment. We also found thatinterannual variation in the birth distribution suggests a degreeof plasticity in the date of conception, and variation in thenumber of calves born each year suggest further synchrony ata timescale longer than a single year.     

Plankton in the River Rhine: structural and functional changes observed during downstream transport

The growth dynamics of phytoplankton, zooplankton and bacterioplanktonin the River Rhine were analysed simultaneously with a numberof environmental factors in order to identify environmentalsteering factors and to describe some of their interrelations.Observations on the metabolic activity (for algae and bacteria)and density (for all organisms) were carried Out three timesin 1990 using successive sampling of the same water parcel duringits transport in the lower 660 km reach of the river. High algaldensities (up to 170.5 µg chlorophyll a l^–1), rotifers(up to 1728 l^–1), crustaceans (up to 65 l^–1) andbacteria (up to 16x10⁹ l^–1) were found. Algae and rotifersshowed a rapid successive development during transport, whilecrustaceans were only abundant in the tidal reach of the river.In May, a vigorous growth of phytoplankton, zooplankton andbacteria was found. The diatom-dominated phytoplankton depletedthe dissolved silicate in the river water and this led to acollapse of the populations, indicated by a decreased specificrate of photosynthesis (measured by the ¹⁴C method) and vigorousbacterial growth (measured by [³H]thymidine incorporation).Subsequently, the remaining phytoplankton diminished to verylow levels near the river mouth. In July and September, it seemedthat biological interactions within the plankton populationsor between plankton and benthos balance the population densitiesso that separate developmental stages, as in spring, were lessprominent. Estimates of the growth rates and loss rates of thephytoplankton were made. Phytoplankton exerted a substantialinfluence on the partitioning of nutrients (nitrogen, phosphorus,silicate) over water and suspended matter (as analysed by elementanalysis). It seems likely that only the reduction of phosphate,as planned under the Rhine Action Programme, and not that ofnitrogen, may restrict the peaks of plankton growth describedhere.     

Qualitative and Quantitative Interactions between Microphytobenthos and Herbivorous Meiofauna on a Brackish Intertidal Mudflat

Micro- and meiofauna are the predominant consumers of diatoms on a brackish intertidal mudflat. The impact of grazing on the benthic diatom populations was studied by field observations and feeding experiments on a few abundant members of the community. Only small fractions of the microphytobenthic biomass and production are converted by herbivores. A hypothesis is presented explaining the growth kinetics and productivity of diatom populations and the inefficient transfer of carbon into herbivore foodchains. Data on feeding rate and population dynamics of the nematode species, Eudiplogaster pararmatus, are discussed in view of the seasonal succession of edible diatom species.