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  • 1
    Electronic Resource
    Electronic Resource
    Responses of phytoplankton to experimental fertilization with ammonium and phosphate in an African soda lake (1982)
    Springer
    Oecologia 52 (1982), S. 321-326 
    Details
    ISSN: 1432-1939
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Summary Phytoplankton abundance in tropical lakes is more often judged to be limited by nitrogen than phosphorus, but seldom does the evidence include controlled enrichments of natural populations. In January 1980 we performed the first experimental fertilization in an equatorial African soda lake, Lake Sonachi, a small, meromictic volcanic crater lake in Kenya. During our study the natural phytoplankton abundance was ca. 80 μg chl a/l, and the euphotic zone PO4 and NH4 concentrations were less than 0.5 μM. In the monimolimnion PO4 reached 180 μM and NH4 reached 4,600 μM. Replicate polyethylene cylinders (5 m long, 1.2 m3) were enriched to attain 10 μM PO4 and 100 μM NH4. Phytoplankton responses were measured as chlorophyll, cell counts and particulate N, P and C. After two days, the chlorophyll increase in the P treatment was significantly higher than the control (P〈0.01) while the N treatment was not. After five days the molar N/P ratio of seston was the same in the N treatment and control (23) but only 6 in the P treatment. The molar N/P ratio of seston in an unenriched Lake Sonachi sample was 21 and in samples from Lakes Bogoria and Elmenteita, two shallow soda lakes in Kenya, the ratios were 12 and 70 respectively. We conclude that limitation of phytoplankton abundance by phosphorus can occur even in some tropical African soda lakes.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00367954
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    Paper (German National Licenses)
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  • 2
    Electronic Resource
    Electronic Resource
    Nitrogen fixation by periphyton and plankton on the Amazon floodplain at Lake Calado (1994)
    Springer
    Biogeochemistry 26 (1994), S. 41-66 
    Details
    ISSN: 1573-515X
    Keywords: Amazon floodplain ; floating meadows ; nitrogen fixation ; periphyton ; plankton ; tropical limnology
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Geosciences
    Notes: Abstract Nitrogen fixation by periphyton and plankton was measured on the Amazon flood-plain using the acetylene reduction method calibrated with15N-N2. The average ratio (± SD) of moles C2H4 reduced per mole N2-N fixed was 3.4 ± 0.7, similar to other studies. Periphyton and plankton had high rates of light-dependent nitrogen fixation, with dark nitrogen fixation averaging 26% of the average rates in the light. The average daily (24 h) rates for periphyton nitrogen fixation in 1989 and 1990 were 1.79 and 0.51 mmol N2-N·m−2·d−1 respectively, which are comparable to summer rates in many temperate cyanobacterial assemblages. Nitrogen fixation was depressed at N03 − concentrations as low as 0.5 μM, and was below detection limits at concentrations of 4 μM, which occurred during periods of river flooding. Planktonic nitrogen fixation rates were high (0.5–0.8 mmol N2-N·m−2·d−1) during the high-water and drainage phases of the annual hydrograph when the floodplain waters were draining towards the river (low NO3 −), but rates were undetectable (〈 0.05 mmol N2-N·m−2·d−1) when there was river flooding (high NO3 −). Nitrogen fixation by periphyton and plankton in 1989–1990 accounted for approximately 8% of previously reported total annual nitrogen inputs to the floodplain at Lake Calado.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02180403
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  • 3
    Electronic Resource
    Electronic Resource
    Modeling the hydrochemistry of the Choptank River Basin using GWLF and Arc/Info: 1. Model calibration and validation (2000)
    Springer
    Biogeochemistry 49 (2000), S. 143-173 
    Details
    ISSN: 1573-515X
    Keywords: GIS ; GWLF ; nonpoint source pollution ; nutrient ; watershed modeling
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Geosciences
    Notes: Abstract We modeled nutrient export in the ChoptankRiver Basin on the coastal plain of the Chesapeakedrainage, using a modified version of alumped-parameter, hydrochemical model (GWLF).Calibration was performed using long-term (WY1980–WY1990)hydrochemistry data from a gauged site. Thecalibrated model reproduced water yields, TN, and TPexport with cumulative errors of 〈1% over the11-year calibration period and with annual RMS errorsof 10–50%. Model validation was done withindependent measurements at the same gauged site(WY1991 to WY1996) and at another nearby independentlygauged site (WY1991 to WY1995). Local adjustment ofthe groundwater recession coefficient and thedissolved N concentration in agricultural stormflowwas essential for successful application at the secondsite. GWLF appears to be a useful model for estimationof fluxes of water, N and P from ungauged areas withaccuracies of 10–50% at annual time scales.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1006375530844
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  • 4
    Electronic Resource
    Electronic Resource
    Solute dynamics in soil water and groundwater in a central Amazon catchment undergoing deforestation (1997)
    Springer
    Biogeochemistry 38 (1997), S. 303-335 
    Details
    ISSN: 1573-515X
    Keywords: Amazon ; deforestation ; hydrologic pathway ; groundwater ; nitrogen ; rain forest ; slash-and-burn agriculture ; solutes ; tropical
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Geosciences
    Notes: Abstract Hydrochemical changes caused by slash-and-burnagricultural practices in a small upland catchment inthe central Amazon were measured. Soluteconcentrations were analyzed in wet deposition,overland flow, shallow throughflow, groundwater andbank seepage in a forested plot (about 5 ha) and anadjacent plot (about 2 ha) which had been deforestedin July 1989 and planted to manioc, and in streamwater in partially deforested and forested catchments. Measurements were made from November 1988 to June1990. The effects of slash-and-burn agriculturalpractices observed in the experimental plot includedincreased overland flow, erosion, and large losses ofsolutes from the rooted zone. Concentrations ofNO3 -, Na+, K+, SO4 2-,Cl- and Mn in throughflow of the experimentalplot were higher than those of the control plot bymore than a factor of 10. Extensive leaching occurredafter cutting and burning, but solute transfers werediminished along pathway stages of throughflow togroundwater, and particularly within the riparian zoneof the catchment. High concentrations of N and P inoverland flow indicate the importance of usingforested riparian buffers to mitigate solute inputs toreceiving waters in tropical catchments.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1005801303639
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    Paper (German National Licenses)
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