Zooplankton biomass and community structure in a Danube River floodplain system: effects of hydrology

1. Zooplankton density and biomass was examined in a Danube River floodplain section with highly variable hydrological dynamics. Temporal patterns were analysed to assess the effects of hydrological conditions on zooplankton community structure and the differential response of the two major zooplankton taxa, rotifers and crustaceans.
2. Calculated floodplain water age was used as an integrated parameter describing hydrological conditions and connectivity.
3. Total zooplankton biomass, crustacean biomass and crustacean species number were significantly positively related to water age. Rotifer biomass followed a hump-shaped relationship with water age, and rotifer species number decreased with increasing water age.
4. Rotifers dominated the community in periods of low to medium water ages. In periods of higher water ages the community was dominated by crustaceans.
5. We propose that the hydrological regime of floodplains is crucial for zooplankton biomass patterns and succession, through the alternation of washing-out effects, taxon-specific potential of reproduction and biological interactions. Flood events and high water levels reset the community to an early successional phase.     

Genome Size inAllium: In Quest of Reproducible Data

ComparingAlliumgenome size measurements of different authors,we noticed that the estimates for certain species diverge morestrongly than one would have expected in view of the methodologicaladvantages of the material. As the matter has theoretical significancefor explaining the biological role of genome size variation,we measured, by Feulgen densitometry, 28 species and altogether57 accessions or cultivars. Flow cytometric measurements supplementedthese data. The current hypothesis of a discontinuous and step-wisedistribution of DNA amounts inAlliumseems questionable, as mostof our DNA values did not appear in the corresponding DNA groupas proposed previously. On the other hand, we can confirm thatthere is a significant negative correlation between genome sizeof a species and its first month of flowering, but only in diploids,or in diploids and polyploids if only the basic genome size(2Cxlevel) is considered. We compared our results with thoseof nine other publications. Only 29 of 60 2C values publishedpreviously deviate less than 10% from our data, the others deviatemore strongly, from 0.44- to 1.44-fold. The more comprehensivedata sets of various authors were compared by correlation analysiswith our data. Positive and mostly significant correlationswere seen in all tests, but nevertheless the degree of incongruencebetween studies was unsatisfactory in view of the much betterintra-laboratory reproducibility of the present data. The presentwork highlights the need for generally agreed improvements instandardization and preparative procedures of cytophotometricgenome size determination.Copyright 1999 Annals of Botany Company Allium, genome size, Feulgen densitometry, flow cytometry, discontinuous DNA content variation, nucleotype hypothesis, flowering time, data reproducibility, correlation analysis.