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  • 1
    Electronic Resource
    Electronic Resource
    Summer phytoplankton assemblages across trophic gradients in hard-water reservoirs (1998)
    Springer
    Hydrobiologia 369-370 (1998), S. 27-43 
    Details
    ISSN: 1573-5117
    Keywords: phytoplankton ; trophic level ; reservoirs ; hypertrophic ecosystems ; nutrients ; diversity
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract Summer phytoplankton assemblages are described and characterised according to their prevalence in a series of hard-water reservoirs of eastern Spain that had been classified in trophic categories on OECD criteria. Distribution patterns of phytoplankton species were ordinated statistically by principal components analysis (PCA). The first component was strongly related to trophic gradient and it particularly discriminated the eutrophic and hypertrophic reservoirs. The second component segregated life-forms, so that (1), on the oligo-mesotrophic side, large dinoflagellates were separated from small centric diatoms, unicellular chrysophytes and filamentous ullotrichales and, on the eu-hypertrophic side (2), colonial greens and large desmids were separated from unicellular volvocales and small centric diatoms. The large differences between eutrophic and hypertrophic reservoirs were also clearly identified in a second PCA, in which physical and chemical factors were used with the principal components solved from the phytoplankton data. From these results, a new trophic category was discerned, for which we propose the name ‘holotrophic’. This category applies to water bodies having the following main features: (1) concentrations of chorophyll, total P and total N in the range of the hypertrophic systems, but with much higher concentrations of dissolved phosphorus and ammonia and (2) phytoplankton predominantly composed by unicellular green flagellates (Pteromonas, Chlamydomonas) and chlorococcales (Scenedesmus), without cyanobacterial blooms.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1017051322293
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  • 2
    Electronic Resource
    Electronic Resource
    Size distribution of Daphnia longispina in the vertical profile (1997)
    Springer
    Hydrobiologia 360 (1997), S. 187-196 
    Details
    ISSN: 1573-5117
    Keywords: Daphnia longispina ; toothed morph ; size distribution ; vertical migration ; predation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract D. longispina of the meromictic lake El Tobar is a round-headed form. It never has a helmet, but in summer a small proportion of immature individuals (0.9–1.2 mm females and males) have one or two neck teeth. The size structure of this Daphnia population, as well as the vertical distribution and migration of different size-classes, were studied in September and November of 1991 and April of 1992. The large variation in mean size and size at first reproduction, as well as the occurrence of different patterns of vertical migration are interpreted as responses to different predator situations. At the end of April, when Daphnia mortality by visually hunting predators is dominating, a typical nocturnal migration is adopted and size distribution is biased to smaller size classes. In November, when mortality is mainly attributed to the nocturnally migrating Chaoborus, Daphnia shows a reversed migration pattern. In September, when the population of Daphnia is responding to both visual (fish) and non-visual predators (Chaoborus), it adopts a pattern of twilight migration. The presence of neck teeth in vulnerable size classes in September might be an additional adaptative response to Chaoborus predation. In September, the size structure of the Daphnia population is shifted to larger classes and the vertical distribution of size classes shows a pronounced segregation between juveniles and adults. Juveniles are found closer to the surface, while adults dwell predominantly in the rich, deep waters near the oxicline. This suggests that an additional advantage of the ascent of the adult Daphnia exploiting those deep resources is the release of young in more oxygenated and warmer waters. The Daphnia population of lake El Tobar is known to be clonally diverse, and the changing frequency of genotypes could play an important part in the observed seasonal differences in behaviour and size.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1003173504463
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  • 3
    Electronic Resource
    Electronic Resource
    Chydorid assemblages in the sedimentary sequence of Lake La Cruz (Spain) subject to water level changes (1997)
    Springer
    Hydrobiologia 360 (1997), S. 277-285 
    Details
    ISSN: 1573-5117
    Keywords: Chydoridae ; paleolimnology ; diversity ; species associations ; cluster analysis ; principalcomponents analysis
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract Changes among chydorid assemblages in Lake La Cruzsediments, from a core of 178 cm length, have beenstudied using community structure indices andmultivariate methods (cluster analysis, principalcomponents analysis).The results revealed the existence of two main sourcesof variation in these assemblages. One (axis I ofPCA1) is associated with the trophic level of thelake, which is hypothetised to be greater in thebottom and upper part of the core. Both zones arecharacterised by an increase in the relative frequencyof Chydorus sphaericus, accompanied by a markedreduction in the relative frequency of the mostabundant species throughout the history of the lake,Acroperus neglectus.The other source of variation (axis II of PCA1) may beinterpreted as the alternation of periods of dryer and wetter weather. Ouranalyses show the separation of two assemblages whichalternatively prevail in the sedimentary sequence, oneconstituted by Pleuroxus laevis, Alona guttata,Graptoleberis testudinaria and, sometimes, Alonarectangula, and another dominated by Chydorussphaericus, Alona quadrangularis, Alona affinis andsometimes Leydigia species. According to theecological preferences of these species, the dominanceof the first chydorid assemblage can be related toepisodes of higher temperature and with very low andfluctuating water level, or with a high water levelbut a very reduced extent of shallow water in thelake. Prevalence of the second group corresponds toepisodes with a colder environment and permanentwaters, the level of which could be variable but insuch a way that a benthic sublittoral zone maydevelop. The structure of the community is stronglyaffected by changes in lake level. High indices offluctuation in community structure and small mean bodysize of the chydorid assemblage marked the period withmore extreme dry conditions, characterised by Alona rectangula maxima. The peaks of C. sphaericuscoincided with peaks of the index offluctuation D0 and prevalence of smaller speciesof the chydorid assemblage.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1003185824027
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