A mesocosm experiment was conducted ''in situ'' in a Chara dominated shallow lake near Valencia (Spain) to study top-down and bottom-up effects on rotifers by means of nutrient and fish additions. Both processes were important in determining rotifer abundance, biomass and diversity. A total of 36 mesocoms were established with triplicate treatment combinations of three fish levels (from no fish to 45 individuals of Gambusia holbrooki males) and four nutrient enrichment levels (from no additions to 10 mg l À1 nitrate-N and 1 mg l À1 phosphate-P). The main effect was a notable increase of planktonic and plant associated rotifers densities with fish. Rotifers benefited from mosquitofish predation on microcrustaceans and chironomids. The results showed a marked negative relationship between rotifer and cyclopoid abundances, indicating the importance of the predatory pressure of cyclopoids on rotifers. Effects on rotifer diversity were also evident, in general rotifer diversity decreased with nutrients and increased with fish. The effects of nutrients analysed at species level showed two contrasting density responses: an increase or a decrease with nutrients, which levelled off at high nutrient concentrations. High-level nutrient additions (from 5 mg l À1 nitrate-N and 0.5 mg l À1 phosphate-P) induced a switch to a turbid state with macrophyte disappearance. Most planktonic rotifer species, as well as plant associated ones, diminished when the turbid state was well established, especially in the mesocosms without fish. In the turbid mesocosms, relative abundance of plant-associated rotifers (as a whole) was higher than that of planktonic rotifers. The changes in rotifer species composition after the switch from a clear to a turbid water state are also described. Species of the genus Anuraeopsis, Trichocerca and Hexarthra, dominant in the clear water state, practically disappeared in the turbid water state, in which Proalides tentaculatus and Lecane nana were the main species.

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The vertical distribution of rotifer populations has been analyzed in a meromictic basin (III) of the karstic Lake Banyoles . This basin had, at the time of the study, an anaerobic monimolimnion from around 17 m in summer or 21 in winter to 25 m, its maximum depth. However, below this layer a warmer microaerophilic zone with suspended marls is found in a chimney to a depth of 130 m .The rotifer distributions of several years are compared . The presence of large populations of rotifers at the oxic-anoxic interface is constant through the years, but highly variable regarding the total abundance and relative proportions of the species, depending on the importance of the vertical mixing in winter and the posterior segregation of water layers as a function of production and decomposition processes .Filinia hofmanni, Filinia longiseta longiseta are species with a restricted occurrence at the oxycline and are not present in the other non meromictic basins of the lake . Filinia terminalis, more broadly distributed, also has dense populations in summer at the oxycline in basin III, but disappears from the plankton during this season in the other basins. Other species, such as Polyarthra dolichoptera, Anuraeopsis fissa, Keratella cochlearis and Kellicottia longispina, also showed maxima at the oxycline in summer, as well as other maxima in the metalimnion . Populations at the oxycline are probably different ecotypes adapted to low oxygen conditions, but having the advantage of high food availability .