| Text | 205994 2011 Rotifer resting eggs Ecotoxicology Resurrection ecology Common garden experiemnts Zooplankton resting eggs in sediment cores Lake Orta S pollution and recovery Piscia, Roberta CNR ISE, Pallanza Verbania Despite a well known ecological importance and prompt reaction to environmental changes, rotifers have been so far overlooked in palaeolimnological studies. In the present study, we tested the possibility to use their abundance,morphotypes diversity and numbers in sediment cores as a proxy for reconstructing long term changes during chronic copper pollution and acidification and after chemical restoration. Lake Orta, a deep, sudalpine lake, provided a good opportunity for this attempt, being its pollution and chemical recovery monitored in detail. Analysis of rotifer s egg bank was performed on two sediment cores ORTA 07/1A and ORTA 07/2A 52.5 and 61.5 cm long, respectively , representative, although with different time resolutions, of pre , during and post pollution phases. The first core allowed for relating increasing abundance and decreasing diversity of Rotifera resting eggs to changes in the environment, in agreement to previously observed results on subfossils Guilizzoni Lami 1988; Manca Comoli et al. 1995; Ruggiu et al. 1998; Cattaneo et al. 1998 . Cladocerans ephippia started to accumulate later and reached a lower level of abundance than rotifer resting eggs. Toxic conditions of the sediments in which they were preserved, did not affect viability of resting eggs Brachionus calyciflorus was successfully hatched from ca. 100 year old eggs, so were cladocerans of different zooplankton taxa from ca. 25 year BP. Surprisingly, and differently from what previously observed for Daphnia, B. calyciflorus from different pollution phases were of the same, most widely distributed, North American clone. Such clone, therefore, persisted for ca. 80 years, all over pollution and recovery phases. We do not know,however, whether this was the only clone able to produce resting eggs or the only clone persisting in the lake active population, as data on zooplankton rotifers are very scanty and mostly qualitative. The low sedimentation rate of core ORTA 07/2A allowed for a finer time resolution and a correlation with long term hydrochemistry data along with pollution and recovery. Rotifer resting eggs abundance was significantly p<0.005 correlated with both, ionic copper concentration positively and water pH negatively , suggesting these eggs might be increasingly produced in response to pollution. Assemblages differed among pre , during and post pollution; diversity and morphotypes number increased after recovery from acid conditions >5.5 units and Cu2 <38 g L 1 thus allowing for hypothesizing thresholds for discriminating among assemblages of pre , during and post lake pollution. Lake Orta also provided a good opportunity for investigating response mechanisms of Daphnia exposure to heavy metals and water acidification. Previous studies Bachiorri et al. 1991; Piscia et al. 2006 detected an extremely low clonal diversity in Daphnia obtusa Kurz and in Daphnia galeata Sars established in the lake in two different phases of lake recovery, the latter being still present in the lake. Acute toxicity tests on D. galeata allowed for estimating that tolerance to ionic copper was three fold that reported in literature EC50 = 67 g·L 1 , while a high sensitivity to acid pH threshold of 5.5 pH units might be a reason for which the species colonized Lake Orta only after pH early recovery i.e, from acid to acidic values . Instantaneous rate of population increase was almost unaltered in individuals recovered after acute exposure 48 hr to enriched Cu2 twice those presently measured on the water column, of 3 g L 1 . It resulted, however, from increasing egg production, counterbalanced by increasing proportion of abortion, thus suggesting that D. galeata was, however, under stress. Given the low clonal diversity of D. galeata population in Lake Orta Piscia et al. 2006 we hypothesized that a strong selective pressure reduced an ability to adapt to conditions different from those which specimens were selected. Life history response of contemporary/past hatched from ephippial eggs 25 years old, when ionic copper concentration ranged between 26 and 38 g L 1 Daphnia galeata clones to ionic copper enrichment 10 and 40 g L 1 was also tested in common garden like experiments. Ephippia were recovered from sediment core ORTA 07/1A. To exclude maternal effects, both ex ephippio and contemporary clones were acclimated to common laboratory conditions for >3 generations. Contemporary Daphnia died after 10 days exposure, few of them soon after releasing non viable eggs, while ex ephippio clones were able to survive and reproduce at both concentrations. Resurrected clones exposed to the less ionic copper enriched treatment E10 had a higher instantaneous rate of population increase a than control, non enriched ones. A slight increase after 40 days of life in survivorship weighted fecundity and in sexual production measured as newborn dry weight of ex ephippio more Cu2 enriched E40 might suggest that, differently from contemporary ones, older clones might be able to acclimate to increasing ionic copper concentrations. Results of lifetime tests also suggest that response patterns may differ from those obtained by means of chronic shorter ecotoxicological tests. The occasional finding of an invasive, non indigenous species Procambarus clarki Girard 1852 provided an opportunity for discussing, among the others, possible impact on biodiversity of macrozoobenthos Gherardi 2007 and potential impact on human health when used for food, as they may accumulate heavy metals from sediments where they live Alcorlo et al. 2006 , still high in Lake Orta.Analyses of Rotifer s resting eggs assemblages suggested that pre and post pollution communities differed; as above mentioned, data on Lake Orta zooplankton rotifers are scanty; it is, however worth to try to exploit those available, for e.g. a comparison with the nearby lake Maggiore, to understand whether, particularly from a functional point of view the two differ and how. Preliminary results on phytoplankton and plankton rotifers seem to suggest that, 11 years after liming, Lake Orta s plankton seasonal dynamics differed substantially form that of the reference lake. Another aspect under study is a comparison between 21 day and whole life based estimates of demographic and reproductive parameters of contemporary/past D. galeata from Lake Orta, under enriched and un riched ionic copper concentrations. Altro prodotto roberta.piscia PISCIA ROBERTA DG.RSTL.094.012 Evoluzione della biodiversita e valutazione della banca degli stadi duraturi di organismi acquatici di laghi remoti |
