Zooplankton resting eggs in sediment cores: Lake Orta'S pollution and recovery

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. galeat