The study of cyst hatching, performed on 5 cyst types under lab conditions, …

• Abstract
• Background
• Results
• Discussion and conclusion
• Methods
• Abbreviations
• References
• Figures

The study of cyst bank of 6 hypersaline lakes allowed us to give a dimension to the unexpressed, potential biodiversity inhabiting such a typology of stressed environments. A sensible portion of the biodiversity of each lake rests during most of the year, and only a portion of the species present result active in the water column at each date. The accumulation of cysts of different species, generations and genotypes with variable hatching rates leads to a mixed cyst bank more bio-diverse than the active community sampled at any one moment.

Available information coming from the Vecchia Salina [10] reported a total of 16 species in the water column (deriving from a sampling effort which lasted over two years). The number of cyst types (17) here reported from the bottom sediments of the same environment is lower than that (24) reported by the same authors, but in that case the investigation was carried out on a total of 14 cores collected in 14 different sites of the lake, while in the present study only the central area of the lake was concerned by the collection of just 3 replicate cores.

It is evident that such a light sampling effort (3 sediment cores at one sampling date) devoted to the sediments gives more information than a two-year period of sample collection in the water. Nevertheless, we cannot ignore the fact that some species do not supply the sediment cyst bank but overcome adverse periods differently (e.g. migrating as insects do, or resting as sub-adults, as some harpacticoids do). Hence the analysis of cyst banks cannot still completely substitute the traditional investigation on active stages. It could be proposed, however, as an indispensable additional source of information to correctly evaluate the biodiversity of water environments [10,13]. A greater diversity of zooplankters are represented in the egg banks in comparison to that present in the current water column, these are dormant phases of the zooplankters life cycle and favourable hatching conditions for those unique cohorts may or may not occur within a given lake. The egg banks stored in sediments of each lake represent its real potential diversity. This does not diminish their importance over time because, as environmental conditions in a lake change its appropriate zooplankter assemblage, it will be more (or less) dominant.

As regards the species composition of each assemblage, it can be noted that Koyashskoe and Nartë were the poorest, while Vendicari pools and Vecchia Salina saltworks were the richest lakes.

Some information about salinity [see Additional file 3] suggests that this could be due to the high salt concentrations characteristic of the first waters. The biodiversity of saline habitats, in fact, seems to be inversely correlated with the salinity value [1,15]. As regards the species composition, we can note that both the environments of Vendicari (Pantano Grande and Pantano Roveto) do not host Artemia cysts, while they are the only habitats where diaptomid eggs have been found. As in the case described by [16], despite the proximity to the sea, the salt is probably not of marine but of athalassohaline origin (underground brines). It has been proved that the quality of salt content affects the biological community [6,7], and a chemical analysis of dissolved anions in the Vendicari lakes water will be the necessary future information to be collected to confirm this rule. The sediments of both Vendicari lakes (Pantano Grande and Pantano Roveto) sampled during the dry season (September 2005), did not show the presence of Artemia cysts, while there have been recognized eggs of other Anostraca (Phallocryptus sp.) and Calanoida (Arctodiaptomus sp.). Although the two lakes are close to the sea, this taxonomic composition of egg bank suggests their salinity is not entirely of marine origin, but influenced by continental inputs.

The wide hatching patterns observed for most of the tested species is thought to be an obligatory adaptation to the extreme variability of the habitat. Hatching tests affirmed that the 5 tested species inhabiting the present lakes dislike freshwater conditions. Hatching has been found to be highly variable, even among cysts coming from the same sediment level, as well as among different layers of each core, probably due to the need to spread the risk of non successful hatching over many attempts, according to the bet hedge theory of [17,18] to ensure the persistence of populations in unpredictably stressed environments. Indeed, most of cyst morphotypes here considered belong to species that live in environments with a high level of stochasticity [19], a condition in which bet-hedging is expected to evolve [20].

Cyst banks form an essential component of plankton ecology. As cyst banks integrate seasonal and year-to-year variations in environmental conditions, they represent the total species and genetic diversity in any community better than the active component sampled at any one time. Cyst banks can be considered the archive of the local habitat, and overlooking the composition of the cyst bank in the study of biodiversity and biogeography may result in erroneous patterns and interpretations of the underlying processes.