Holocene to recent ostracoda of Lake Sevan, Armenia : biodiversity and ecological controls

The morphology and dynamics of Lake Sevan, Armenia, have abruptly changed on several occasions during the Holocene. Arguably the most significant events took place about 4000 years BP, when a phase of aridity greatly reduced the size of the lake and about 2100BPwhen the combined effect of an episode of volcanic activity that dammed the single outlet to the basin and the initiation of a wetter climatic phase, culminated in the development of the modern lake. Archaeological evidence indicates the extent of the first event, while the development of aquatic communities and increase in diversity of, for example, ostracods, characterises the second. Recent environmental stress, caused by a significant drop in water level due to human activity has resulted in changes in the ecosystem and food chain, resulting in a decline in ostracod diversity compared to both the earlierHolocene and sub-Recent. The living fauna is characterised by abundant Candona neglecta and Cyprideis torosa, common sexually reproducing Limnocythere inopinata, and less common Fabaeformiscandona caucasica, Candona candida, Cyclocypris ovum, Cypria ophthalmica and Darwinula stevenson

The impact of late Holocene environmental change on lacustrine Ostracoda in Armenia

Lake Sevan, Armenia, is an ancient lake, although it evolved into its present form only in the Holocene. The ostracod population was dominated by extant, spatially widespread species that have been recorded throughout Europe and Asia (e.g. Limnocythere inopinata, Ilyocypris bradyi, Cyprideis torosa, Candona candida and Candona neglecta), although a small proportion of the association is restricted to the Caucasus (e.g. Fabaeformiscandona caucasica). Diversity appears to reflect the gradual increase in water depths and environmental heterogeneity during the Sub-boreal European climatic phase (late in the Khvalynian climatic phase of the Caspian region). The muds, silts and sands penetrated by the boreholes contain shallow water ostracods indicating water depths of about 5 to 20 m. None of the boreholes penetrated deeper water deposits, so that Fabaeformiscandona dorsobiconcava, which lives at depths below c.40 m in the modern lake, was not found. Salinity was no greater than oligohaline; and water temperatures appear to have been cool. As is the case today, there were a number of streams transporting ostracods (such as Prionocypris zenkeri and Ilyocypris bradyi) into the lake and weeds were abundant. The Holocene association of Lake Sevan is unusual in that it is one of the few in which bisexual populations of Limnocythere inopinata occur