Diversity and structure of the winter rotifer assemblage in a shallow eutrophic northern temperate Lake Võrtsjärv

Abstract

In the eutrophic Lake Võrtsjärv (Central Estonia, area 270 km 2 , mean depth 2.8 m) rotifers form ca. 90% of total abundance and 80% of biomass in winter zooplankton community. The winter rotifer assemblage was dominated by Polyarthra dolichoptera, both in abundance and in biomass. Synchaeta verrucosa and Keratella quadrata were the subdominants. Thus, winter rotifer community had low diversity and high dominance of a few species. This pattern probably refers to the period of extreme environmental conditions where the rotifer assemblage is composed of few well-adapted species, and the low diversity here was not indicating instability of community structure, but the scarcity of suitable niches. These community structure indices indicate that the winter rotifer assemblage of L. Võrtsjärv was very similar to autumn assemblage, but very different from the spring one. In winter, small raptors were the most important functional group. The second place is occupied by larger raptors. Marginal role of fine particle sedimentators, absence of suckers and high proportion of large raptors were contrasting features of the winter trophic structure in comparison with the other seasons. Changes have taken place in the winter rotifer assemblage in L. Võrtsjärv in 1990Võrtsjärv in -2007 Against the background of diminishing rotifer abundance, the dominant species has become even more prevalent, and the diversity of the winter rotifer assemblage has decreased. Shifts in the community trophic structure were also observed.

Food spectrum of the omnivorous rotifer Asplanchna priodonta in two large northeastern European lakes of different trophy

“…This predator is generally considered to be omnivorous, preferring to feed on other smaller rotifers (Guiset 1977, Pourriot 1977, Salt 1977. It also has been considered a predominantly zoophagous species among metazooplankton in L. Võrtsjärv and L. Peipsi (Haberman 1997, Nõges et al 1998, Laugaste & Haberman 2005, Agasild et al 2007, Virro et al 2009). Due to the specific algal composition, which comprises mainly large and filamentous forms, the zooplankton community is generally considered to have little grazing impact on the phytoplankton in these two lakes (Agasild et al 2007).…”

“…Asplanchna priodonta is considered an important predator of rotifers, and is believed to efficiently regulate the density and biomass of its prey, as well as affect the size structure of the rotifer assemblage (Dumont 1977, Hessen & Nilssen 1985, Virro et al 2009). However, Salt et al (1978) argue that A. priodonta relies heavily on different kinds of algae rather than larger prey because it has weak trophi.…”

“…Rotifers, crustaceans, and ciliates dominate the zooplankton community. Usually the most abundant members of the zooplankton community are rotifers; these are perennial in the lake, but planktonic ciliates also have an important role in L. Võrtsjärv, as they account for almost two-thirds of the total zooplankton biomass , Zingel & Haberman 2008, Virro et al 2009). …”

“…The samples were preserved in 4% formaldehyde solution. The methods of collecting and analyzing samples, including biomass calculations, are described in detail in Laugaste et al (2001), Haberman (2001) and Virro et al (2009).…”

“…In the lakes of the study area, A. priodonta is supposed to feed on algae and small rotifers and in doing so may effectively control these populations (Virro et al 2009). Previously, when calculating zooplankton production and food ratio A. priodonta was considered semi-predaceous (i.e., consuming algal and animal food in equal amounts) in the studied lakes (Haberman et al 2000, Haberman 2001, Agasild et al 2007, Virro et al 2009). This assumption may have led to inaccurate assessments of the trophic dynamics at the basal consumer level, as differences between the diets of populations of the same Asplanchna species often occur (Chang et al 2010).…”