Paradox of planktonic rotifers: similar structure but unique trajectories in communities of the Great Masurian Lakes (Poland)

For decades ecologists have attempted to characterize zooplankton community structure (GHILAROV & TIMONIN 1972, SCHMID-ARAYA 1998, CASTRO et al. 2005). Many studies use indices such as species richness (S) and species diversity (H'). Unfortunately these kinds of studies only provide a snapshot perspective of community composition, whereas sophisticated computer models of community dynamics (e.g., FUSSMANN & HERBER 2002) may require training or resources beyond the means o f some researchers, hindering important research. With studies linking biodiversity to ecosystem processes (LOREAU & HECTOR 2001, LoREAU et al. 200 l), assessing diversity may be key to advancing our understanding of community structure and function. Thus, we need new ways to assess zooplankton community dynamics that are not overly prohibitive in their reliance on sophisticated statistical tools. Although our approach precludes examination of interactions among multiple trophic levels, here we explore ways for analyzing community composition and diversity of zooplankton through a focused analysis of rotifers, microscopic metazoans critical to freshwater food webs (WALLACE et al. 2006). Synchronization o f rotifer populations has been demonstrated in the field (Lake Constance; VASSEUR & GAEDKE 2007) and in a laboratory study of monoclonal populations (e.g., FoNTAINE & GoNZALEZ 2005). Lake proximity may increase environmental similarity and facilitate dispersal, both of which can lead to synchronization (FoNTAINE & GoNZALEZ 2005). Anticipating that communities in apposed lakes will exhibit sirnilar trends, we tested the null hypothesis of no difference in community trajectories. To compare community structure among lakes and years, we applied several diversity indices to rotifer communities in 4 of the Great Masurian Lakes o f northeastem Poland._Indices evaluated were: effective number o f species (ENS); guild ratio (GR); Index ofFaunal Originality (IFO); Shannon-Wiener index (H'); species richness (S); and Species Tumover Index (STI; Table l). To our knowledge, these parameters have never been applied together to freshwater community composition. Previous studies have employed a variety of approaches to statistically analyze or model community dynamics (HARVEY et al. 1983, BJ0RNSTAD 2001, RusAK et al. 2002, WHITE et al. 2006), but we explored the use o f simple indices and graphical analysis to give an initial impression of community trends. We also introduce a new index (GR) of rotifer feeding types, thus including a functional trait analysis with diversity indices to characterize community structure.