Lake Tahoe vs. Lake Kinneret phytoplankton: comparison of long-term taxonomic size structure consistency

Both theoretical ecology and lake management practices acutely need quantitative assessment tools for the analysis of structural changes taking place in the plankton community. Size spectrum, a tool allowing such assessment, is usually based on size distributions of organisms irrespective of their taxonomy. The size-frequency distribution of taxonomic units in an assemblage, named by us 'traditional taxonomic size spectrum' (TTSS), has been applied for over 70 years, but seldom in aquatic ecology. Longterm consistency of phytoplankton TTSS, evidenced even during pronounced ecosystem changes, was described for the subtropical and eutrophic Lake Kinneret, Israel. In the present study, we examine whether consistent TTSS patterns prevail across ecosystems, and apply the TTSS to the phytoplankton of the temperate and oligotrophic Lake Tahoe, USA. A typical annual TTSS pattern was revealed, and its details were analyzed quantitatively by hierarchical cluster analysis. The Tahoe TTSS similarity level during 4 years (Pearson r=0.92 to 0.99) is comparable to that of the Kinneret during its stable period; even for pairs divided by > 20 years, r > 0.8. While the Tahoe TTSS general pattern resembles that of Lake Kinneret, the two lakes are distinguishable by means of cluster analysis. A high similarity (r=0.91) was found between the eight-year averaged TTSSs of the two lakes. The above results let us suppose that the longterm consistency of the aquatic assemblage taxonomic size structure pattern is a general phenomenon. This pattern deserves special attention at times of accelerated global climate change, acerbated by burgeoning anthropogenic impacts.