Seasonality of intra-specific cell size in the phytoplankton of Lake Kinneret

A distinct pattern of seasonal fluctuations in intra-specific cell size and/or colony size was observed in a large number of phytoplankton species from Lake Kinneret, Israel. The same species showed larger cell size or colony size in late winter and smaller size in late summer, with intermediate sizes in the interim periods. This phenomenon was exhibited by species of chlorophytes, dinoflagellates and cyanobacteria that were abundant enough to be sampled (fortnightly) and measured throughout the year. The annual pattern of fluctuations in size repeated itself over 8 consecutive years (2004-2012). The size fluctuations were independent of the temporal changes observed in cell abundance of each species. Rather, peak sizes coincided with lowest water temperature and highest nutrient availability, minimum sizes with highest water temperature and lowest nutrient availability. These observations fit well with current ecological theory on organism size, where larger organisms occur in colder climates and the reverse in warmer climates.