Size as a factor in centric diatoms distribution: The Spanish reservoirs as an example

Centric diatoms are relatively abundant in the phytoplankton of Spanish reservoirs, and therefore they can be considered as indicative of the phytoplankton populations as a whole. Taxa distribution may often be related to sorne major environmental characteristics of the reservoirs. In the case of sorne species of the genus Aulacoseira (formerly belonging to Melosira), A. distans, A. italica and A. subarctica are confined to N-NW reservoirs, where there are waters of low mineral content, while A. granulata is more widespread, mainly preferring higher mineral content and/or nutrient richer waters. Among the smaller centrics, Cyclotella comensis and C. radiosa are the most frequent, but occur only in the hard-water reservoirs (East Spain, alkalinity > 1 meq r 1 ). C. radiosa was located in northern reservoirs, while C. comensis preferred warmer waters. Other taxa had a more restricted distribution, but their presence was indicative of peculiar environmental conditions. For example, Chaetoceros muelleri is restricted to the phytoplankton of a merornictic, brackish water reservoir. Sorne morphological observations of the cell valve (size, number of striae or areolae, number and disposition of fultoportulae) have been made with SEM on C. comensis and C. radiosa populations. The data come from a large number of reservoirs in the east of Spain, and were tentatively related to environmental variables of the reservoirs using statistical analyses. Cell di ame ter was revealed as the most directly related morphological character. Differences in size between populations of each species were consistent, as confirmed through an analysis of variance. Using the certainty that size has characteristic values in every population, its relationship with environmental variables has been explored using a stepwise regression analysis. Temperature is in fact a common factor affecting the size of organisms. Dependence on calcium content seems more unusual, and it may be considered as an accurate expression of the effect of ionic strength of the water on diatom cell size.