THE TIME-COURSE OF PHYTOPLANKTON BIOMASS AND RELATED LIMNOLOGICAL FACTORS IN SHALLOW AND DEEP LAKES - A MULTIVARIATE APPROACH

Weekly studies of phytoplankton biomass and environmental variables we re made over one year in a shallow stratifying, hypertrophic El Porcal lake near Madrid (Spain). Data were collected on abiotic factors, pri mary production, biomass and phytoplankton losses and subjected to red uction by means of several principal component analyses. Furthermore, weekly data on the same variables were gathered from published studies on Uberlinger See, an embayment of the much deeper, mesotrophic Lake Constance (Central Europe), and treated in the same way. The two first principal components of PCAs on biological variables explained more t han 60% of overall variance in both lakes. They could be ascribed to p hytoplankton production + biomass and photosynthetic physiology + phyt oplankton losses, respectively. The ordination of the biological traje ctories in the data space of the two first principal components reveal ed six stable states of phytoplankton biomass in the shallow lake and seven in the deep lake. The breakpoints between stable states could be due to environmental, abiotic variables in some cases but biological interactions were suspected to be the cause of the other breakpoints. The abiotic effects on phytoplankton biomass took longer to occur in t he deep lake. Also, short-term dynamics (one-three weeks) were demonst rated for both phytoplankton communities. A preliminary comparison bet ween phytoplankton biomass dynamics in stratifying, shallow and deep l akes suggests that differences may be attributed partly to differences in depth.