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 were made over one year in a shallow stratifying, hypertrophic El Porcal lake near Madrid (Spain). Data were collected on abiotic factors, primary production, biomass and phytoplankton losses and subjected to reduction by means of several principal component analyses. Furthermore, weekly data on the same variables were gathered from published studies on Überlinger 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 than 60% of overall variance in both lakes. They could be ascribed to phytoplankton production + biomass and photosynthetic physiology + phytoplankton losses, respectively. The ordination of the biological trajectories in the data space of the two first principal components revealed 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 the deep lake. Also, short-term dynamics (one-three weeks) were demonstrated for both phytoplankton communities.A preliminary comparison between phytoplankton biomass dynamics in stratifying, shallow and deep lakes suggests that differences may be attributed partly to differences in depth.