Environmental factors: water salinity, temperature, incident light, dissolv
ed inorganic nitrogen (DIN), dissolved reactive phosphorus (DRP), and bioti
c factors: Ulva rigida biomass in field and relative growth rates (RGR) in
cages of the two Mediterranean stations (Chanel of the Thau lagoon, France
and Venice Lido, Italy) were compared. Principal Componant Analysis of envi
ronmental variables shows the linkage " salinity-temperature " in the eutro
phicated Thau lagoon, and the linkage " salinity-incident light " in Venice
Lido. Due to the water temperature the annual Ulva rigida RGR curve is bim
odal (spring and autumn peaks) in contrast to the unimodal classical sheme.
The PCA of biological and environmental variables underlines the DRP as th
e most limiting factor in spring at Venice Lido station and the incident li
ght as a strigent limiting factor for autumn growth in the Thau lagoon.