The influence of incubation periods on photosynthesis-irradiance curves

In phytoplankton primary production studies, samples for the determination of photosynthesis versus irradiance relationship (P-I) are usually incubated at several irradiance levels during a fixed time period, commonly 2-4 h. However, it is not clear if the use of this fixed incubation time is appropriate to study the P-I relationship in any given ecosystem. The aim of this work was to study the influence of incubation time on the P-I relationship in natural phytoplankton populations from three different coastal ecosystems: an open coastal area, an estuary, and a coastal lagoon. Physical and chemical variables, phytoplankton biomass, species composition, and P-I curves were analysed. The results showed that, in the coastal area and in the estuary, P-I relationships were time dependent, whereas in the coastal lagoon different incubation periods produced the same P-I curve. An underestimation of daily primary production, ranging from 13% to 42.5%, was calculated when data from standard incubation times (2-4 h) were used in ecosystems where P-I curves present a dynamic time-dependent behaviour. This work suggests that the response of the P-I curves to the incubation time varies with the characteristics of the ecosystem and is related to the light regime to which phytoplankton cells are adapted.