MEPS 412:29-44 (2010)  -  DOI: https://doi.org/10.3354/meps08669

ABSTRACT: The timescales of variability in photosynthetic parameters and the relationships between environmental variables and photosynthetic response are important issues for an accurate estimation of phytoplankton primary production, a major feature of aquatic systems. In the present study, variability in phytoplankton photosynthetic parameters of the Douro estuary was analyzed using data from photosynthesis-irradiance (P-E) incubations performed during seasonal tidal cycles, as well as during monthly surveys carried out along a salinity gradient. Tidal surveys were conducted in October 2003 and January, May, and July 2004. Monthly surveys were conducted from December 2002 to December 2003. Diel variability in P-E parameters was highest during the summer survey. The highest values of the biomass-specific maximum photosynthetic rate P_max were generally observed around noon or in the afternoon, and correlated positively with photosynthetic efficiency α. Seasonal variability in P_max and α was mostly related to temperature and light, whereas photoacclimation seemed to be the most important factor in diel variability. Spatially, variability increased upstream for mean P_max and α values, associated with higher chlorophyll a concentration and temperature. By combining a temperature-limiting function with the Steele P-E equation, 60% of P_max variation was explained. Analysis of inter-seasonal, intra-seasonal, and diel variability of photosynthetic parameters suggests that, in the Douro, photosynthetic parameters P_max and α are most variable at a seasonal timescale, while for optimal light intensity E_opt, diel variability was, as expected, highest. Thus, predictive models of primary productivity should take into account this seasonal variability when looking at larger timescales.

KEY WORDS: P-E parameters · Phytoplankton · Seasonal and diel variability · Primary production · River discharge · Douro estuary