Light as a driver of phytoplankton growth and production in the freshwater tidal zone of a turbid estuary

Light is usually the main driver of phytoplankton growth in turbid estuaries, but it has received far less attention than nutrients as a bottom-up factor. This study presents the first experimental analysis of light limitation of phytoplankton growth and production and its seasonal variability in the freshwater tidal reaches of the turbid Guadiana estuary, SE Portugal/SW Spain. Natural phytoplankton communities were exposed to different photosynthetically active radiation (PAR) intensities. Short-term incubations with addition of 14HCO 3 − were used to estimate photosynthetic parameters and long-term incubations allowed the evaluation of the effects of light on phytoplankton composition and growth. Light limitation of phytoplankton growth occurred throughout the year in the freshwater tidal reaches of the estuary and no photoinhibition was observed at least up to 615 μmol photons m −2 s −1. In the summer, co-limitation by nutrients prevented a positive response of phytoplankton to light enrichment. Diatoms were the most light-limited group, whilst cyanobacteria were the only group acclimated to low-light conditions. Green algae and dinoflagellates responded positively to higher PAR exposures. High saturating irradiances, high light-saturated rates of primary production and low photosynthetic efficiencies suggest that phytoplankton community was not acclimated to the low-light conditions that prevail in the Guadiana estuary. ► Light is usually the main driver of phytoplankton in turbid estuaries. ► We carried out light enrichment experiments under controlled conditions with phytoplankton from the freshwater tidal reaches of the Guadiana estuary. ► Phytoplankton growth was light-limited throughout the year. ► Diatoms were the most light-limited groups, in contrast to cyanobacteria. ► P–E curves suggest that phytoplankton was not acclimated to the low-light conditions that prevail in the Guadiana estuary.