Differential responses of two Phytoplankton Communities from the Chubut River Estuary (Patagonia, Argentina) to the combination of UVR and elevated temperature

Abstract

The combined impact of solar ultraviolet radia- tion (UVR) and elevated temperature on the photosynthesis of phytoplankton communities from the two end members of the Chubut River estuary was studied using short- and long-term incubations. Samples from the river and seawater sections of the estuary were collected during the austral Spring (November 2011) and exposed to solar radiation (with and without UVR) under two temperatures: in situ and increased by 5 °C. The long-term incubations were designed to assess the potential acclimation of each phyto- plankton community to the experimental conditions. Short- term incubations, at day 1 and day 5 of the acclimation period, included the determination of Photosynthesis vs. irradiance curves, Biological Weighting Functions and daily cycles of Chl-a fluorescence measurements, and were used as an estimation of the ability to cope with either increased temperature or UVR. There was a synergistic effect be- tween UVR and temperature in the seawater community, with samples demonstrating a better photosynthetic re- sponse in the absence of UVR and at the in situ tempera- ture. In contrast, temperature had an antagonistic effect when combined with UVR in the river community, with increased temperature counteracting the negative impact of UVR. Despite this, both communities showed an acclima- tion capacity toward UVR and increasing temperature. Our results point to a differential impact of global change variables on phytoplankton communities within the estuary, which affect photosynthesis and might influence the species composition with potential repercussions on the food web.