Abstract
Increases in rainfall, continental runoff, and atmospheric dust deposition are reducing water transparency in lakes worldwide (i.e. higher attenuation Kd). Also, ongoing alterations in multiple environmental drivers due to global change are unpredictably impacting phytoplankton responses and lakes functioning. Although both issues demand urgent research, it remains untested how the interplay between Kd and multiple interacting drivers affect primary productivity (Pc). We manipulated four environmental drivers in an in situ experiment—quality of solar ultraviolet radiation (UVR), nutrient concentration (Nut), CO2 partial pressure (CO2), and light regime (Mix)—to determine how the Pc of nine freshwater phytoplankton communities, found along a Kd gradient in Mediterranean ecosystems, changed as the number of interacting drivers increased. Our findings indicated that UVR was the dominant driver, its effect being between 3–60 times stronger, on average, than that of any other driver tested. Also, UVR had the largest difference in driver magnitude of all the treatments tested. A future UVR × CO2 × Mix × Nut scenario exerted a more inhibitory effect on Pc as the water column became darker. However, the magnitude of this synergistic effect was 40–60% lower than that exerted by double and triple interactions and by UVR acting independently. These results illustrate that although future global-change conditions could reduce Pc in Mediterranean lakes, multiple interacting drivers can temper the impact of a severely detrimental driver (i.e. UVR), particularly as the water column darkens.
Highlights
Solar radiation, including the ultraviolet (UVR, 280–400 nm), constitutes the main energy source for aquatic autotrophic microorganisms[1], especially phytoplankton
Together with solar ultraviolet radiation (UVR), three other major drivers alter phytoplankton community responses: (1) nutrient (Nut) concentration in surface waters due to more continental runoff and/or atmospheric dust deposition, as currently being registered in tropical, temperate, and polar lakes[3,4], (2) increasing concentrations of atmospheric carbon dioxide ( pCO2) derived from burning of fossil fuels by h umans[5], and (3) recurrent changes in mixing conditions (Mix) due to the increasing in the frequency and intensity of extreme events[6]
We used the attenuation of PAR (400–700 nm) as a measure of transparency of the water column; we found low median values of KdPAR in aquatic ecosystems worldwide (~ 0.50 m−1), implying generally high transparency at these wavelengths of solar radiation (Fig. 1)
Summary
Solar radiation, including the ultraviolet (UVR, 280–400 nm), constitutes the main energy source for aquatic autotrophic microorganisms[1], especially phytoplankton. The attenuation of solar radiation depends on several factors: the water itself; the presence of chromophoric dissolved organic (DOM) and inorganic matter; the concentration of organic and inorganic particles; and the density of phytoplankton which can act as a self-shading agent[2] Because these factors alter the intensity, and spectral composition of the underwater light environment, they can be considered key modulators of the phytoplankton responses to UVR. The net effects of opposing drivers are species s pecific[15], depending on an organism’s capability to overcome environmental stress, as well as context d ependent[16] In this sense, Carrillo et al.[17] found a synergistic UVR × Nut effect that stimulated PP and biomass in a highly transparent Mediterranean lake, but an antagonistic effect in a less transparent Andean lake. DOM can stimulate the microbial loop, by providing a nutrient subsidy to bacteria, and by reducing the exposure of protists to damaging UVR26
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