Phosphorus inputs unmask negative effects of ultraviolet radiation on algae in a high mountain lake

Abstract

Ultraviolet solar radiation (UVR) and atmospheric nutrient loads to pristine ecosystems are global climate change phenomena that simultaneously affect aquatic organisms in ways not easily predicted by single factor studies. Plankton in a high mountain lake was exposed in situ to increasing phosphorus (P) concentrations (mimicking atmospheric pulses) in absence or presence of UVR in order to identify their interactive effect on functional [primary production, organic carbon (C) release (EOC), and percentage of C released (%EOC)], growth rate, structural-physiological (algal biomass, sestonic C, P content, chlorophyll a (Chl a), and Chl a : C ratio, P cell quota, cell-specific Chl a), and stoichiometric (autotroph C : P ratio) traits. The availability of P after the pulse determined the intensity of responses by primary producers to UVR stress. All structuralphysiological and functional variables significantly increased by up to two orders of magnitude in response to P enrichment. UV radiation, over a long-term scale, exerted significant deleterious effects on most structural-physiological variables when inorganic P was added at high levels (! 30 lg P L À1 ). The subsequent unexpected negative synergistic UVR Â P effect on algal development did not support our initial hypothesis that P input might buffer the harmful UVR effect. UVR exerted a weak negative effect on primary production but strongly enhanced the absolute and percentage excretion of C (up to 60%), mechanism responsible of a significant reduction in autotroph C : P ratios. We propose that low sestonic C : P ratios are the outcome of an adaptive strategy of algae in environments with high UVR exposure and extreme nutrient limitation and have important implications for C flux through grazing vs. microbial food webs in oligotrophic systems.

Synergistic effects of UVR and simulated stratification on commensalistic phytoplankton–bacteria relationship in two optically contrasting oligotrophic Mediterranean lakes

“…The results presented here indicate increased susceptibility to UVR of bacteria and phytoplankton communities relatively less exposed to UV-B during their life cycles (Pakulski et al, 2007;Harrison and Smith, 2011a). Interestingly, the UVR effect on %EOC was only significant in the high-UVR lake; the release of C has been described as a protective mechanism to prevent photosystem damage from reducing power excess under high irradiance of PAR (Wood and Van Valen, 1990) as well as UVR (Carrillo et al, , 2008. The lack of this "escape valve", which helps to prevent overexcitation of PSII, might be the final cause of the higher sensitivity of phytoplankton communities in the low-UVR lakes.…”

“…La Caldera is a mixed oligotrophic (total phosphorus [TP] < 0.3 µM and chlorophyll a [Chl a] < 5 µg L −1 ) high-mountain lake above the treeline on a siliceous bedrock in a glacial cirque (Carrillo et al, 2006). This lake has a surface area of 2 ha and a mean depth of 4.3 m, with a maximum depth interannually varying from 2 to 14 m. UVR of considerable intensity penetrates deeply in the lake due to the high transparency of the water and low values of dissolved organic carbon ([DOC] < 0.08 mM) as reported in Carrillo et al (2008) and Helbling et al (2013). Therefore, this lake is hereafter called the "high-UVR" lake.…”

“…It has also been demonstrated that UVR exposure in the upper layers of the water column can increase the proportion of photosynthate released as exudates (Carrillo et al, 2008;Korbee al., 2012), which would stimulate the growth of UVR-resistant bacteria (Xenopoulos and Schindler, 2003) and give rise to a coupled phytoplankton-bacteria relationship in clear oligotrophic lakes . Coupling between phytoplankton and bacterioplankton has been defined as the capacity of the carbon (C) released by phytoplankton to support the bacterial carbon requirement (Morán et al, 2002) and will therefore differ depending on (i) the availability of alternative (allochthonous or autochthonous) carbon sources (Gasol et al, 2009) and (ii) the supply of inorganic nutrients (Medina-Sánchez et al, 2010;López-Sandoval et al, 2011).…”

Phytoplankton are more tolerant to UV than ­bacteria and viruses in the northern South China Sea

“…Medina-Sánchez et al (2002) also reported that UVB radiation inhibited bacteria when they were strongly P-deficient, whereas UVB exerted no direct effect on bacterial activity when they were not P-limited. A recent study showed that phosphorus amendment and UVR exposure had a synergistic negative effect on phytoplankton (Carrillo et al 2008).…”

“…For example, nutrient stress may weaken or mask the UVB stress (Xenopoulos et al 2002), while a nutrient amendment (e.g. P addition) had a negative effect on phytoplankton subjected to UVR exposure (Carrillo et al 2008). …”