Interactive effects of vertical mixing, nutrients and ultraviolet radiation: in situ photosynthetic responses of phytoplankton from high mountain lakes in Southern Europe

[摘要] Global change, together with human activities, has resulted in increasingamounts of organic material (including nutrients) that water bodies receive.This input further attenuates the penetration of solar radiation, leading tothe view that opaque lakes are more "protected" from solar ultravioletradiation (UVR) than clear ones. Vertical mixing, however, complicates thisview as cells are exposed to fluctuating radiation regimes, for which the effectshave, in general, been neglected. Furthermore, the combined impacts of mixing,together with those of UVR and nutrient inputs are virtually unknown. In thisstudy, we carried out complex in situ experiments in three high mountainlakes of Spain (Lake Enol in the National Park Picos de Europa, Asturias, andlakes Las Yeguas and La Caldera in the National Park Sierra Nevada, Granada),used as model ecosystems to evaluate the joint impact of these climate changevariables. The main goal of this study was to address the question of howshort-term pulses of nutrient inputs, together with vertical mixing andincreased UVR fluxes modify the photosynthetic responses of phytoplankton.The experimentation consisted in all possible combinations of the followingtreatments: (a) solar radiation: UVR + PAR (280–700 nm) versus PAR (photosynthetically active radiation)alone (400–700 nm); (b) nutrient addition (phosphorus (P) and nitrogen(N)): ambient versus addition (P to reach to a final concentration of30 μg P L⁻¹, and N to reach N:P molar ratio of 31); and (c)mixing: mixed (one rotation from surface to 3 m depth (speed of1 m 4 min⁻¹, total of 10 cycles)) versus static. Our findings suggestthat under ambient nutrient conditions there is a synergistic effect betweenvertical mixing and UVR, increasing phytoplankton photosynthetic inhibitionand excretion of organic carbon (EOC) from opaque lakes as compared to algaethat received constant mean irradiance within the epilimnion. The oppositeoccurs in clear lakes where antagonistic effects were determined, with mixingpartially counteracting the negative effects of UVR. Nutrient input, mimickingatmospheric pulses from Saharan dust, reversed this effect and clear lakesbecame more inhibited during mixing, while opaque lakes benefited from thefluctuating irradiance regime. These climate change related scenarios ofnutrient input and increased mixing, would not only affect photosynthesis andproduction in lakes, but might also further influence the microbial loop andtrophic interactions via enhanced EOC under fluctuating UVR exposure.

[学科分类] 地球化学与岩石