Abstract
Some of the most important effects of global change on coastal marine systems include increasing nutrient inputs and higher levels of ultraviolet radiation (UVR, 280–400 nm), which could affect primary producers, a key trophic link to the functioning of marine food webs. However, interactive effects of both factors on the phytoplankton community have not been assessed for the Mediterranean Sea. An in situ factorial experiment, with two levels of ultraviolet solar radiation (UVR+PAR vs. PAR) and nutrients (control vs. P-enriched), was performed to evaluate single and UVR×P effects on metabolic, enzymatic, stoichiometric and structural phytoplanktonic variables. While most phytoplankton variables were not affected by UVR, dissolved phosphatase (APAEX) and algal P content increased in the presence of UVR, which was interpreted as an acclimation mechanism of algae to oligotrophic marine waters. Synergistic UVR×P interactive effects were positive on photosynthetic variables (i.e., maximal electron transport rate, ETRmax), but negative on primary production and phytoplankton biomass because the pulse of P unmasked the inhibitory effect of UVR. This unmasking effect might be related to greater photodamage caused by an excess of electron flux after a P pulse (higher ETRmax) without an efficient release of carbon as the mechanism to dissipate the reducing power of photosynthetic electron transport.
Highlights
Coastal marine ecosystems contribute about 30–35% of the global production of phytoplankton in oceanic waters [1] and play a key ecological and economic role for human populations [2]
Changes with depth of the downwelling irradiance of the different spectral bands as well as surface solar radiation during the experimental period are shown in Fig 1A and 1B
Enzymatic antioxidant activities were not measured in our experiment, the absence of violaxanthin or the decrease in the Dt:(Dt+Dd) ratio, pigments related to the xanthophyll cycle, involved in the thermal dissipation of excess light [74], could partly support the idea of the existence of metabolic stress
Summary
The aim of the present study was to quantify single and combined effects of UVR flux and changes in the P supply on primary producers, paying special attention to the physiological mechanisms involved in the responses of the phytoplankton communities
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