Lara S. Garcia-Corral

Ultraviolet radiation enhances Arctic net plankton community production

In this study we report the response of net community production (NCP) of plankton communities in the Arctic surface waters exposure to natural ultraviolet radiation (UVR) conditions. A possible bias in previous measurements performed using borosilicate glass bottles (opaque to most UVR) can underestimate NCP. Here we show that 77% of the sampled communities suffer, on average, 38.5% of net increase in NCP when exposed to natural UV-B condition, relative to values when UV-B radiation is excluded. UV-B tends to shift communities toward autotrophy, with the most autotrophic communities responding the strongest. This is likely explained by the inhibition of bacterial respiration during the continuous day period of the Arctic summer, corroborated by experiments where bacterial production influenced by UV-B directly affect NCP. Whereas Arctic warming is expected to lead to lower NCP, our results show that increased UV-B radiation may partially compensate this negative effect in surface waters.

Experimental assessment of cumulative temperature and UV-B radiation effects on Mediterranean plankton metabolism

The Mediterranean Sea is a vulnerable region for climate change, warming at higher rates compare to the global ocean. Warming leads to increased stratification of the water column and enhanced the oligotrophic nature of the Mediterranean Sea. The oligotrophic waters are already highly transparent, however, exposure of Mediterranean plankton to ultraviolet radiation (UV-B and UV-A) may increase further if the waters become more oligotrophic, thereby, allowing a deeper UV radiation penetration and likely enhancing impacts to biota. Here we experimentally elucidate the cumulative effects of warming and natural UV-B radiation on the net community production (NCP) of plankton communities. We conducted five experiments at monthly intervals, from June to October 2013, and evaluated the responses of NCP to ambient UV-B radiation and warming (+3oC), alone and in combination, in a coastal area of the northwest Mediterranean Sea. UV-B radiation and warming lead to reduced net community production and resulted in a heterotrophic (NCP<0) metabolic balance. Both UV-B radiation and temperature, showed a significant individual effect in NCP across treatments and time. However, their joint effect showed to be synergistic as the interaction between them (UV x Temp) was statistically significant in most of the experiments performed. Our results showed that both drivers, would affect the gas exchange of CO2-O2 from and to the atmosphere and the role of plankton communities in the Mediterranean carbon cycle

Continuous daylight in the high-Arctic summer supports high plankton respiration rates compared to those supported in the dark

Plankton respiration rate is a major component of global CO2 production and is forecasted to increase rapidly in the Arctic with warming. Yet, existing assessments in the Arctic evaluated plankton respiration in the dark. Evidence that plankton respiration may be stimulated in the light is particularly relevant for the high Arctic where plankton communities experience continuous daylight in spring and summer. Here we demonstrate that plankton community respiration evaluated under the continuous daylight conditions present in situ, tends to be higher than that evaluated in the dark. The ratio between community respiration measured in the light (Rlight) and in the dark (Rdark) increased as the 2/3 power of Rlight so that the Rlight:Rdark ratio increased from an average value of 1.37 at the median Rlight measured here (3.62 µmol O2 L−1 d−1) to an average value of 17.56 at the highest Rlight measured here (15.8 µmol O2 L−1 d−1). The role of respiratory processes as a source of CO2 in the Arctic has, therefore, been underestimated and is far more important than previously believed, particularly in the late spring, with 24 h photoperiods, when community respiration rates are highest.

Temperature dependence of plankton community metabolism in the subtropical and tropical oceans: Temperature Dependence of Subtropical Plankton Metabolism

Here we assess the temperature dependence of the metabolic rates (gross primary production - GPP, community respiration - CR and the ratio GPP/CR) of oceanic plankton communities. We compile data from 133 stations of the Malaspina 2010 Expedition, distributed among the subtropical and tropical Atlantic, Pacific and Indian oceans. We used the in vitro technique to measured metabolic rates during 24 h incubations at three different sampled depths: surface, 20% and 1% of the photosynthetically active radiation measured at surface. We also measured the % of ultraviolet B radiation (UVB) penetrating at surface waters. GPP and CR rates increased with warming, albeit different responses were observed for each sampled depth. The overall GPP/CR ratio declined with warming. Higher activation energies (Ea´s) were derived for both processes (GPPChla = 0.97; CRChla = 1.26; CRHPA= 0.95 eV) compared to those previously reported. The Indian Ocean showed the highest Ea (GPPChla = 1.70; CRChla = 1.48; CRHPA= 0.57 eV), while the Atlantic Ocean showed the lowest (GPPChla = 0.86; CRChla = 0.77; CRHPA= -0.13 eV). We believe that the difference between previous assessments and the ones presented here can be explained by the overrepresentation of Atlantic communities in the previous data sets. We found that UVB radiation also affects the temperature dependence of surface GPP, which decreased rather than increased under high levels of UVB. Ocean warming, which causes stratification and oligotrophication of the subtropical and tropical oceans, may lead to reduced surface GPP as a result of increased penetration of UVB radiation.