Artificial Upwelling in Singular and Recurring Mode: Consequences for Net Community Production and Metabolic Balance

Artificial upwelling of nutrient-rich waters and the corresponding boost in primary productivity harbor the potential to enhance marine fishery yields and strengthen the biological pump for sequestration of atmospheric CO2. There is increasing urgency to understand this technology as a “ocean-based solution” for counteracting two major challenges of the 21st century—climate change and overfishing. Yet, little is known about the actual efficacy and/or possible side effects of artificial upwelling. We conducted a large-scale off-shore mesocosm study (∼44 m3) in the oligotrophic waters of the Canary Islands to identify the community-level effects of artificial upwelling on a natural oligotrophic plankton community. Four upwelling intensities were simulated (approx. 1.5/3/5.7/10 μmol L–1 of nitrate plus phosphate and silicate) via two different upwelling modes (a singular deep-water pulse vs. recurring supply every 4 days) for 37 days. Here we present results on the response of net community production (NCP), metabolic balance and phytoplankton community composition (<250 μm). Higher upwelling intensities yielded higher cumulative NCP. Following upwelling onset, the phytoplankton community became dominated by diatoms in all treatments, but other taxa such as Coccolithophores increased later in the experiment. The magnitude of effects on the metabolic balance scaled with the amount of added deep water, leading to (i) a balanced to net-heterotrophic system in the singular and (ii) a net-autotrophic system in the recurring upwelling treatments. Accordingly, the mode in which nutrients are supplied to an oligotrophic system plays a crucial role in the ecosystem response, with recurring upwelling leading to higher long-term positive NCP than singular upwelling. These results highlight the importance of empirically measured local responses to upwelling such as community structure and metabolism, with major implications for the potential employment of artificial upwelling as an ocean-based solution to generate (primary) production.

Physiological Responses of Contrasting Maize (Zea mays L.) Hybrids to Repeated Drought

Maize is the most important crop worldwide in terms of production and yield, but every year a considerable amount of yield is lost due to drought. The foreseen increase in the number of drought spells due to climate change raises the question whether the ability to recover quickly after a water pulse may be a relevant trait for overall drought resistance. We here address the following hypotheses: (i) different maize hybrids exhibit distinct physiological adaptive responses to drought stress and (ii) these responses affect the ability to recover from the stress. (iii) The relative biomass production of maize hybrids, which show severe drought symptoms but are able to recover quickly after a water pulse, is comparable to those hybrids, which invest more energy into tolerance mechanisms. The physiological responses of eight maize hybrids to repeated drought were elucidated employing physiological parameters such as electrolyte leakage, osmolality, relative water content, growth rate and gas-exchange measurements. Only one hybrid was able to maintain biomass production under drought conditions. Amongst the others, two hybrids with similar growth inhibition but contrasting physiological responses were identified by a PCA analysis. Both strategies, i.e. stabilization of leaf water content via resistance mechanisms versus high recovery potential were equally effective in maintaining aboveground biomass production in the scenario of a long drought intermitted by a water-pulse. However, each strategy might be advantageous under different drought stress scenarios. Overall, the recovery potential is underestimated in drought resistance under natural conditions, which includes periodic cycles of drought and rewatering, and should be considered in screening trials.

Variations of ocean biogeochemistry in a transient deglacial simulation with MPI-ESM

<p>Variations in ocean-atmosphere carbon exchange, in response to varying physical and biogeochemical ocean states, is one of the major causes of the glacial-interglacial atmospheric CO<sub>2</sub> changes. Most of the existing modelling studies use time-slice simulations with Earth System Models to quantify the proposed mechanisms, such as the impact of a weakened Southern Ocean westerlies and a massive discharge of freshwater from ice sheet melting on the deglacial atmospheric CO<sub>2</sub> rise. We present the variations of ocean biogeochemistry in a transient deglaciation (21 – 10 kB.P.) simulation using the Max Planck Institute Earth System Model. We force the model with reconstructions of atmospheric greenhouse gas concentrations, orbital parameters, ice sheet and dust deposition. In line with the physical ocean component, we account for the automatic adjustment of all marine biogeochemical tracers in response to changing bathymetry and coastlines that relate to deglacial melt water discharge and isostatic adjustment. We include a new representation of the stable carbon isotope (<sup>13</sup>C) in the ocean biogeochemical component to evaluate the simulation against δ<sup>13</sup>C records from sediment cores.</p><p>The model reproduces several proposed oceanic CO<sub>2</sub> outgassing mechanisms. First, the net primary production (NPP) in the North Atlantic Ocean dramatically decrease (by 40 – 80%) during the first melt water pulse (15 – 14 kB.P.) which is caused by the weakening in the strength of the Atlantic Meridional Overturning Circulation from 21 to 3 Sv. However, globally the oceanic NPP only slightly decreases by 8% as oceanic NPP in the South Hemisphere increases during the same period. Second, during the melt water pulse in the Southern Ocean the ventilation of intermediate waters, which has high DIC content and low alkalinity concentration, is slightly enhanced. Third, the surface alkalinity decreases due to dilution and due to episodic shifts between CaCO<sub>3</sub> production and opal production by phytoplankton. Lastly, CO<sub>2</sub> solubility decreases with increasing deglacial sea surface temperature. The increase of surface pCO<sub>2</sub> caused by the above mechanisms is, however, smaller than that of the prescribed atmospheric CO<sub>2</sub>. Thus, the ocean is a weak carbon sink in this deglacial simulation.</p>

Late Quaternary Chrysophycean stomatocysts in a Southern Carpathian mountain lake, including the description of new forms (Romania)

In this study we present results of a low-resolution chrysophyte stomatocyst analysis that followed a high-resolution diatom analysis of a mountain lake sediment sequence from the Retezat Mountains, in the south Carpathians (Romania). The stomatocyst assemblages of the previously distinguished ten diatom assemblage zones of Lake Gales were studied with the aim to describe stomatocyst composition and create a taxonomical basis for detailed stratigraphical analysis in the future. We report 83 stomatocyst forms, and 7 of them are formally described here as new for science. An abrupt shift in cyst as well as diatom assemblages were recorded around 9200 cal yr BP during the 15,000 years long history of the Lake Gales. This Lake Gales event could be linked to the 9.3-ka widespread significant climatic anomaly, which was triggered by a melt water pulse into the North Atlantic.