Impact of dust enrichment on Mediterranean plankton communities under present and future conditions of pH and temperature: an experimental overview

Abstract

Abstract. In Low Nutrient Low Chlorophyll areas, such as the Mediterranean Sea, atmospheric fluxes represent a considerable external source of nutrients likely supporting primary production especially during stratification periods. These areas are expected to expand in the future due to lower nutrient supply from sub-surface waters caused by enhanced stratification, likely further increasing the role of atmospheric deposition as a source of new nutrients to surface waters. Yet, whether plankton communities will react differently to dust deposition in a warmer and acidified environment remains an open question. The impact of dust deposition both in present and future climate conditions was assessed through three perturbation experiments in the open Mediterranean Sea. Climate reactors (300 L) were filled with surface water collected in the Tyrrhenian Sea, Ionian Sea and in the Algerian basin during a cruise conducted in May/June 2017 in the frame of the PEACETIME project. The experimental protocol comprised two unmodified control tanks, two tanks enriched with a Saharan dust analog and two tanks enriched with the dust analog and maintained under warmer (+3 °C) and acidified (−0.3 pH unit) conditions. Samples for the analysis of an extensive number of biogeochemical parameters and processes were taken over the duration of the experiments (3–4 d). Here, we present the general setup of the experiments and the impacts of dust seeding and/or future climate change scenario on nutrients and biological stocks. Dust addition led to a rapid and maximum input of nitrate whereas phosphate release from the dust analog was much smaller. Our results showed that the impacts of Saharan dust deposition in three different basins of the open Northwestern Mediterranean Sea are at least as strong as those observed previously in coastal waters. However, interestingly, the effects of dust deposition on biological stocks were highly different between the three investigated stations and could not be attributed to differences in their degree of oligotrophy but rather to the initial metabolic state of the community. Finally, ocean acidification and warming did not drastically modify the composition of the autotrophic assemblage with all groups positively impacted by warming and acidification, suggesting an exacerbation of effects from atmospheric dust deposition in the future.

Impact of dust addition on the metabolism of Mediterranean plankton communities and carbon export under present and future conditions of pH and temperature

“…This was confirmed by the 14 C-based particulate production rates, which were about half the ones measured at the other two stations. The community at TYR was most likely relying on regenerated nutrients, as shown by the highest levels of ammonium (NH4 + ) measured at the start of this experiment (Gazeau et al, 2020). As discussed in Guieu et al…”

“…Phytoplankton communities at stations TYR and ION were dominated by Prymnesiophytes followed by Cyanobacteria, while, at station FAST, the phytoplanktonic community was clearly dominated by photosynthetic prokaryotes. At all three stations, the proportion of pigments representative of larger species was very small (< 5%; Gazeau et al, 2020). Heterotrophic prokaryotes were the most abundant at station FAST (6.15 x the percentage of extracellular release (%PER) did not exceed 45%.…”

“…Initial conditions in terms of the chemical and biological standing stocks measured while filling the tanks at the three stations are fully described in Gazeau et al (2020). Briefly, the three experiments were conducted with surface seawater collected during stratified oligotrophic conditions typical of the open Mediterranean Sea at this period of the year (Table 1).…”

Spatial patterns of ectoenzymatic kinetics in relation to biogeochemical properties in the Mediterranean Sea and the concentration of the fluorogenic substrate used

“…Thus, the difference between Km 1 and Km all might reflect adaptive strategies to spatial and/or temporal patchiness in the distribution of suspended particles. Freshly sinking material was probably not present in our incubations because of the small volume of water used but could have contributed to the release of free bacteria, small suspended particles and DOM within its associated plume (Azam and Long, 2001;Tamburini et al, 2003;Grossart et al, 2007;Fang et al, 2015). Baltar et al (2009a) also suggested that hot spots of activity at depth were associated with particles.…”

“…We compared the in situ LAP hydrolysis rates to the N demand of heterotrophic prokaryotes (which was based on BP data assuming no active excretion of nitrogen and a C/N ratio of 5). Similarly, the in situ rates of TAAs plus TCHOs were compared to the bacterial carbon demand (based on a bacterial growth efficiency of 10 %; Gazeau et al, 2020;Céa et al, 2014;Lemée et al, 2002). Using the global model, in situ hydrolysis of TAAs by LAP contributed only 25 % ± 22 % of the bacterial N demand in epipelagic layers and 26 % ± 24 % in deep layers.…”