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

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 periods of

stratification. These areas are expected to expand in the future due to

lower nutrient supply from sub-surface waters caused by climate-driven

enhanced stratification, likely further increasing the role of atmospheric

deposition as a source of new nutrients to surface waters. Whether plankton

communities will react differently to dust deposition in a warmer and

acidified environment remains; however, an open question. The potential

impact of dust deposition both in present and future climate conditions was

investigated in 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 the frame of the PEACETIME project in May–June 2017. The

experiments comprised two unmodified control tanks, two tanks enriched with

a Saharan dust analogue and two tanks enriched with the dust analogue 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 (3–4 d)

of the experiments. Dust addition led to a rapid release of nitrate and

phosphate, however, nitrate inputs were much higher than phosphate. 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, all performed in coastal waters. The

effects of dust deposition on biological stocks were different for 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. Ocean acidification and warming did not drastically modify the

composition of the autotrophic assemblage, with all groups positively

impacted by warming and acidification. Although autotrophic biomass was more

positively impacted than heterotrophic biomass under future environmental

conditions, a stronger impact of warming and acidification on mineralization

processes suggests a decreased capacity of Mediterranean surface plankton

communities to sequester atmospheric CO2 following the deposition of

atmospheric particles.