The significance of the episodic nature of atmospheric deposition to Low Nutrient Low Chlorophyll regions

Abstract

In the vast Low Nutrient Low-Chlorophyll (LNLC) Ocean, the vertical nutrient supply from the subsurface to the sunlit surface waters is low, and atmospheric contribution of nutrients may be one order of magnitude greater over short timescales. The short turnover time of atmospheric Fe and N supply (<1 month for nitrate) further supports deposition being an important source of nutrients in LNLC regions. Yet, the extent to which atmospheric inputs are impacting biological activity and modifying the carbon balance in oligotrophic environments has not been constrained. Here, we quantify and compare the biogeochemical impacts of atmospheric deposition in LNLC regions using both a compilation of experimental data and model outputs. A metadata-analysis of recently conducted field and laboratory bioassay experiments reveals complex responses, and the overall impact is not a simple "fertilization effect of increasing phytoplankton biomass" as observed in HNLC regions. Although phytoplankton growth may be enhanced, increases in bacterial activity and respiration result in weakening of biological carbon sequestration. The application of models using climatological or time-averaged non-synoptic deposition rates produced responses that were generally much lower than observed in the bioassay experiments. We demonstrate that experimental data and model outputs show better agreement on short timescale (days to weeks) when strong synoptic pulse of aerosols deposition, similar in magnitude to those observed in the field and introduced in bioassay experiments, is superimposed over the mean atmospheric deposition fields. These results suggest that atmospheric impacts in LNLC regions have been underestimated by models, at least at daily to weekly timescales, as they typically overlook large synoptic variations in atmospheric deposition and associated nutrient and particle inputs. Inclusion of the large synoptic variability of atmospheric input, and improved representation and parameterization of key processes that respond to atmospheric deposition, is required to better constrain impacts in ocean biogeochemical models. This is critical for understanding and prediction of current and future functioning of LNLC regions and their contribution to the global carbon cycle.

Estimating chemical composition of atmospheric deposition fluxes from mineral insoluble particles deposition collected in the western Mediterranean region

“…Extreme Zn mass fluxes (122.63 ± 1765.23 µg m −2 wk −1 ) were observed at Lampedusa, which need to be thoroughly investigated, but were likely due to the large use of non-ferrous metal manufacturing in this environment strongly impacted by sea spray. Knowing that a good representation of these intense fluxes is critical to estimate the role of atmospheric deposition on marine biosphere in modelling (Guieu et al, 2014), the parameterization of wet deposition of nutrients and trace metals needs to be improved.…”

The Impact of Atmospheric Dry Deposition Associated Microbes on the Southeastern Mediterranean Sea Surface Water following an Intense Dust Storm

“…These atmospheric inputs provide a variety of nutrients and trace metals (Table S1 and reviewed in Guieu et al, 2014), which are required for microbial cellular metabolism, enzymatic activity and growth (e.g., Cvetkovic et al, 2010;Huertas et al, 2014). In addition to nutrients and trace metals, atmospheric deposition may also introduce a wide array of airborne microorganisms to surface seawater (reviewed in Griffin, 2007;Polymenakou, 2012).…”

“…Overall, the impacts observed following desert dust or aerosol additions are diverse and cannot all be explained by the inducement of a "fertilization response" (Guieu et al, 2014). Although variability in aerosol composition and changes in ocean hydrography and ecosystem structure at the time of deposition have been invoked in order to explain the diverse responses (Paytan et al, 2009), another possible explanation is the impact of the airborne microbes delivered with the added dust/dry aerosol deposition.…”

Response of the Eastern Mediterranean Microbial Ecosystem to Dust and Dust Affected by Acid Processing in the Atmosphere

“…Atmospheric dust and aerosols represent an important source of dissolved nutrients to the offshore global ocean and can increase primary and bacterial productivity and thus carbon uptake (reviewed in Guieu et al, 2014). This is particularly important in regions of the ocean that are strongly impacted by desert dust such as the Central N. Atlantic, the NW Pacific Ocean and the Mediterranean Sea (Mahowald et al, 2005(Mahowald et al, , 2008.…”