In open-ocean, nutrient-depleted ecosystems, atmospheric dust is often considered a vital source of biologically important trace metals and limiting nutrients. By supplying elements such as phosphorous, iron, and nitrogen, this dust appears to increase primary productivity, which in turn modifies carbon and other biogeochemical cycles. Yet despite marine ecosystems’ apparent dependence upon atmospheric dust, few studies have documented a direct, long-term connection between it and surface productivity.
To clarify this relationship, Torfstein and Kienast compared in situ surface concentrations of atmospheric dust and chlorophyll a—a common measure of ocean productivity—in the Gulf of Aqaba, a long, narrow extension of the nutrient-depleted Red Sea, limited in both phosphorous and nitrogen.
To overcome this complication, the team compared continuous, daily records of these parameters between 2012 and 2016, with the unique advantage of evaluating the impact of discrete dust storms on marine productivity. The high-resolution results show that no correlation exists between the gulf’s surficial dust and chlorophyll a concentrations, even when the researchers account for lags in the marine ecosystem’s response, between 1 and 10 days.
This finding contradicts the results of previous studies that indicated the importance of dust-derived nutrients for phytoplankton growth. This inconsistency suggests that the role of atmospheric dust in controlling ocean productivity in the northern Red Sea—and possibly other nutrient-poor regions—may have been overestimated. Because the impact of the dust extends throughout the water column, rather than solely at the surface, the authors suggest that future work should evaluate the relationship between these parameters across a greater depth range.
In the past, it has also been suggested that too much dust could have toxic effects on phytoplankton growth. In that case, one could expect a negative correlation between the two, but that was not observed by the team either. Because the Gulf of Aqaba is an important analogue for other warm, nutrient-depleted ocean waters, these study findings are likely to have important implications for understanding biogeochemical cycles in warmer and increasingly arid subtropical oceans. (Journal of Geophysical Research: Biogeosciences, https://doi.org/10.1002/2017JG004063, 2018)
—Terri Cook, Freelance Writer