Dust deposition and seawater fertilization in the Benguela current

Abstract

The atmospheric transport of windblown dust is a vital mechanism delivering essential nutrients to ecosystems, particularly influencing primary oceanic productivity through its impact on phyto-plankton growth and the carbon cycle. The trace metal iron (Fe) is essential and plays a crucial role in the growth of oceanic biota. The scarcity of Fe has been linked to the limitation of phyto-plankton growth in high-nutrient, low-chlorophyll (HNLC) regions. Eastern boundary upwelling systems, exemplified by the Benguela Upwelling System (BUS), are thought to share similarities with HNLC oceans and are suggested to face nutrient limitations, including Fe. Notably, desert dust, constituting approximately 95% of the global atmospheric budget, is a primary carrier of atmospheric Fe. However, given the frequency of dust-emitting sources surrounding the BUS, which are high in Fe, it’s feasible to assume that the BUS is not Fe-limited. Previous remote sens-ing studies have highlighted the frequent dust emission from the southern African region, depos-iting in the adjacent BUS. Despite the potential significance of dust inputs to the BUS, the impact on phytoplankton biomass has only recently come under scrutiny. Recent research utilizing dust monitoring and chlorophyll a (Chl-a) remote sensing has observed increased chlorophyll concen-trations in the BUS daily. Chl-a is commonly used as a proxy for phytoplankton changes in the ocean. While some research has examined the response of phytoplankton to individual nutrient treatments in the northern BUS, the reaction of BUS phytoplankton to a combined sample of Na-mib dust has not yet been explored. There is no existing long-term, high-resolution dust deposition modelling study for the BUS. It also remains unknown whether the seasonal dust deposition cycle potentially aligns with the seasonal phytoplankton changes in the region at the finer scale. In addition, a very recent dust emission event, visible on NASA satellite images, prompted the inves-tigation of potentially new dust-emitting sources in the southwestern Angolan region. These lim-itations impede our understanding of dust deposition and seawater fertilization in the BUS. This study explores the dust problem over the western southern African coast by answering four key questions: 1) Is there a dust problem? 2) What are the sources of the dust? 3) Where is the dust being transported to? and 4) What are the effects of dust deposition on the BUS? In pursuit of these objectives, the study aims to i) Characterize high and low aerosol episodes over the Namibian coast through ambient monitoring; ii) Identify preliminary new southern African dust sources; iii) Investigate the relationship between ambient Fe concentrations over the Namibian coast and sat-ellite Chl-a data; iv) Evaluate total dust deposition to the BUS using a Conformal Cubic Page | ii Abstract Atmospheric Model (CCAM) simulation; and v) Investigate the response of BUS phytoplankton to Namib desert dust addition in a land-based mesocosm experiment. During high aerosol episodes (HAEs), prevailing winds were southerly and south-westerly, originating from the adjacent BUS. Daytime Fe concentrations varied between 3.00 and 3687.00 ng/m³, averaging 351.67±496.58 ng/m³, while nighttime concentrations were typically lower, averaging 362.06±410.95 ng/m³. Monthly mean Fe concentrations showed variations, with the most extreme concentration events (ECEs) occurring in March. The locations of 47 new potential southern African dust sources were identified, and their landform types are suspected to be similar to the Namibian sources to the south. All detected plumes from the sources stretched over 100 km, suggesting a substantial po-tential for emission and deposition. Notably, the Benguela Low-Level Jet drove emissions from these sources. Two domains of high dust deposition were identified: one to the north adjacent to southwestern Angola and the other in the south surrounding Lüderitz. No seasonal correlation and agreement were found in comparing the CCAM dust deposition with the observed Chl-a concen-trations separately for both identified domains. Namib desert dust addition resulted in a positive response from the BUS phytoplankton, given the higher Chl-a concentrations in the mesocosm subject to dust addition compared to the control mesocosm. The observation that adding dust from the Namib desert stimulated the dust mesocosm is both significant and surprising. Given that this experiment was conducted in an upwelling system near the continental shelf and within a region frequently exposed to substantial aeolian dust supply, it is plausible to expect that phytoplankton in the Benguela would be nutrient-replete. The Chl-a increase was unrelated to Fe, Si, PO43−, or light intensity, given weak correlation coefficients.