Abstract
Abstract. Productivity in the Southern Ocean (SO) is iron-limited, and supply of iron dissolved from aeolian dust is believed to be the main source from outside the marine environment. However, recent studies show that icebergs could provide a comparable amount of bioavailable iron to the SO as aeolian dust. In addition, small-scale areal studies suggest increased concentrations of chlorophyll, krill, and seabirds surrounding icebergs. Based on previous research, this study aims to examine whether iceberg occurrence has a significant impact on marine productivity at the scale of the SO, using remote sensing data of iceberg occurrences and ocean net primary productivity (NPP) covering the period 2002–2014. The impacts of both large and small icebergs are examined in four major ecological zones of the SO: the continental shelf zone (CSZ), the seasonal ice zone (SIZ), the permanent open ocean zone (POOZ), and the polar front zone (PFZ). We found that the presence of icebergs is associated with elevated levels of NPP, but the differences vary in different zones. Grid cells with small icebergs on average have higher NPP than other cells in most iron-deficient zones: 21 % higher for the SIZ, 16 % for the POOZ, and 12 % for the PFZ. The difference is relatively small in the CSZ where iron is supplied from meltwater and sediment input from the continent. In addition, NPP of grid cells adjacent to large icebergs on average is 10 % higher than that of control cells in the vicinity. The difference is larger at higher latitudes, where most large icebergs are concentrated. From 1992 to 2014, there is a significant increasing trend for both small and large icebergs. The increase was most rapid in the early 2000s and has leveled off since then. As the climate continues to warm, the Antarctic Ice Sheet is expected to experience increased mass loss as a whole, which could lead to more icebergs in the region. Based on our study, this could result in a higher level of NPP in the SO as a whole, providing a possible negative feedback for global warming in near future.
Highlights
Iron, the fourth most abundant crustal element, has received widespread attention since the late 1980s due to its particular relevance for biogeochemical cycles in the ocean, especially in the high-nutrient low chlorophyll (HNLC) oceanic areas where plenty of macronutrients, such as nitrate and phosphate, are present, but with limited Fe (Falkowski et al, 1998)
The majority of small icebergs come from the dislocation and breaking of large icebergs and serve as an important diffuse process for transport of freshwater and nutrients including iron (Tournadre et al, 2016). We examine both large and small icebergs to determine if similar impacts on the ocean net primary productivity (NPP) could be observed at the Southern Ocean (SO) scale
When only productive water is considered, productivity is the highest in the continental shelf zone (CSZ) (113.5 + 3.7 gC m2 yr−1) near the coast of Antarctica. It gradually decreases in the seasonal ice zone (SIZ) (63.0 + 1.6 gC m2 yr−1), reaching the minimum values in the permanent open ocean zone (POOZ) (57.5 + 1.0 gC m2 yr−1)
Summary
The fourth most abundant crustal element, has received widespread attention since the late 1980s due to its particular relevance for biogeochemical cycles in the ocean, especially in the high-nutrient low chlorophyll (HNLC) oceanic areas where plenty of macronutrients, such as nitrate and phosphate, are present, but with limited Fe (Falkowski et al, 1998). The iron hypothesis stimulated new interests into the iron-enrichment experiments ranging from microcosmic (e.g., de Baar et al, 1990) to large-scale studies (e.g., de Baar et al, 2005) in either natural or artificial settings (Blain et al, 2007; Smetacek et al, 2012). These studies have demonstrated that Fe serves as a major control over both the primary productivity and the planktonic community structure in HNLC waters in more than 25 % of the world oceans. De Baar and de Jong (2001) estimate it between 0.1 and 0.6 nM. Klunder et al (2011) reported that within the upper surface mixed layer of the Atlantic sector of the SO, the concentrations of dissolved Fe vary between 0.1
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