δ⁵⁶Fe in seabird guano reveals extensive recycling of iron in the Southern Ocean ecosystem

Abstract

Availability of iron limits productivity in the Southern Ocean where vast regions of high‐nitrogen low‐chlorophyll conditions are punctuated by hotspots of production surrounding the subAntarctic islands. Here, we compare patterns in accumulation of iron, as indicated by [Fe] and Fe : C (from Wing et al. ), with proxies for uptake of iron within food webs, as indicated by Fe : Al and δ56Fe within the subAntarctic Auckland Island ecosystem. We compare these proxies for dynamics of iron within egested material collected from six seabird species, representing three distinct foraging guilds. Fractions of biogenic iron, estimated from Fe : Al, were lowest in the coastal foraging guild (0.83 ± 0.01), and higher in the oceanic and predator guilds (0.94 ± 0.01 and 0.95 ± 0.01), consistent with high rates of biological recycling. The isotopic ratio of iron 56Fe/54Fe, reported as δ56Fe, is depleted in the heavy isotope 56Fe by recycling in the microbial loop, and is likely further depleted in 56Fe when taken up by animals within the food web. δ56Fe was most negative for the oceanic foraging group indicating intensive recycling (−1.16 ± 0.16‰), most positive in the coastal group where direct lithogenic sources of iron are more available (−0.14 ± 0.15‰), and intermediate in the predatory group, representing a mixture and possible fractionation within food webs (−0.37 ± 0.14‰). These data reveal strong contrasts in bioaccumulation and recycling of iron between coastal and oceanic food webs, and important interactions between marine food web structure and biogeochemical processes supporting productivity in the Southern Ocean.

Pelagic Iron Recycling in the Southern Ocean: Exploring the Contribution of Marine Animals

“…This group of animals inexorably draws nutrients closer to the surface. Such recycling of excess nutrients that were ingested but not assimilated has been demonstrated in seabirds (Wing et al, 2014(Wing et al, , 2017Shatova et al, 2016), as well as land and pelagic dwelling marine mammals (Smetacek and Nicol, 2005;Nicol et al, 2010;Ratnarajah et al, 2014Ratnarajah et al, , 2017Wing et al, 2014). Vertebrate fecal material has been linked to Fe enrichments within their local environment and this can stimulate phytoplankton growth (Shatova et al, 2016).…”

“…The spring-time melting of sea ice (Sedwick and Di Tullio, 1997;Lannuzel et al, 2007), ice shelves (Herraiz-Borreguero et al, 2016), and icebergs (Smith et al, 2007;Lancelot et al, 2009;Lin et al, 2011;Duprat et al, 2016) release the trapped Fe into the surface waters. Other sources of Fe can include: hydrothermal vents (Tagliabue et al, 2010;Klunder et al, 2011), upwelling (de Baar et al, 1995Watson, 2001), deep winter mixing (Tagliabue et al, 2014), biogenic mixing (Katija and Dabiri, 2009;Katija, 2012), vertical mixing (Webb and Suginohara, 2001;Cisewski et al, 2005;Frants et al, 2013), the weathering of shelf sediments (Sedwick et al, 2008;Bowie et al, 2009), sediment resuspension (Moore et al, 2004;Blain et al, 2007;Dulaiova et al, 2009) and pelagic recycling (Tovar-Sanchez et al, 2007;Ortega-Retuerta et al, 2009;Schmidt et al, 2011Lehette et al, 2012;Ratnarajah et al, 2014Ratnarajah et al, , 2016aRatnarajah et al, ,b, 2017Wing et al, 2014Wing et al, , 2017Shatova et al, 2016;Laglera et al, 2017).…”

“…Laboratory dissolution experiments of whale fecal material using natural seawater suggests that Fe continues to leach from the fecal particles over a 12 h time period (Ratnarajah et al, 2017). Despite the high Fe concentrations measured in krill (Schmidt et al, 2011Ratnarajah et al, 2016b), seabird (Wing et al, 2014(Wing et al, , 2017Shatova et al, 2016) and whale (Nicol et al, 2010;Ratnarajah et al, 2014Ratnarajah et al, , 2017Wing et al, 2014), fecal material bioavailability of the fecal Fe will depend on the size of the fecal particle, and/or the dissolution of fecal particles over time. In comparison, salp fecal pellets contain high concentrations of Fe, but the pellets are highly refractory (Cabanes et al, 2017).…”

The collapse and recovery potential of carbon sequestration by baleen whales in the Southern Ocean

“…We also show that whalemediated carbon sequestration is predominantly due to the fertilization pathway. Our study suggests that this often-neglected carbon sequestration performed by all marine mammals 27 but also seabirds [27][28][29] and fish [30][31][32] could be considered as a NCS where populations are restored.…”