The Biogeochemical Role of Baleen Whales and Krill in Southern Ocean Nutrient Cycling

Abstract

The availability of micronutrients is a key factor that affects primary productivity in High Nutrient Low Chlorophyll (HNLC) regions of the Southern Ocean. Nutrient supply is governed by a range of physical, chemical and biological processes, and there are significant feedbacks within the ecosystem. It has been suggested that baleen whales form a crucial part of biogeochemical cycling processes through the consumption of nutrient-rich krill and subsequent defecation, but data on their contribution are scarce. We analysed the concentration of iron, cadmium, manganese, cobalt, copper, zinc, phosphorus and carbon in baleen whale faeces and muscle, and krill tissue using inductively coupled plasma mass spectrometry. Metal concentrations in krill tissue were between 20 thousand and 4.8 million times higher than typical Southern Ocean HNLC seawater concentrations, while whale faecal matter was between 276 thousand and 10 million times higher. These findings suggest that krill act as a mechanism for concentrating and retaining elements in the surface layer, which are subsequently released back into the ocean, once eaten by whales, through defecation. Trace metal to carbon ratios were also higher in whale faeces compared to whale muscle indicating that whales are concentrating carbon and actively defecating trace elements. Consequently, recovery of the great whales may facilitate the recycling of nutrients via defecation, which may affect productivity in HNLC areas.

Understanding the variability in the iron concentration of Antarctic krill

“…The higher Fe/C ratio in the stomach and digestive gland compared to the rest of the body suggests that krill are ingesting more Fe than they require. The Fe/C ratio for whole krill from this study (10.3 ± 4.3 μ mol mol −1 ) is much lower compared to our previous estimate (69.0 μ mol mol −1 in Ratnarajah et al ) based on samples collected at a different location (Table ). Iron‐to‐carbon ratios in whole krill from the recent study are also similar to diatoms (6 μ mol mol −1 ), autotrophic flagellates (8.7 μ mol mol −1 ) and heterotrophic flagellates (14.1 μ mol mol −1 ) growing in Fe deplete conditions (Twining and Baines ).…”

“…The Fe concentrated in krill is transferred to higher order marine animals when they are eaten, and ultimately recycled through the defecation of excess Fe into Fe‐poor surface waters of the Southern Ocean (Nicol et al ; Ratnarajah et al ). Adult whales, like all mammals, do not require all the Fe that they ingest and, because they cannot excrete Fe once it has been absorbed, most of it is defecated (Smetacek ; Nicol et al ; Ratnarajah et al ). Baleen whales take in Fe that is unavailable for phytoplankton growth (because it is locked up in the bodies of krill) and convert it to faecal slurry that could be a stimulant for phytoplankton growth (Smith et al ).…”

“…Recent studies have demonstrated that in the Southern Ocean, krill could play an important role in storing iron (Fe) obtained through their diet (Nicol et al 2010;Ratnarajah et al 2014). Lower trophic level crustaceans store Fe in their body muscle and cuticle for physiological requirements (Marsden and Rainbow 2004;Nicol et al 2010;Ratnarajah et al 2014). Nicol et al (2010) estimated that the Southern Ocean krill population could contain approximately 24% of the total Fe in the surface waters in the region, acting as a Fe reservoir.…”

Global nutrient transport in a world of giants

“…We also found vast regional differences: Vertical transport capacity in the Southern Ocean is now ∼16% of its historical value, but there are higher values in the North Pacific (34%) and the North Atlantic (28%). We compare our estimates of P movement at natural capacity by marine mammals with quantities of ocean P concentrations that were measured by the Ocean Climate Laboratory (details are provided in SI Appendix) and estimate that on a yearly basis, in the past, marine mammals could have increased surface concentrations by up to 1% per year in the Southern Ocean [2.5 kg·km −2 ·y −1 added to a mean concentration of 248 kg·km −2 , although other calculations have suggested that the effect on trace elements could be even higher (29)], which could result in considerable stock changes in surface P over time.…”

Iron in sea ice: Review and new insights

“…However, if sea ice is not present, the Fe delivered from glaciers, land masses and sediments will not have the opportunity to be stored over winter and released into surface waters in spring, at a time ideal for phytoplankton growth; i.e., the link that sea ice makes between the continental sources (where Fe is replete) and the offshore high-nutrient low-chlorophyll surface ocean (where Fe is needed) would be cut. Also, higher trophic levels, such as krill and whales that rely on sea ice and its associated food web, have been recognized to play a role in remineralizing Fe in surface waters (Tovar-Sánchez et al, 2007;Lavery et al, 2010;Nicol et al, 2010;Ratnarajah et al, 2014). As such, it will be difficult to predict what effect a predicted reduction in sea-ice extent and volume will have on the Fe cycle given the multiple interlinked positive and negative feedback loops.…”