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.
Highlights
IntroductionLarge regions of the Southern Ocean are characterized by low phytoplankton biomass despite high concentrations of major nutrients (e.g. nitrate, phosphate and silicate), and have been characterised as High Nutrient Low Chlorophyll (HNLC) waters [1]
Large regions of the Southern Ocean are characterized by low phytoplankton biomass despite high concentrations of major nutrients, and have been characterised as High Nutrient Low Chlorophyll (HNLC) waters [1]
One factor responsible for limiting the accumulation of phytoplankton in HNLC waters has been the availability of essential trace elements, iron (Fe), that are required for biochemical processes such as photosynthesis and respiration, as well as in the reduction of inorganic nitrogen species [7]
Summary
Large regions of the Southern Ocean are characterized by low phytoplankton biomass despite high concentrations of major nutrients (e.g. nitrate, phosphate and silicate), and have been characterised as High Nutrient Low Chlorophyll (HNLC) waters [1]. Phytoplankton forms the base of the marine food chain, supporting everything from microscopic animals to large marine mammals [2,3,4]. It plays an important role in carbon sequestration by converting carbon dioxide (CO2) to biomass through photosysnthesis, and through sinking, transferring the carbon to the deep ocean and sea floor sediments [5, 6]. The major sources of trace elements in marine ecosystems are from atmospheric deposition, continental run-off, shelf sediments, hydrothermal vents and ocean crust [8]. As Fe, Mn, and Cu have a short residence time, while Cd, Zn and P have an intermediate residence time in oxygenated waters [11,12,13,14], any mechanism that can increase the persistence of trace elements in surface waters should enhance overall marine primary productivity
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