Links Between the Phytoplankton Community Composition and Trace Metal Distribution in Summer Surface Waters of the Atlantic Southern Ocean

Abstract

This study assessed changes in the phytoplankton community related to macronutrient and bioactive trace metal distribution in surface waters of the Atlantic sector of the Southern Ocean, between Cape Town and Antarctica along the GEOTRACES GIPY_05 (mainly along prime meridian) transect in summer 2014-2015. Several general community structure features were reaffirmed, such as the restriction of cyanobacteria to the northern Subtropical Zone, while haptophytes, such as Phaeocystis, along with diatoms, dominate the community north of the Polar Front, and diatoms clearly dominate south of the Polar Front. These community structure changes were often linked with macro-and micro-nutrient composition changes. For example, the concentration of diatoms increased southwards with the availability of silica whereas the cyanobacterial contribution in the northern Subtropical Zone appeared to be linked with labile cobalt depletion. However, these links were not consistent along the entire transect, i.e., no individual nutrient, such as silica or iron, was linked to community composition changes across all water masses. Each station showed a rather unique combination of nutrient and community compositions. Our findings also indicated impacts on the phytoplankton community through trace metal distributions that could be related to a deep mixing event at ∼54 • S and to ice melt at ∼65 and 68 • S. The timing of sampling after such trace metal fluxes proved to be an important consideration, particularly where iron appeared to be preferentially depleted to near-limiting concentrations, possibly driving utilization of other metals. The study highlights the importance of considering a suite of trace metals when assessing controls of phytoplankton variability in the open ocean and emphasizes the need for higher resolution trace metal sampling and multi-element incubation studies to further study the complex relationships between phytoplankton and nutrients.

Iron and manganese co-limit the growth of two phytoplankton groups dominant at two locations of the Drake Passage

“…Thus, due to different cellular Fe and Mn requirements and TMs availability 6 , not all members of a community may be affected equally by co-limitation (also defined as community co-limitation 25 ). As a consequence, this can result in regional and interspecific differences in cellular phytoplankton stoichiometries and possibly lead also to a cascade effect of limitation by trace metals other than Fe and Mn 6 , 24 .…”

“…Only recently, clear evidence for Fe-Mn co-limitation of phytoplankton growth in the Drake Passage was provided by Browning et al 23 , a hypothesis previously proposed by Martin et al 7 . Due to different TM availability in the seawater and different TMs requirements by the cells, phytoplankton can be limited by multiple TMs 6 , 24 this leading to co-limitation by different TMs at the community level 25 . A strict definition of nutrient co-limitation was given by Liebig’s law 26 describing that an organism is co-limited when one nutrient is limiting followed by a second nutrient limitation.…”

Biogeochemical controls on wintertime ammonium accumulation in the surface layer of the Southern Ocean

“…Iron is not directly involved in NH₄⁺ assimilation but is required for electron transport during photosynthesis and respiration (Raven, 1988). While iron limitation is widespread across the Southern Ocean (Janssen et al, 2020;Pausch et al, 2019;Viljoen et al, 2019), iron availability appears to be higher in winter than during other seasons (Mtshali et al, 2019;Tagliabue et al, 2014) due to enhanced mixing, storms, and increased aeolian deposition (Coale et al, 2005;Honjo et al, 2000;Sedwick et al, 2008). The fact that ρNO₃⁻ and ρNH₄⁺ were generally similar across the transect (Fig.…”

Biogeochemical controls on ammonium accumulation in the surface layer of the Southern Ocean

“…Iron is not directly involved in NH + 4 assimilation but is required for electron transport during photosynthesis and respiration, as well as for chlorophyll synthesis (Raven, 1988). While iron limitation is widespread across the Southern Ocean (Janssen et al, 2020;Pausch et al, 2019;Viljoen et al, 2019), iron availability appears to be higher in winter than during other seasons (Mtshali et al, 2019;Tagliabue et al, 2014) due to enhanced mixing, storms, and increased aeolian deposition (Coale et al, 2005;Honjo et al, 2000;Sedwick et al, 2008). The fact that ρNO − 3 and ρNH + 4 were generally similar across the transect (Fig.…”