Evaluation of iron sources in the Ross Sea

https://doi.org/10.1016/j.jmarsys.2020.103429Get rights and content

Highlights

  • Control of Ross Sea phytoplankton by dFe was simulated with a one-dimensional model.

  • Input of dFe from sea ice melt initiates Phaeocystis antarctica blooms in early spring.

  • Resuspension of iron-rich sediment supports phytoplankton growth near shallow banks.

  • Mid- and deep-water dFe sources support diatom blooms following P. antarctica blooms.

  • P. antarctica contributes more chlorophyll but less POC than do diatoms.

Abstract

A one-dimensional numerical model that includes the complex life cycle of Phaeocystis antarctica, diatom growth, dissolved iron (dFe) and irradiance controls, and the taxa's response to changes in these variables is used to evaluate the role of different iron sources in supporting phytoplankton blooms in the Ross Sea. Simulations indicate that sea ice melt accounts for 20% of total dFe inputs during low light conditions early in the growing season (late November-early December), which enhances blooms of P. antarctica in early spring. Advective inputs of dFe (60% of total inputs) maintain the P. antarctica bloom through early January and support a diatom bloom later in the growing season (early to mid-January). In localized regions near banks shallower than 450 m, suspension of iron-rich sediments and entrainment into the upper layers contributes dFe that supports blooms. Seasonal dFe budgets constructed from the simulations show that diatom-associated dFe accounts for the largest biological reservoir of dFe. Sensitivity studies show that surface input of dFe from sea ice melt, a transient event early in the growing season, sets up the phytoplankton sequencing and bloom magnitude, suggesting that the productivity of the Ross Sea system is vulnerable to changes in the extent and magnitude of sea ice.

Introduction

Iron is central to growth and productivity of Antarctic waters, both in the deeper waters of the Antarctic Circumpolar Current and on continental shelves (Martin et al., 1990; Boyd et al., 2000; Coale et al., 2004; Marsay et al., 2017), similar to its role in other high nutrient, low chlorophyll regions (de Baar et al., 2005). Iron is supplied to Antarctic surface waters from atmospheric dust (Li et al., 2008; Tagliabue et al., 2009), sea ice ablation (Grotti et al., 2005; Lannuzel et al., 2010), glacial melt (Gerringa et al., 2012; Alderkamp et al., 2012; Sherrell et al., 2015), and vertical entrainment of deeper iron-rich waters (Marsay et al., 2014; McGillicuddy et al., 2015). Despite these potential pathways of supply, iron supply rates during the Antarctic spring/summer are low.

The primary productivity in the Ross Sea is estimated to be ca. 179 g C m−2 yr−1, which is deemed to be the greatest biomass production of any coastal region in the Southern Ocean (Arrigo et al., 2008), and iron supply is critical in regulating phytoplankton growth, productivity and composition of this region (Smith et al., 2014). Atmospheric deposition rates of iron are low in the Ross Sea (Cassar et al., 2007), sea ice inputs are episodic, and vertical resupply is reduced by the stratification that persists throughout the growing season. The low supply rates lead to broad patterns of iron limitation, which have been confirmed by experimental manipulations (Sedwick et al., 2000; Olson et al., 2000; Feng et al., 2010) and field observations using iron-limitation proxies (Smith Jr. et al., 2011, Smith Jr. et al., 2013; Gerringa et al., 2015; Kustka et al., 2015; Hatta et al., 2017; Marsay et al., 2014, Marsay et al., 2017).

Iron budgets constructed for the Ross Sea provide estimates of the relative importance of input pathways (McGillicuddy et al., 2015; Gerringa et al., 2015). McGillicuddy et al. (2015) showed that sea ice melt and resupply from deep waters via sediment and Circumpolar Deep Water (CDW) account for most of the iron that supports primary production. Gerringa et al. (2015) found that dissolved iron supplied by sediments was the primary input supporting phytoplankton growth in early summer. The deep-water dissolved iron pathway was confirmed with tracer studies implemented with a high-resolution Ross Sea circulation model (Mack et al., 2017). Other studies showed that Modified Circumpolar Deep Water (MCDW), which is a cooler and saltier version of CDW found on the Ross Sea continental shelf, and benthic sources provide dissolved iron that is important for supporting phytoplankton blooms (Kustka et al., 2015; Hatta et al., 2017). However, uncertainties remain about taxon-specific responses to iron additions, as well as spatial and temporal variations in supply and removal.

Phytoplankton assemblages in the Ross Sea are dominated by the haptophyte Phaeocystis antarctica and diatoms. Blooms of P. antarctica generally occur in austral spring, yet growth continues in parts of the Ross Sea throughout summer (Smith et al., 2014; Smithand Jones, 2015). Growth of the haptophyte in spring is facilitated by elevated iron concentrations and reduced irradiance imposed by ice, relatively deep vertical mixing, and low solar angles (Arrigo et al., 1999; Kropuenske et al., 2009). Conversely, diatoms grow in more stratified conditions, such as in areas with melting sea ice. Sedwick et al. (2011) showed that iron was reduced to low concentrations (ca. 0.06 nM) in spring by P. antarctica growth, and these concentrations persisted throughout the summer. Large diatom blooms observed in summer (Peloquin and Smith, 2007; Kaufman et al., 2014; Smithand Kaufman, 2018) are dependent on either new sources of iron or greatly increased carbon/iron ratios under high light conditions. A direct comparison of iron strategies between diatoms and P. antarctica is unavailable.

The many environmental variables that are changing simultaneously (e.g., irradiance, iron concentrations, losses due to grazing and passive sinking, variable elemental ratios of plankton) make determining their interactions and controls on production via experimental manipulations challenging (Boyd et al., 2015). A taxon-specific numerical model provides one approach for assessing the role of irradiance and various pathways of iron supply in controlling biological production. Such models have proven to be powerful tools to understand the interactions among controlling variables and phytoplankton (e.g., Kaufman et al., 2017), and to begin to understand how regions will respond to future climate change. Models for P. antarctica have described its growth (e.g., Wang and Moore, 2011), and other models have focused on diatomaceous growth (e.g., Lancelot et al., 2000). Arrigo et al. (2003) assessed the temporal evolution of P. antarctica and diatom growth in different regions of the Ross Sea in response to the interactions of light and nutrients (NO3 and iron). However, this model did not differentiate the two stages of P. antarctica life cycle. In this study a one-dimensional numerical model that includes the complexities of the P. antarctica life cycle, diatom growth, nitrate, iron, silicate and irradiance controls, and each taxa's response to the changes in these variables is implemented to evaluate the role of different iron sources and the two phytoplankton assemblages in primary production of the Ross Sea.

Section snippets

Model overview

The biogeochemical model used in this study (Fig. 1) is based on Fiechter et al. (2009), with modifications appropriate for the Ross Sea. The model includes dynamics for the macronutrients nitrate-nitrogen (NO3) and silicate (Si), and the micronutrient dissolved iron (dFe). Iron and silicate consist of dissolved and phytoplankton-associated components (Fep, Sip), through inclusion of Fe: C and Si: C ratios, respectively. Primary producers are represented by diatoms and haptophytes, which are

Base case simulation

The base case simulation provides calibration and verification of the biogeochemical model using observations from a high primary production region of the Ross Sea (Smith et al., 2011). The simulated vertical distribution of nitrate, averaged over the simulation, shows the expected increase in concentration with depth (Fig. 2A). Nitrate concentrations in the upper 200 m range between 17 and 32 μM, which agree with observations. The simulated vertical profiles of dissolved Si and biogenic Si (

Productivity, iron demand and recycling

The highest rates of simulated net production, 285 and 347 mmol C m−2 d−1, were obtained from simulations that included dFe inputs from melting sea ice cover. Inputs of dFe from MW sources sustain a simulated primary production of 147–274 mmol C m−2 d−1. Sediment sources of dFe supported production rates of 52–112 mmol C m−2 d−1. Arrigo et al. (2008) estimated mean daily primary productivity rates between 1997 and 2006 in the Ross Sea that ranged from 13 to 237 mmol C m−2 d−1. Smith and Kaufman

Conclusions

Currently, phytoplankton blooms in the Ross Sea result from short- and long-term processes that supply dFe to the surface waters. The short-term supply of dFe to surface waters via sea ice melt early in the growing season enhances the dFe provided by the winter reserve and stimulates a bloom of P. antarctica. Subsequent diatom blooms are supported by dFe that is provided from mid- and deep-water sources. Thus, this combination and sequencing of dFe inputs promotes development of phytoplankton

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper

Acknowledgments

This study is a contribution to the Processes Regulating Iron Supply at the Mesoscale-Ross Sea (PRISM) project, which was supported by NSF Antarctic Sciences grant numbers ANT-0944174 and ANT-0944254, and NSF Office of Polar Programs grant OPP-1643652. We thank an anonymous reviewer for helpful and insightful comments that substantially improved an earlier version of the manuscript. We also thank Dr. A. Piola for editorial assistance in review of this manuscript.

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