Satellite remote sensing of the island mass effect on the Sub-Antarctic Kerguelen Plateau, Southern Ocean

Abstract

The presence of the Kerguelen Plateau and surrounding bathymetric features has a strong influence on the persistently eastward flowing Antarctic Circumpolar Current (ACC), resulting in enhancement of surface chlorophyll-a (Chl-a) in the downstream section of the plateau along the polar front (PF). The phenomenon is reported in this paper as the island mass effect (IME). Analysis of climatological Chl-a datasets from Aqua-Moderate Resolution Imaging Spectroradiometer (Aqua-MODIS) and Sea-viewing Wide Field-of-view Sensor (SeaWiFS) shows distinct bloomy plumes (Chl-a>0.5 mg/m³) during austral spring-summer spreading as far as ~1800 km offshore up to 98°E along the downstream of the north Kerguelen Plateau (NKP). Similar IME phenomena is apparent over the south Kerguelen Plateau (SKP) with the phytoplankton bloom extending up to 96.7°E, along the southern boundary of ACC. The IME phenomena are pronounced only during austral spring-summer period with the availability of light and sedimentary source of iron from shallow plateau to sea surface that fertilizes the mixed layer. The NKP bloom peaks with a maximum areal extent of 1.315 million km² during December, and the SKP bloom peaks during January with a time lag of one month. The blooms exist for at least 4 months of a year and are significant both as the base of regional food web and for regulating the biogeochemical cycle in the Southern Ocean. Even though the surface water above the Kerguelen Plateau is rich in Chl-a, an exception of an oligotrophic condition dominated between NKP and SKP due to apparent intrusion of iron limited low phytoplankton regime waters from the Enderby basin through the north-eastward Fawn Trough Current.

Keywords

island mass effect / Antarctic Circumpolar Current / Aqua-MODIS / SeaWiFS

Introduction

The occurrence of enhanced primary production off an island in the wake of an eddy flow is reported as the island mass effect (IME) ( Doty and Oguri, 1956), and the phenomenon can be exhibited to investigate natural iron enrichment for high-nutrient low-chlorophyll (HNLC) systems. Iron (Fe) is essential for marine phytoplankton growth ( Sunda, 1989). The Southern Ocean is known as the largest HNLC region of the global ocean (Martin et al., 1990; Minas and Minas, 1992). Earlier mesoscale iron fertilization investigations revealed strong influence of micronutrient iron on phytoplankton biomass and community composition in the Southern Ocean ( Boyd et al., 2000; Coale et al., 2004). Direct deposition of micronutrient iron from mineral dust is low into the Southern Ocean (Luo et al., 2003; Zender et al., 2003). There are locations in the vicinity of islands and their down-streams where iron from islands and surrounding shallow plateau enhances phytoplankton biomass owing to IME (Venables and Moore, 2010). This can contribute to productivity and fishery resources in and around islands ( Heywood et al., 1990; Signorini et al., 1999), and also for the global CO₂ budget ( Heywood et al., 1996). Satellite observations from Coastal Zone Color Scanner (CZCS) and Sea-viewing Wide Field-of-view Sensor (SeaWiFS) data have shown the IME off Madeira Island ( Caldeira et al., 2002), Galapagos Islands ( Palacios, 2002), and Crozet Archipelago ( Bakker et al., 2007). The present work focuses on the scope of satellite remote sensing observations from Aqua-Moderate Resolution Imaging Spectroradiometer (Aqua-MODIS) and SeaWiFS to study the IME phenomena around the Kerguelen Plateau and characterizes the monthly evolution of phytoplankton blooms during austral spring-summer period.

Data analysis and methodology

The Kerguelen Plateau is an extensive submarine topographic high in the Indian Ocean sector of the Southern Ocean bounded by deep ocean basins. Figure 1(a) illustrates the bathymetry of the region mapped using Earth Topography One Arc-Minute Global Relief Model, 2009 (ETOPO1) ice surface version geo-referenced data (21601 by 10801 cells) obtained from www.ngdc.noaa.gov. Northeast shifting of pixel location in ETOPO1 was rectified as the method specified in Jena et al. (2012). The maximum length of the Plateau is approximately 2350 km (restricted between 45°S and 63.5°S latitude) with overall orientation of a Northwest to Southeast trend towards the Antarctic continent. The meridional extent stretches from 61°E to 86°E. The Plateau can be divided into two parts such as the north Kerguelen Plateau (NKP) and the south Kerguelen Plateau (SKP), which is separated by the Fawn Trough (Fig. 1(a)). The FT allows the eastward Antarctic Circumpolar Current (ACC) to flow through the narrow channel. The ACC comprises three major circumpolar fronts: the subantarctic front (SAF), polar front (PF), and southern ACC front (sACCF), which corresponds to water mass boundaries as well as deep-reaching jets of eastward flow ( Orsi et al., 1995). Locations of various Southern Ocean climatological fronts were obtained from the Australian Antarctic Data Centre ( Orsi et al., 1995). In order to understand the circulation pattern in and around the Kerguelen Plateau, the mean dynamic topography (MDT) raster datasets ( Maximenko and Niiler, 2005) were converted to contours and overlaid an Aqua-MODIS entire mission chlorophyll-a (Chl-a) composite image (2002‒2013) (Fig. 1(b)). The MDT dataset was derived from combined analysis of drifters, satellite altimetry, wind, and GRACE (Gravity Recovery and Climate Experiment) satellite based geoid (www.aviso.altimetry.fr). In addition, the trajectories from ARGO float (Argo-1900046, 1901329, 1900314, 2901303, 2901306, 7900310) that crossed the entire Plateau were overlaid to illustrate the surface current patterns in and around the Plateau (Fig. 1(b)).

In-situ observations of biophysical parameters over vast portions of the southern ocean and adjoining Indian Ocean are significantly under-sampled, both temporally and spatially (Jena et al., 2013; Mishra et al., 2015). This limitation is partially overcome by the routine measurement of parameters by means of space-borne radiometers which offer synoptic and temporal sampling advantages (Moore et al., 1999; Jena et al., 2006, 2010; Kumar et al., 2012). In order to characterize the IME phenomena, Aqua-MODIS and SeaWiFS derived Chl-a concentration datasets based upon global standard algorithms were acquired from National Aeronautics and Space Administration (NASA) Goddard Space Flight Center (NASA’s GSFC). Aqua-MODIS Level-2 data during 25th February 2013 (9:50hrs UTC, Ascending pass), was processed to generate the high spatial resolution (~1 km) Chl-a concentration image (Fig. 2). Level-3 climatological Chl-a composites from Aqua-MODIS (2002‒2013) and SeaWiFS (1998‒2010) were generated after discarding sea-ice and cloudy pixels. An unsupervised classification based on the ISODATA algorithm developed by Memarsadeghi et al. (2007) was performed on the Aqua-MODIS Chl-a composite image (2002‒2013) to obtain the robust features associated with IME phenomenon around the Kerguelen Plateau (Fig. 1(b)).

Satellite datasets are known to underestimate Chl-a concentration at elevated in-situ values. For example, Moore et al. (1999) validated SeaWiFS derived Chl-a with in-situ Chl-a_fluorescence (Chl-a_flu) for a range of 0.1–1.5 mg/m³ and the results revealed that SeaWiFS underestimated the in-situ values. Investigations by Szeto et al. (2011) noticed that SeaWiFS Chl-a, using global OC4v4 algorithm, consistently underestimates Chl-a value by 50%, even though HPLC based measurements were considered. Earlier works defined the phytoplankton bloom as Chl-a concentration>0.8 mg/m³ ( Fitch and Moore, 2007) and>1 mg/m³ ( Moore and Abbott, 2000). After considering the underestimation tendency of satellite observations, a phytoplankton bloom is defined in this work as the pixels exceeding Chl-a concentration of 0.5 mg/m³. Further, monthly composite images from Aqua-MODIS (2003‒2013) and SeaWiFS (1998‒2010) were used to characterize the monthly evolution of phytoplankton blooms (Chl-a>0.5 mg/m³) (Figs. 3 and 4). While estimating the areal extent of blooms, the geographic boundaries for NKP are set as 46°S to 56.5°S and 65°E to 100°E. And for SKP it is restricted from 56.5°S to coastal Antarctica and 65°E to 100°E. In this work, Austral spring-summer periods are refereed as the months of November, December, January, and February.

Results and discussion

Figure 1(b) indicates that wide spread low surface Chl-a concentration prevailed (<0.3 mg/m³) in the western part of the Kerguelen Plateau with the typical characteristics of HNLC water. In contrast, distinct phytoplankton blooms (Chl-a>0.5 mg/m³) observed above the Kerguelen Plateau and its downstream section along the polar front, which is attributed to IME phenomena (Figs. 1(b), 2, and 3). The presence of the Kerguelen Plateau stands as a topographic barrier to eastward ACC (Fig. 1(a)), allows a mixing process developed through turbulence and eddies which can bring subsurface iron-rich water from shallow Plateau to the sea surface, fertilizing the mixed layer. In addition, the interaction between high-energy internal tidal waves and bathymetry boost-up the vertical eddy diffusivity above the Kerguelen Plateau, and pumps subsurface iron to the sea surface ( Park et al., 2008). An experiment carried out in October 1995 during the ANTARES 3/F-JGOFS cruise reported enrichment of dissolved iron (5.3‒12.6 nmol/L) in the coastal surface waters of the Kerguelen Island and the region was associated with elevated phytoplankton biomass ( Bucciarelli et al., 2001). The offshore waters were also affected by trace-metal inputs from coastal and continental shelf origin with reported dissolved iron concentrations of 0.46 to 0.71 nmol/L. Another experiment carried out for the Kerguelen Plateau and Ocean compared Study during 2004 revealed that the surface water having low iron (less than 0.09 nmol/L) on the Kerguelen Plateau was enriched by relatively high iron waters (0.19–0.51 nmol/L) emanating from a 500 m depth through a mixing process ( Blain et al., 2007; Park et al., 2008). Once the mixed layer on the Plateau is fertilized, the waters flow eastward by the persistently flowing eastward ACC. These physical processes in turn trigger enhancement of Chl-a concentration on the Kerguelen Plateau and its downstream sections, which provide evidence of the IME phenomena (Figs. 1(b), 2 and 3).

Earlier studies revealed wide spread low phytoplankton biomass in the Southern Ocean region, which is attributed to lack of micronutrient iron ( De Baar et al., 1995; Coale et al., 1996; Gordon et al., 1997; Takeda, 1998), light limitation (Mitchell and Holm-Hansen, 1991; Nelson and Smith, 1991; Boyd et al., 2001), and strong grazing pressure ( Gall et al., 2001; Selph et al., 2001; Zeldis, 2001). The artificial iron fertilization experiments revealed increase in phytoplankton biomass after addition of iron into HNLC regions ( Coale et al., 1996, 2004; Boyd et al., 2000, 2007; Tsuda et al., 2003). The naturally occurring phytoplankton blooms on the Kerguelen Plateau are demonstrated in Figs. 1(b), 2 and 3, owing to IME. The light-limitation is expected to prevent phytoplankton blooms in the Southern Ocean during austral winter. Even though the light is not a limiting factor during austral summer, blooms were restricted above the Kerguelen Plateau, its downstream section, and coastal Antarctic region where enhanced iron flux to surface water is expected (Figs. 1(b) and 3).

Analysis of Aqua-MODIS and SeaWiFS monthly climatological datasets revealed that the IME phenomena is pronounced only during the austral spring-summer period, likely due to the availability of light and iron-rich water from the shallow plateau (Figs. 3 and 4). In the NKP region, the phytoplankton bloom initiates during November with an areal extent of 0.876 million km² and spreads to the downstream plateau section along the polar front (Figs. 3 and 4). During December, the bloom spreads further eastward as far as 1800 km offshore (up to 98°E) with an extensive areal extent of 1.315 million km². An offshore flow was observed with the ARGO float trajectories (Argo-1900046, 1901329) (Fig. 1(b)). The areal extent reduced considerably to 0.357 million km² during January (Fig. 4) and did not show its eastward existence during subsequent months except small patches near the northern part of Kerguelen Island (Fig. 3). The northwest extension of bloom along the eastern flank of SKP has in-phase correspondence with the flow pattern of deep western boundary current (DWBC) as inferred after analyzing the MDT datasets, ARGO float trajectory (Argo-2901306), and earlier reports from Donohue et al. (1999), Aoki et al. (2008) (Figs. 1(b) and 3). The flow direction of DWBC is northwestward throughout the water column emerging from the confluence of westward flow along the Antarctic continental slope and eastward flow of Weddell Basin waters through the Princess Elizabeth Trough to the south of the SKP ( Donohue et al., 1999). Besides a supply of iron from the shallow plateau of SKP, the DWBC current can bring iron from the Marginal Ice Zone (MIZ) through sea-ice melting during the austral summer. Although direct deposition of iron from mineral dust is generally low in the MIZ (Luo et al., 2003; Zender et al., 2003), some dust particles from the atmosphere accumulate during winter snow fall that solidifies with the sea-ice providing a pulse input of iron during melting ( Sedwick and DiTullio, 1997; Grotti et al., 2005). This mechanism can lead to bloom development at SKP. IME phenomena is apparent over SKP when the phytoplankton bloom initiates during November (0.191 million km²) (Figs. 3 and 4). The SKP bloom peaks during January (1.245 million km²) through December (0.859 million km²) and spreads eastward along the southern boundary of ACC (Figs. 3 and 4). The bloom peaks with a time lag of one month compared to the NKP bloom (Fig. 4). The SKP bloom areal extent declines during February (1.064 million km²) and significantly loses its existence in the subsequent period. The areal extent estimated using monthly Aqua-MODIS composite image (2002‒2013) matches well with SeaWiFS (1998‒2010), thus supporting the validity of observations (Fig. 4). The result revealed that the Kerguelen Plateau blooms exist at least for 4 months of a year (November, December, January and February). Even though the surface water above the Kerguelen Plateau is rich in Chl-a, an exception of oligotrophic condition dominated in between NKP and SKP at Fawn Trough region (Fig. 1). Sedimentary source of iron for this surface water is unlikely as the bathymetry of Fawn Trough region typically deeper than 2000 m (Fig. 1(a)). The observed oligotrophic condition could be due to apparent intrusion of iron limited low phytoplankton regime waters from the Enderby basin through the northeastward Fawn Trough Current (Fig. 1(b)). The float trajectories (Argo-1900314 and 2901303) provided evidence on the northeastward movement of the surface waters from Enderby basin to Kerguelen Plateau through Fawn Trough.

Conclusions

The IME phenomenon was developed after the eastward ACC encounters with the Kerguelen Plateau topography. The sedimentary source of iron from the islands and surrounding shallow plateau fertilizes the mixed layer, and possibly causes the phytoplankton bloom. Extensive blooms were observed on the Kerguelen Plateau and its downstream section for a significant period of four months in a year (November, December, January, and February). The observed physical forcing effect of biological alteration on the Kerguelen Plateau and surrounding region could draw a significant amount of CO₂ from the atmosphere, creating direct effect on the biogeochemical cycle. Satellite remote sensing datasets are well suited to observe and quantify the seasonal areal extent of phytoplankton blooms in the Kerguelen Plateau ecosystem which supports large assemblage of phytoplankton, zooplankton, seabirds, seals, and whales. Although several studies have been carried out for iron budget and carbon cycle within and outside the blooms during KEOPS cruises, the present work provided scope to study the IME phenomena using remote sensing observations.

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