Characterizing the vertical phytoplankton distribution in the Philippine Sea off the northeastern coast of Luzon

https://doi.org/10.1016/j.ecss.2021.107322Get rights and content

Highlights

  • Chlorophyll profiles were characterized by functional data analysis.

  • Distinct clusters representing offshore waters, cyclonic eddy area and shelf water.

  • Vertical distribution was influenced by temperature in 2011 and salinity in 2012.

  • Phytoplankton abundances declined in 2012 due to shift in bifurcation latitude.

  • Phytoplankton composition in 2012 became species poor due to influence of NEC.

Abstract

The vertical distribution of phytoplankton in the open ocean shows an increase in biomass at a depth referred to as the Subsurface Chlorophyll Maximum (SCM) that contributes significantly to the primary production of the water column. Hence, it is important to understand the dynamics that lead its formation and maintenance. This study examines the SCM in the Philippine Sea off the northeast coast of Luzon, utilizing bio-optical and empirical phytoplankton data from two oceanographic cruises conducted northeast of the island of Luzon in May/June 2011 and April/May 2012. Chlorophyll (Chl) profiles were converted to smoothed chlorophyll functions by using a b-spline basis. In 2011, the mean SCM depth was 97.24 m ± 22.33 m with mean SCM concentration of 0.43 μg/L ± 0.09 μg/L while in 2012, mean SCM was deeper at 115.45 m ± 24.25 m and mean SCM concentration of 0.31 ± 0.09 μg/L. Functional principal component analysis showed that the first principal component (PC) explained variability in the SCM depth, the second PC showed variability in the magnitude of the SCM concentration while the third PC accounted for the presence of multiple peaks. K-means clustering using the principal components resulted in three clusters which represented the offshore stations with the deepest SCM, stations within an observed cyclonic eddy with intermediate SCM and stations with coastal and shelf waters showing shallow SCM. Correlation analyses between Chl and physico-chemical and bio-optical parameters showed that Chl was positively correlated to beam attenuation, a bio-optical property that has been used as an alternative proxy for phytoplankton. This suggests that the observed SCMs represent actual increase in phytoplankton biomass. When the influence of the Kuroshio recirculation gyre was dominant in 2011, cooler temperature in surface waters was seen to significantly increase surface Chl. In 2012, highly saline waters from the tropical North Equatorial Current (NEC) waters appeared to lower the Chl distribution, particularly at the SCM. Phytoplankton abundance was recorded to be higher at the SCM than the surface in both years. In 2011, different species of diatoms dominated all clusters, except at the SCM of the coastal and shelf cluster wherein the dinoflagellate Gyrodinium grossestriatum was dominant. Most dominant species from 2011 were conspicuously absent in 2012 and there was a shift to the diatoms Fragilariopsis (surface), Thalassiosira and Rhizosolenia spp. in all clusters. These provide new insights on the phytoplankton community in relation to the changes in the oceanic circulation from subtropical North Pacific water in 2011 to tropical NEC water in 2012.

Introduction

Tropical waters are generally stable with quasi-permanent stratification. The typical tropical structure profile consists of a subsurface chlorophyll maximum (SCM) that coincides with the thermocline and nutricline where the balance of light and nutrients are ideal (Herbland and Voituirez 1979, Mann and Lazier 2013, Cullen and Eppley 1981). As the term implies, chlorophyll-a at the SCM is found to be at its maximum within the water column and primary production generally reaches its peak above the SCM. This maximum contributes a significant proportion of primary production within the upper water column (Sathyendranath et al., 1995; Tremblay et al., 2008), thus, global estimates of primary production may be grossly underestimated by as much as 58% (Weston et al., 2005) when using remotely-sensed surface chlorophyll that is limited to the optical depth. Furthermore, SCMs may also play a noteworthy role in the global carbon cycling in terms of supporting secondary production and carbon export to the deeper ocean (Silsbe and Malkin 2016).

In temperate regions, the SCM was previously seen only during spring and summer but it has been recently observed in winter when increased turbulence leads to decreased zooplankton grazing, thus allowing phytoplankton growth and productivity at depth (Behrenfeld 2010; Romagnan et al., 2015). Formation and maintenance of the SCM may be influenced by vertical stability of the water column (Navarro et al., 2006; Coyle et al., 2008) or turbulence (Macias et al. 2013; Liccardo et al., 2013; Hahn-Woernle et al., 2014). Cullen (2015) provided a comprehensive review on how the ecological significance of the SCM varies among studies such as a high fraction of the sinking diatoms from spring bloom (Booth et al., 2002; Heiskanen and Keck 1996) or an actively growing community at the optimal depth within the water column (Palmer et al., 2013; Hill and Cota 2005; Tremblay et al., 2008). While there is a multitude of papers that describe the SCM structure such as those that have been cited, there are limited contributions describing their mechanisms within the tropical region.

The Philippine Sea is located northeast of the island of Luzon, facing the Pacific Ocean. This region is where the North Equatorial Current (NEC) bifurcates with the northward current feeding into the Kuroshio Current while the southward current feeds into the Mindanao current. As the NEC serves as an important pathway for heat and water mass exchange between the low- and mid-latitude North Pacific Ocean, its location therefore plays a crucial role in the large-ocean circulation and climate variability of the Pacific (Qiu et al., 2015; Lien et al., 2014; Talley, 2007). Gordon et al. (2014) conducted two in-depth oceanographic cruises in 2011 and 2012 and recorded the nascent Kuroshio Current enclosed by an anticyclonic eddy in the northeast and a cyclonic eddy to its southwest. Moreover, they observed a shift from subtropical to tropical stratification regimes between the two cruises. In 2011, a northerly bifurcation of the NEC at 13°-14°N injected the western North Pacific subtropical thermocline and North Pacific Intermediate Water from the Kuroshio recirculation gyre into the Lamon Bay. The observation corroborated previous circulation models (Kim et al., 2004) and satellite altimetry analysis (Wang and Hu 2006; Qiu and Chen 2010). In 2012, a southerly bifurcation (10°-11°N) brought warm NEC tropical thermocline water into Lamon Bay. A time series mooring deployed in 2011 recorded the change in water mass regime in December 2011. Cabrera et al. (2015) gave further details on how the change in the bifurcation latitude between the two years influence the water masses and ultimately the chlorophyll distribution in the upper thermocline.

In the Philippines, intricate bathymetry coupled with a monsoon climate and high exposure to tropical cyclones creates a complex pelagic ecosystem. Several studies have examined phytoplankton distributions in Philippines seas (Peñaflor et al., 2007, Villanoy et al., 2011, Cabrera et al., 2015, Primavera and Villanoy, 2007), but mechanistic understanding of phytoplankton distributions remains rudimentary. Cordero et al. (2004) analyzed chlorophyll profiles in the seas around the Philippines and found the SCM to be deepest on the Pacific Seaboard and the shallowest within the Visayan seas. Their study was able to estimate integrated phytoplankton biomass from surface chlorophyll in Philippine waters. However, they were unable to report the corresponding phytoplankton community structure that contributed to the observed profiles. Using the same data used by Gordon et al. (2014) and Cabrera et al. (2015), this study extends their findings by focusing on the spatial and vertical variation in the chlorophyll data and we contribute new results on the phytoplankton communities to provide a more in-depth look into the productivity of the Philippine Sea. The main objectives of this study were to (1) identify characteristic chlorophyll profiles in the Philippine Sea with the use of functional data analysis, (2) determine the physico-chemical parameters that may influence the observed profiles, and (3) identify the natural populations of phytoplankton within the subsurface chlorophyll-maximum layer. This work will contribute to knowledge gap on the vertical biological dynamics of the region and provide vital information essential to estimate the primary productivity of the Philippine Sea.

Section snippets

Data collection

A total of 121 stations were occupied across two cruises (Fig. 1) in 20 May – 6 June 2011 (45 stations) and 25 April – 5 May 2012 (76 stations) aboard the R/V Roger Revelle. A Seabird SBE911 CTD system equipped with SBE temperature, conductivity and pressure sensors recorded physical parameters (temperature [Temp], salinity [Sal] and dissolved oxygen [DO]) while bio-optical parameters of chlorophyll [Chl] and beam attenuation coefficient [c] were obtained using Seapoint Chlorophyll Fluorometer

General variation in chlorophyll profiles

In 2011, Chl values ranged from 0.06 to 0.66 μg/L, with the peak depth of the SCM (ZSCM) varying between 50 and 125 m (mean = 97.24 ± 22.33 m). The smoothed Chl curves can be seen in Fig. 2a with SCM concentrations ranging from 0.30 to 0.50 μg/L (mean = 0.34 μg/L). In 2012, although Chl ranged from 0.01 to 0.70 μg/L with ZSCM at deeper depths of 80–145 m (mean = 115.44 ± 24.25 m), most of the smoothed profiles (Fig. 2b) exhibited lower Chl concentration compared to 2011, and, with SCM

Variability of chlorophyll between years and across space

The variability in the chlorophyll between the two years corresponded to a change in subtropical waters from the Kuroshio Recirculation Gyre in 2011 to equatorial NEC waters in 2012 that was previously reported by Gordon et al. (2014) and Cabrera et al. (2015) (Fig. 9). The change in water masses were attributed to the southern movement of the bifurcation latitude from 13 to 14°N to 10-11°N in 2012. Gordon et al. (2014) further described that the subsurface waters showed distinct difference in

Conclusions

Our paper provides the first report on Chl profiles and differing phytoplankton assemblages within three spatial clusters from offshore to nearshore of the Philippine Sea located northeast of Luzon in two different years. Vertical phytoplankton profiles and community structure distinctly responded to variations in the oceanographic features and conditions, wherein the NEC bifurcation shifted to the south and corresponded to a decrease in Chl levels as the water masses changed from predominantly

Funding

Funding support for the research cruises was provided by the Office of Naval Research (ONR) Grant No. N62909-10-1-7126. We are grateful for the support and assistance of the crew and officers of the R/V Roger Revelle, volunteers and students that were actively involved in the two cruises. During the writing of the study, Ms. Cordero-Bailey received financial support from the Department of Science and Technology – Science Education Institute (DOST-SEI) Accelerated Science and Technology Human

Author statement

KCB, LD and AY contributed in Conceptualization, Methodology and Project Administration. Formal analysis, Visualization and Writing of the original draft was done by KCB. All authors contributed during Investigation and Writing – Review & Editing.

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 work is Marine Science Institute contribution number 482. We would like to acknowledge Dr. Cesar Villanoy and Dr. Olivia Cabrera and the anonymous reviewers who provided invaluable inputs for the development and improvement of the paper.

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      They associated their findings to the colder, Chl-a rich waters from the Kuroshio recirculation in 2011 while 2012 was representative of the intrusion of warmer, oligotrophic NEC water. Using the same dataset, Cordero-Bailey et al. (2021) characterized the observed Chl-a profiles and augmented the phytoplankton community structure that defined the profiles from in the offshore, intermediate (eddy) and coastal areas of the study site. A previous study of Cordero et al. (2004) were able to define distinct Chl-a profiles from Philippine waters and they noted that the profiles at the Pacific Seaboard exhibited the deepest SCM but with the lowest concentration, while Sulu Sea profiles showed shallower Chl-a maxima with higher SCM peaks.

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