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

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.