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Geographical changes in surface Chl a and primary productivity among the
three transections of the southern South China Sea, northern Java Sea and
eastern Indian Ocean in April 2011
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Key Laboratory of Topical Marine Bio-resources and Ecology, South China Sea
Institute of Oceanology, CAS, Guangzhou, Guangdong, 510301, China
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* Correspondence for: Tan Yehui, [email protected]; Li Gang, [email protected];
Phone, +86-20-89023219.
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Abstract
Results are presented about the geographical changes in chlorophyll a density, carbon
fixation as well as nutrient levels in surface waters of the three transections of the
southern South China Sea (SCS), northern Java Sea (JS) and eastern Indian Ocean (IO)
during April 5 to16 of 2011. The in situ Chl a concentration as well as carbon fixation
showed decreasing trends from high to low latitude among these three investigated
transections, while the photosynthetic rate of phytoplankton estimated from 14C
incorporation displayed an inconsistent variation. Chl a concentration and carbon
fixation in IO water was lower than that in JS water. Higher salinity and lower
contents of dissolved inorganic nitrogen (DIN) and silicate (SiO32-) characterized the
IO water as compared to the SCS or JS waters, and the PO43- content was lower in
former than later waters in most of stations. Our results also indicate the importance
of DIN and SiO32- concentrations for the geographical changes in phytoplankton
biomass and primary productivity among the SCS, JS and IO waters.
Key words: Carbon fixation; phytoplankton; South China Sea; Java Sea; Eastern
Indian Ocean
YI Rong, KE Zhi-xin, SONG Xing-yu, SHEN Ping-ping, WANG Sheng-fu, FAN
Yan-zhi, HUANG Liang-min, TAN Ye-hui*, LI Gang*
Submit to: Journal of Tropical Oceanography March 21, 2014
Received date:
Revised date:
Editor:
Foundation item: The National Natural Science Foundation of China under contract
Nos. 41130855, 41206132 and 41276162; the CAS Strategic Pilot Science and
Technology under contact Nos. XDA11020200 and XDA05030403; the National
Project of Basic Sciences and Technology under contact Nos. 2012FY112400 and
2013FY111200); and the CAS Knowledge Innovation Program under contract No.
SQ201115.
Biography：YI Rong (1990−), female, from Yichun of Jiangxi Province, master,
research on environmental monitoring and assessment, E-mail:[email protected].
Corresponding authors:
TAN Ye-hui, E-mail, [email protected];
LI Gang, E-mail: [email protected].
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1. Introduction
Marine phytoplankton species that photosynthetically utilize solar energy to fix
CO2 and ultimately to produce organic matters, can contribute to ~50% of global
primary production[1]. According to Carr et al.[2], the amounts of the production
fluctuate greatly in the regional scale, e.g., from less than 0.10 g C m-2 d-1 in pelagic
oceans to more than 3.0 g C m-2 d-1 in coastal water. Simultaneously, various studies
in field, in laboratory or using model have showed that the environmental factors such
as temperature, salinity and nutrients affect marine primary production through
influencing the physiological processes of phytoplankton[3-6]. The great differences of
field environments in regional scale have been extensively reported as well, that leads
to a great geographical change of primary production in marine ecosystems[1,2].
The southern South China Sea (SCS) is generally characterized by shallower
depth and lower nutrient levels[3,7], that connects southerly to northern Java Sea (JS)
by Karimata strait. The JS is mainly a continental shelf with average depth of ~40 m
and is plentiful of land-derived discharges[8-10]. The eastern Indian Ocean (IO) that
connects to JS by Sunda strait is often identified by lower temperature and higher
salinity in sea surface due to the annual occurrence of upwelling that is driven by
seasonal southwest monsoon (May to November)[11,12]. The great environmental
differences aforesaid undoubtedly have resulted in a great change in phytoplankton
primary productivity among the SCS, JS and IO waters. However, little has been
documented about the regional changes of phytoplankton biomass and primary
productivity among these three waters, although it can provide important information
to deeply understand their ecological processes. In this paper, we exhibited the
geographical changes of nutrient contents and phytoplankton chlorophyll a (Chl a)
concentration and carbon fixation rate in surface waters of three transections among
the southern South China Sea, northern Java Sea and eastern Indian Ocean.
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2. Materials and Methods
2.1 Study area and sampling protocol
During a cruise from April 5 to 16 of 2011, we carried out a investigation on
board of RV “Shiyan I” that covered the southern South China Sea (SCS), northern
Java Sea (JS) and eastern Indian Ocean (IO) (Fig. 1). A total of 20 stations (water
depth of 30-3000 m) were occupied with the intervals of 150-200 km (Fig. 1). At each
station, surface seawater was collected with an acid-cleaned (1 N HCl) polycarbonate
carboy after measuring the salinity and temperature. The collected water samples were
treated within 15 min as below, to determine the nutrient contents, phytoplankton
chlorophyll a (Chl a) concentration, photosynthetic carbon fixation and species
compositions.
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Figure 1: Map of the three sampling transections among the southern South China Sea (SCS,
L1-L7, black squares), Java Sea (JS, L8-L11, black triangles) and eastern Indian Ocean (IO,
L12-L20, white cycles) during the cruise dated from April 5 to 16 of 2011. The lines with the
number indicate the bathymetry.
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2.2 Environment measurements
Surface seawater temperature (SST) and salinity (SSS) were measured with a
Sea-bird CTD during the cruise. For nutrient measurements, the collected water
sample was dispensed into 80 ml polycarbonate bottle, frozen and stored at -20 oC for
later analysis. Based on the protocols of Kirkwood et al.[13], the frozen water samples
were thawed and analyzed for dissolved inorganic nitrogen (DIN), phosphate (PO43-)
and silicate (SiO32-) contents using a Quickchem 8500 nutrients-autoanalyzer (Lachat
Instruments, USA). This device has less than 10% analysis error and was calibrated
against CSK standard solutions before the sample measurements.
2.3 Determination of photosynthetic carbon fixation
The collected seawater was pre-filtrated through a 200 μm pore-size mesh (to
remove most large zooplankton), dispensed into triplicate 500 ml polycarbonate
bottles (Nalgene®) and inoculated with 100 μl-4 μCi (0.148 MBq) of NaH14CO3
solution (ICN Radiochemicals); then, the bottles containing water samples were
incubated for 24 h under solar radiation in a water bath, wherein the running-through
surface seawater was pumped to control temperature as similar to SST (25-29 oC). In
addition, duplicate bottles were wrapped in black foil and incubated as control. After
the incubation, each sample was filtered onto a Whatman GF⁄ F glass fiber filter (25
mm in diameter), which was immediately frozen and stored at -20 oC for later
analyses. The frozen filters were then put into 20 ml scintillation vials, thawed and
inoculated with 0.5 ml-0.5 mol L-1 HCl and left uncapped overnight to expel the
non-fixed 14C. After this, 3 ml scintillation cocktail was added to each vials and the
incorporated 14C was counted with a liquid scintillation counter (LS 6500, Beckman
Coulter, USA). The photosynthetic carbon fixation rate was calculated with JGOFS
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C-protocols[14]. A total of 20 incubations were performed during the cruise.
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2.4 Chl a and species analyses
Chl a concentration was determined by filtrating 800 ml surface seawater onto a
Whatman GF⁄ F glass fiber filter, that was then wrapped in aluminum foil and stored
at -20 °C for later analysis. The filters were thawed and extracted in 10 ml 90%
acetone (v/v) for 24 h at 4 °C; after centrifuging at 3500 g for 15 min, the supernatant
was measured with a Turner Design 10 fluorometer. Chl a content was calculated
following the descriptions by Parsons et al.[15].
For taxonomic analysis of phytoplankton assemblages, 1 L water sample (fixed
with Lugol’s solution to 1.5% final concentration) was condensed to 20 ml through
settling for 24 h and siphoning the supernatant; a whole of 0.5 ml concentrated sample
was used for the species analyses under a compound microscope (CX21, OLYMPUS,
Japan) with Utermöhl method[16].
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2.5 Statistical analyses
Mean and standard deviations were used to present in figures. One way-ANOVA
was used to determine the significant differences among the variables (p<0.05). A
Kendall’s τ test was used to establish the relationships of environments and primary
productivity.
3. Results
Geographical changes in surface seawater temperature (SST) and salinity (SSS)
among the JS, SCS and IO waters are shown in Figure 2. The SST increased from
25.81 oC (L1) in SCS water to 29.31 oC (L8) in JS water, while the SSS decreased
from 33.28 to 32.85 (Fig. 2). In IO water the SST was similar to that in JS water but
higher than SCS water (Fig. 2A); whereas the SSS was significantly higher than the
latter two waters (Fig. 2B, C).
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Figure 2: Geographical changes in surface seawater temperature (A, SST, oC) and salinity (B,
SSS), and the temperature vs. salinity (C, SST vs. SSS) of the three investigated waters.
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Concentrations of dissolved inorganic nitrogen (DIN) decreased from 4.10 μmol
L (L1) in SCS water to 3.26 μmol L-1 (L6) in JS water, followed by an increase to
5.79 μmol L-1 (L11) (Fig. 3A); while phosphate (PO43-) varied from 0.23 to 0.31 μmol
L-1 and exhibited no clear spatial variations from SCS to JS water (Fig. 3B). Content
of silicate (SiO32-) increased from 2.22 μmol L-1 in SCS water to 10.8 μmol L-1 in JS
water (Fig. 3C). Generally, the nutrient levels in IO water were significantly lower
than JS and SCS waters (p<0.05), except PO43- at stations L12 to L14; and the DIN
and SiO32- contents showed a slight increasing trend from low to high latitude, nut not
for PO43- (Fig. 3).
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Figure 3: Geographical changes in the contents (μmol L-1) of dissolved inorganic nitrogen (A,
DIN), phosphates (B, PO43-) and silicate (B, SiO32-) in surface seawater of the investigated
regions.
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Coinciding with nutrient variations, Chl a biomass decreased sharply from 0.41
μg L (L1) in SCS water to 0.10 μg L-1 (L6) in JS water (Fig. 4A), followed by an
increase up to 0.32 μg L-1 (L11). Chl a in IO water varied from 0.04 to 0.36 μg L-1 and
was latitudally lower than JS water (Fig 4A), except L20 wherein the blooms of
Trichodesmium erythraeum occurred during the investigated period. According to Li
et al. (2012) and Ke et al. (2012), the microscopic species changed greatly among the
three investigated waters i.e. dinoflagellates (e.g. Gyrodinium dominans, Amphidinium
carterae and Gonyaulax spp.) dominated SCS water; whereas the same dinoflagellates
species, together with diatoms (e.g. Thalassionema nitzschioides, Rhizosolenia spp.
and Chaetoceros spp.) were mainly groups in JS water. Similar to the SCS or JS
waters, the dinoflagellates dominated in IO water but with differential species (e.g.
Alexandrium sp.), indicating the effects of environmental changes (data not shown).
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Figure 4: Geographical changes in biological characteristics among the SCS, JS and IO waters: A)
Chl a biomass (μg L-1, data of L15 to 20 are obtained from Li et al.[17]), phytoplankton carbon
fixation (B, μg C L-1 d-1) and photosynthetic rate [C, μg C (μg Chl a)-1 h-1]. Vertical bars represent
the standard deviations (n=3).
Photosynthetic carbon fixation showed a similar trend as Chl a, and decreased
from 5.70 (L1) to 1.95 μg C L-1 d-1 (L8) in SCS water, then increased to 7.60 μg C L-1
d-1 (L11) in JS water (Fig. 4B). The carbon fixation ranged from 1.18 to 6.30 μg C L-1
d-1 in IO water, and was latitudinally lower than JS water (except L20) (Fig. 4B). The
ranges of photosynthetic rate were 1.37-2.20, 2.76-2.39 and 3.5-1.41 μg C (μg Chl a)-1
h-1 in SCS, JS and IO waters, respectively; and the photosynthetic rate was higher in
JS than SCS waters (Fig. 4C).
When we plotted the biological parameters against SST, SSS of these three
investigated transections, a positive correlation (p<0.05) of SST to photosynthetic rate
was found, as well as the negative correlations (p<0.05) of SSS to Chl a or carbon
fixation (Table 1). Moreover, the DIN and SiO32- contents were recorded to positively
correlate (p<0.05) to Chl a biomass and carbon fixation, indicating the importance of
DIN and SiO32- contents for the geographical changes of primary production among
the SCS, JS and IO waters. No significant correlations were observed between the
biological parameters to N/P ratio, nor to N/Si ratio (Table 1).
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Table 1: R and p values of plotting Chl a content (μg L-1), carbon fixation rate (C-fixed, μg C L-1
d-1) or photosynthetic rate [AsN, μg C (μg Chl a)-1 h-1] against the SST (oC), SSS, concentrations
(μmol L-1) of DIN, PO43- and SiO32-, N/P and N/Si ratios during the cruise. The stars (*) indicate
significant differences (p<0.05).
SST
SSS
r
PO43-
DIN
r
p
p
Chl a
-0.27
0.068
-0.47
0.041*
C-fixed
0.24
0.33
-0.66
AsN
0.64
0.003*
0.23
r
SiO32-
p
r
p
0.47
0.042*
0.30
0.21
0.002*
0.61
0.006*
0.45
0.33
-0.19
0.43
-0.54
r
N/P
N/Si
p
r
p
r
p
0.48
0.039*
-0.07
0.97
-0.32
0.17
0.057
0.57
0.011*
0.20
0.41
0.04
0.88
0.066
0.04
0.89
0.28
0.23
-0.18
0.45
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4. Conclusion and discussion
Geographical locations of the investigated transections had provided us a unique
opportunity to study the regional changes of phytoplankton biomass and primary
productivity among the southern South China Sea, northern Java Sea and eastern
Indian Ocean. We found that there were significant regional variations in surface Chl
a concentration and carbon fixation, as well as nutrient levels among these three
investigated areas. Moreover, the DIN and silicate contents appeared to be more
important for these geographical changes in primary production among the SCS, JS
and IO waters.
Environmental factors differed greatly among the SCS, JS and EIO waters (Figs.
2 and 3). One of the main causes for these environmental changes could be the
geographical locations, i.e., the offshore water of the SCS is generally oligotrophic
due to the severe stratification and less land-derived runoffs[4,18,19]. The northern JS
water that is surrounded by land had higher nutrient levels (Fig. 3), mainly due to the
enrichment by human activities[8, 9] as well as the land-runoffs as indicated by lower
salinity (Fig. 2BC). Together with far-off-land, southwest monsoon that often
prevails in IO water for the study period[11] might bring in the oligotrophic offshore
water, thus leading to the lower nutrient contents and higher salinity as shown in
Figs. 2B and 3. Finally, the contents of DIN, PO43- and SiO32- appeared to be
latitudally higher in JS than IO waters (Fig. 3), consistent with the spatial changes in
Chl a biomass and carbon fixation, which indicated the regulations of nutrients on
marine primary production.
Geographical changes in physico-chemical factors, e.g., nutrients, temperature or
salinity could account for the biological variations. Phytoplankton biomass and
productivity varied greatly from the SCS, JS to IO waters (Fig. 4). Macronutrients
such as nitrogen that varied greatly among the investigated waters (Fig. 3) is
detected to be one of the major regulators for the regional variability of marine
primary production (Table 1), since it is the required material for phytoplankton to
synthesize cellular substances like nucleic acid, proteins as well as all kinds of
enzymes[20,21]. The nutrient SiO32- is usually believed to benefit the growth of
diatoms[22,23], which might lead to their dominance in JS water[3,10,24]. Benefits from
dissolved silicate content are also found on carbon fixation by phytoplankton
assemblages in this study (Table 1) or in Malacca Strait water[5]. According to
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Redfield[25], proper N/P and N/Si ratios of 16 and 1 are needed for the well growth of
phytoplankton, however, higher N/P (24±6.9) and N/Si ratios (2.7±0.78) in
investigated waters could not favor the phytoplankton growth, thus resulting in the
insignificant correlations between the ratios and primary production (Table 1). On
the other hand, temperature often affects phytoplankton productivity[26,27] through
influencing the activities of enzymes involved in the processes of
photosynthesis[20,21], which could explain the positive correlation between
photosynthetic rate to SST (Table 1). It also explained why photosynthetic rate was
higher in the JS and IO waters, where the temperature was higher than that of the
SCS water. Moreover, the inconsistent variations of photosynthetic rate with the
nutrient levels (Figs. 3 and 4C) could be mainly due to the fluctuation of sunlight[5];
unfortunately, solar radiation has not been obtained in this study due to the
unavailability of photoradiometer. Finally, the grazing pressure might have also
contributed to the spatial variations of phytoplankton biomass and production[28]; in
this study however, pre-filtration of seawater could have removed most of the
zooplankton, thus eliminating their effects on the measured carbon fixation.
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Acknowledgments
The authors are very thankful for comments and suggestions of two anonymous
reviewers that helped to improve this manuscript. Thanks are also given to Dr.
Miranda Corkum for polishing English, to Kaizhi Li and Shuai Xing for their
experimental assistance, and to the captain and crews of Shiyan I ship for logistic
support.
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