Picophytoplankton (0.2 - 2 μm), including
Picophytoplankton dominate in subtropical and tropical open oceans, where
The responses of primary production and Chl
Picophytoplankton account for the majority of primary productivity in the South China Sea (SCS) (Chen et al. 2009), and mesoscale eddies are ubiquitous in the northern SCS (Nan et al. 2011). Phytoplankton in open areas of the SCS are dominated by
The SCS is a semi-enclosed marginal sea of the western Pacific, connected with the Java Sea via the Karimata Strait to the south, and with the Pacific Ocean via the deep Luzon Strait in the northeast (Fig. 1a). The SCS is part of the East Asian monsoon system. Driven by monsoonal winds, oceanic circulation in the upper layers shows strong seasonal variability and is predominantly cyclonic in winter and anticyclonic in summer (Liu et al. 2001; Wang et al. 2003). Mesoscale eddies are ubiquitous in the northern SCS; they are frequently generated in the northeastern SCS and most propagate westward, inclining slightly to the south near the continental slope (Liu et al. 2013). The SCS is an oligotrophic “mini-ocean” (Du et al. 2013) with primary production in the range of 16-46 mmol C m-2 d-1, and eddy activity important in the biogeochemistry of the SCS basin. For example, primary production was increased to >90 mmol C m-2d-1 by a cyclonic eddy in the northern SCS (Chen et al. 2007). Lin et al. (2010) observed that eddies can deliver nutrients from coastal areas into the oligotrophic basin and induce algal blooms.
(a) Locations of sampling stations (kj01–kj10) during the cruise of August 2012. (b) Map of weekly average sealevel anomalies during the sampling period in August 12-19, 2012; the 200-, 1000-, 2000-, and 3000-m isobaths are marked. HN, Hainan Island; TW, Taiwan; LZ, Luzon
Sampling was performed at 10 stations (kj01–kj10) along the 18°N transect from August 12 to 19, 2012. Water samples were collected and salinity and temperature were measured using a 12-bottle rosette sampler equipped with a
For Chl
To measure nutrient concentrations, the seawater was pre-filtered through Whatman GF/F filters and dispensed into 80-ml polycarbonate bottles, which were immediately frozen and stored at –20°C for later analysis. Nitrate, phosphate, and silicate concentrations were analyzed with a Quickchem 8500 nutrient autoanalyzer (Lachat Instruments, USA). The detection limits for nitrate/ nitrite, phosphate, and silicate were 0.014, 0.005, and 0.075 μmol l-1, respectively. Detailed methods for nutrient analysis are described by Peterson et al. (2005) and Li & Hansell (2008).
Seawater samples were pre-filtered through 20-mm mesh netting, fixed with glutaraldehyde (final concentration, 1%) in 2-ml cryotubes (Vaulot et al. 1989), quick-frozen in liquid nitrogen until the analysis in the laboratory. For the determination of photosynthetic picophytoplankton after rapid thawing, 0.5-ml water samples were analyzed using a FACSCalibur flow cytometer (Becton Dickinson, USA). Samples were run at a medium speed and 15 000 events were acquired in a log mode. Following standard protocols (Olson et al. 1985; Olson et al. 1990; Marie et al. 1999; Jiao et al. 2005), fluorescence at wavelengths above 650 nm (FL3 channel) was ascribed to Chl
Picophytoplankton cytograms of a sample from 75 m at station kj03 in this study. The symbols of pink, green, blue, deep red and black dots show
Statistical analysis were performed using the SPSS 18.0 software (SPSS, Inc., Chicago, USA). The vertical distributions of picophytoplankton abundance and nutrients were plotted using the Ocean Data View 4 software (Alfred Wegener Institute, Germany).
Images of the sampling stations superimposed onto Sea Level Anomaly (SLA) data produced by the French Archiving, Validation, and Interpolation of Satellite Oceanographic (AVISO) project showed a discernible warm eddy during the sampling period (Fig. 1b). The maximum SLA for the warm eddy was 35 cm during the study period; the eddy centered on 117°E and spanned two degrees of longitude during the sampling period in agreement with Nan et al. (2011) and Zhou et al. (2013). Isothermal and isohaline surfaces appeared as clear depressions in the warm eddy compared to the surrounding water, consistent with previous descriptions of anticyclonic eddies (Fig. 3) (McGillicuddy et al. 2007; Nan et al. 2011). The eddy was characterized by lower salinity (<32.7) at the surface relative to that of the surrounding water (>33.2). The water was well mixed in the upper 25 m; the downward displacement of isohaline and isothermal water began at 50 m. For example, the temperature at 50 m was 27.5°C in the eddy core and 22.5°C outside the eddy (Fig. 3a). Isohaline water was also uplifted at the eddy edges (e.g. salinity at stations kj09 and kj10 at 50 m was >34.2 at the edges but <33.5 in the eddy core (Fig. 3b).
Distribution of temperature (a) and salinity (b) in the upper 200 m at sampling stations kj01–kj10 across the warm eddy (18°N) in the northern SCS in summer 2012. The stations are denoted at the top of each panel.
Sampling stations kj01–kj10 were grouped by aquatic environment (eddy core, EC; eddy edge, EE; and reference sites, Ref) as defined by SLA, temperature, and salinity, using PRIMER cluster analysis (Fig. 4). The analysis revealed that stations kj05 and kj06 represented the eddy core, while kj04 and kj07 were located at the eddy edge. The reference sites consisted of stations kj01–kj03 and kj08–kj10.
Dendrogram of the three station groupings (EC - eddy core, EE - eddy edge, and Ref - reference sites) according to cluster analysis
Nitrate, phosphate, and silicate concentrations were low in the upper 50 m of the core of the warm eddy, and increased with depth below 50 m (Fig. 5). The nitrate and phosphate isopleths were uplifted at the eddy edges in the surface water. For example, the nitrate concentration at 50 m in depth was approximately 7.5 μmol l-1 at the edge, compared to 2.5 μmol l-1 in the eddy core (Fig. 5a), and the phosphate concentration at the same depth increased from 0.5 μmol l-1 in the eddy core to 0.8 μmol l-1 at the eddy edge (Fig. 5b). Silicate displayed a similar trend as nitrate and phosphate in the eddy core, but the downward displacement of this nutrient was not as intense (Fig. 5c). Water-column integrated nitrate, phosphate, and silicate concentrations were lower in the eddy core than at the eddy edges or the reference stations; phosphate concentration was significantly lower in the core than at the edges (Table 1 and Fig. 6).
Vertical distribution of nitrate (a), phosphate (b), and silicate (c) in the upper 200 m at sampling stations kj01–kj10 across the warm eddy (18°N) in the northern SCS in summer 2012. The stations are denoted at the top of each panel.
Spatial distribution of water-column integrated nitrate (a), phosphate (b), and silicate (c) from 0 to 200 m. The sampling stations (kj01–kj10) were classified as Ref, EE, and EC 50 m, the abundance of
Water-column integrated nutrient, Chl
Parameter SCM refers to subsurface Chl |
Unit | Ref | EE Parentheses show P values obtained from analysis of variance (ANOVA) for (EE + EC) vs. Ref (α = 0.05). |
EC Parentheses show P values obtained from ANOVA for EC vs. Ref (the first value) or EE (the second one) (α = 0.05); symbol “—” means P>0.05. |
b(EE + EC) |
---|---|---|---|---|---|
Nitrate | μmol l-1 | 12.56±3.97 | 16.94±3.81 | 11.88±0.51 | 14.41±3.67 |
Phosphate | 0.78±0.08 | 0.99±0.002 (0.03) | 0.74±0.01 (—, 0.030) | 0.87±0.15 | |
Silicate | 11.43±0.95 | 12.14±0.26 | 9.34±0.83 (—, 0.039) | 10.74±1.69 | |
Chl |
μg l-1 | 0.16±0.04 | 0.09±0.01 | 0.08±0.02 | 0.09±0.02 (0.005) |
Total Chl |
0.05±0.01 | 0.06±0.002 | 0.05±0.01 | 0.06±0.01 | |
× 103 cells ml-1 | 2.71±0.63 | 2.55±0.12 | 1.56±0.04 (0.031, —) | 2.05±0.58 | |
Picoeuk | 0.92±0.15 | 0.60±0.11 (0.037) | 0.28±0.01 (0.001, —) | 0.44±0.19 (0.001) | |
25.10±2.32 | 15.75±1.78 (0.002) | 9.67±0.23 (0, 0.05) | 12.71±3.66 (0) | ||
Syn-carbon | × 105 fgC ml-1 | 6.76±1.58 | 6.38±0.29 | 3.89±0.11 (0.031, —) | 5.13±1.45 |
Picoeuk-carbon | 19.40±3.18 | 12.64±2.26 (0.037) | 5.78±0.08 (0.001, —) | 9.21±4.19 (0.001) | |
13.30±1.23 | 8.35±0.94 (0.002) | 5.12±0.12 (0, 0.05) | 6.74±1.94 (0) | ||
Pico-carbon | 39.47±2.59 | 27.37±1.02 (0.011) | 14.79±0.07 (0, 0.022) | 21.08±7.28 (0) |
As expected, Chl
The vertical distribution of picophytoplankton abundance varied along the 18°N transect across the warm eddy.
Vertical distribution of Chl
Vertical profiles of Chl
Spatial distribution of water-column integrated Chl
Picoeuk were primarily distributed in the upper 50 m (Fig. 7d). The maximum abundance of Picoeuk occurred at 50 m at the edge and reference stations, and at 75 m in the eddy core, and Picoeuk abundance was approximately five times lower in the eddy compared to the reference stations (Fig. 8d). In addition, the water-column integrated Picoeuk abundance was significantly lower in the eddy core and at the edge than at the reference stations, and did not differ significantly between the core and edge (Table 1 and Fig. 9d).
A previous study showed that picophytoplankton contributed less to the total Chl
Nitrate and phosphate concentrations were significantly higher at the eddy edge than at the core or outside the eddy (Fig. 5 a and b, respectively); phosphate and silicate were significantly lower in the core of the upper nutricline, but were elevated at the edges (Fig. 5 b and c, respectively). This result was consistent with previous findings (Bibby et al. 2008; Mizobata et al. 2002; Peterson et al. 2005; Sweeney et al. 2003) and may be attributed to upwelling and accumulation of nutrients at the edges of warm eddies (Kim et al. 2012; Mizobata et al. 2002; Wang et al. 2008). In anticyclones, the shear generated between the eddy periphery and the surrounding waters enhances the net spin of the fluid in the direction of the Earth’s rotation, leading to the upwelling towards the inner edge of the eddy and downwelling towards the outer edge under the influence of wind forcing (Mahadevan & Archer 2000; Mahadevan et al. 2008). In addition, phosphate levels were higher at the edges than at the eddy core or reference stations, while nitrate and silicate levels were comparable to those at the reference stations. Elevated nutrients at the edges of the warm eddy can cause shifts in the dominant taxa from dinoflagellates to diatoms and in the dominant size class from picoplankton to nanoplankton, especially when more than one nutrient is limited (Ning et al. 2008). Our results are consistent with this dynamics, as described below.
Mesoscale eddies can affect rates of nutrient supply to the euphotic zone through upwelling and downwelling, thereby altering the phytoplankton productivity and particle export. Nutrient ratios in the open ocean indicate that these areas are nitrogen limited (Peterson et al. 2011). At 50 m, the ratio of nitrate to phosphate (N/P) was much lower than that predicted by the Redfield ratio (Redfield 1958) (Table 2). The ratio of N/P in the eddy core was lower than that at the edge or reference stations, suggesting that nitrogen was more limiting in the core and that picophytoplankton abundance would be lower there. The Si/N and Si/P ratios indicated nutrient conditions in the core were more conducive to diatom growth compared to the edge and reference stations (Table 2). This result was consistent with the SCM and maximum
Nutrient ratios (N - nitrate, P - phosphate, and Si - silicate) at 50 m and 75 m depths at EC, EE, and Ref
Depth | N/P | Si/N | Si/P | |
---|---|---|---|---|
Ref | 50 m | 5.82±1.01 | 0.83±0.29 | 4.75±1.21 |
75 m | 11.93±5.61 | 1.54±0.55 | 14.63±2.83 | |
EE | 50 m | 6.56±2.85 | 0.77±0.18 | 4.79±1.02 |
75 m | 9.02±3.49 | 1.09±0.33 | 9.28±0.88 | |
EC | 50 m | 4.78±0.33 | 0.97±0.02 | 4.63±0.22 |
75 m | 12.67±0.99 | 0.62±0.14 | 7.91±2.34 |
Mesoscale warm eddies affect the distribution of phytoplankton groups through vertical and horizontal transport (Mahadevan et al. 2008). Picophytoplankton dominate the primary productivity in the open ocean (Casey et al. 2007; Liu et al. 2007b). Our results showed that the warm eddy played an important role in reducing the picophytoplankton abundance (especially those of