addition, low salinity surface water in the coastal area was also noted,
which was attributed to a large amount of freshwater input during the
warm season (Figs. 2 and 3).
3.2. Nutrients and Chl a
3.2.1. Nutrient dynamics
During the cold season, both the coastal and central areas showed
high nutrient concentrations (average value for the euphotic zone) as
a result of the well-mixed structure in the water column (Table 2).
The concentrations of nitrate and phosphate during the March cruise
reached 6 μmol L−1 and 0.6 μmol L−1, respectively, whereas they
were less than 1 μmol L−1 and 0.1 μmol L−1 during August in the central
YS (Fig. 4). Additionally, the nutrient-rich water observed inMarchwas
consistentwith the occurrence of the YSWC (Figs. 3 and 4). Based on the
results of the 35°N transect, relative higher nutrient concentrations
were present in the central area rather than the coastal area (Fig. 4). Nutrient
distribution patterns in April were generally similar to that in
March, apart from reduction in phosphate concentration within the
euphotic zone during April (Fig. 4E). There was a significant difference
between the phosphate concentrations in the central YS and the coastal
area (Table 2).
The diapycnal nutrient flux was restrained by the development of
stratification during summer, and thus quite low concentrations of
surface nutrients were observed in the central area (Table 2 and
Fig. 4). The concentrations of nitrate (1.1 ± 0.8 μmol L−1, mean ± SD)
and phosphate (0.09 ± 0.04 μmol L−1, mean ± SD) in the euphotic
zone (~30 m) of the YSCWM area (n = 15) during the October cruise
were significantly lower than those in the coastal area (p b 0.01, n =
18) (Table 2).
3.2.2. Chl a
Two contrasting spatial patterns were observed for surface Chl a
concentrations during the April and October cruises in 2006 (Fig. 2). A
clear spring phytoplankton bloomwas observed during the April cruise
in the northeast area (Fig. 2E). During this cruise, the surface Chl a
ranged from 0.89 mg m−3 (at the southern stations) to 12.62 mg m−3
(at Station A5), with a mean of 2.97 ± 2.96 mg m−3 (n = 31). In