Nitrate-based New Production Research Articles

Spatial and seasonal variations of nitrate-based new production and primary production in the South China Sea

The results from eight cruises that were conducted between 2000 and 2003 are used to elucidate the spatial and temporal dynamics of both nitrate-based new production (NP) and primary production (PP) in the northern South China Sea (SCS). The spatial variation study, which was based on the results of three cruises, compares the differences among the shelf, slope and basin of the SCS in the spring and fall. Temporal (seasonal) variation was studied during eight cruises at a basin site and two shelf sites in the SCS. Nutrient enrichment experiments were conducted concurrently with field biochemical surveys to evaluate the role of nitrogen, phosphorus and iron in limiting phytoplankton growth. The results clearly show that winter was the most productive season of the year in the basin and possibly on the shelf as well. Integrated primary production (IPP) and nitrate-based new production (INP) of the basin were 0.55 and 0.26 g C m −2 d −1, respectively, in winter; 0.26 and 0.07 g C m −2 d −1, respectively, in spring; 0.19 and 0.03 g C m −2 d −1, respectively, in summer; and 0.28 and 0.05 g C m −2 d −1, respectively, in fall. The f-ratio (INP/IPP) was as high as 0.47 in winter, decreasing to 0.29–0.36 in early mid-March and 0.14–0.20 from late March to October, and fluctuated coincidentally with abundance of nitrate. The presence of nutrients in the surface layer, supporting the winter productivity, could be attributed to the shallow nitracline, which made nutrient mixing possible when a cold and strong northwest monsoon prevailed, reflecting the vital role of nitrate availability in modulating new production in the SCS. Spatial variation of INP or IPP among the basin, slope and shelf are not as obvious as the seasonal variations because of great local variability. In general, however, the shelf was more productive than the slope and the basin. In the spring, mean IPP and INP in the shelf, slope and basin were 0.72 and 0.16, 0.34 and 0.10, 0.49 and 0.15 g C m −2 d −1, respectively. In the fall, IPP and INP in the shelf and basin were 0.45 and 0.14 g C m −2 d −1, and 0.34 and 0.09 g C m −2 d −1, respectively. There exists a positive correlation between INP and IPP for the spatial data set in either the spring or the fall. f-Ratios are not statistically different between spring (0.28) and fall (0.29). While both nitrate availability and irradiance significantly modulated the spatial variations of production, effects of nitrate availability were more prevalent than light intensity. Diatoms and coccolithophores prospered in winter in the basin or on the shelf in both winter and summer, when surface water was rich in nitrate, and were more abundant than in summer in the basin when nitrate was scarce. The results of the nutrient enrichment experiments also supported the role of nitrogen availability in productivity of the SCS water. In these experiments, chlorophyll a concentration and primary productivity responded to nitrate supplementation, while iron and phosphorus supplementation did not stimulate phytoplankton growth. The predicted ratio of nitrate-based NP to N 2-fixation-based NP from Trichodesmium is greater than 50:1, indicating that nitrate derived from the subsurface water was the main source of new nitrogen, which supported phytoplankton growth in the SCS.

Nitrate-based new production and its relationship to primary production and chemical hydrography in spring and fall in the East China Sea

Abstract The purposes of the present research were to identify the factors modulating the dynamics of nitrate-based new production (NP) and primary production (PP) in the East China Sea (ECS) during spring and fall, the transitional periods between winter and summer when phytoplankton production in the ECS is limited by light and nutrient, respectively. Two cruises were carried out in March (spring) and October (fall) of 1998, respectively. In the fall when solar irradiance was relatively high, phytoplankton productions in terms of NP and PP were related to light attenuation and the degree of stratification of water column, indicating that light availability for phytoplankton growth was decided by water property. But in the spring, when solar irradiance was very low and water column was transparent, production was related to solar irradiance. In addition to light, nitrate abundance modulated spring productivity, but not in the fall when upward diffusion of nutrients supported phytoplankton growth but the ambient nitrate concentrations were mostly not detectable. Thus, ambient nitrate concentration was not an effective indicator of nutrient availability in this transitional period. Spatial and seasonal variations in production in the ECS were pronounced. While water-column-integrated PP (IPP) ranged between 0.14 and 1.64 g C m−2 d−1 in the fall and between 0.08 and 1.20 g C m−2 d−1 in the spring, integrated NP (INP) was 0.05–0.59 g C m−2 d−1 in the fall and 0.02 -0.42 g C m−2 d−1 in the spring. In the fall, both INP and IPP were higher in the mid-shelf than in the China coastal waters. In the spring, however, both productions were higher in the western half of the ECS shelf than in its eastern half. In the ECS, the coastal water adjacent to the Changjiang mouth showed the most significant seasonal variations in both phytoplankton productivity and f-ratio. INP and IPP in other China coastal waters were higher in spring and summer than in fall and winter, and opposite was true in the mid-shelf ECS waters. Year-round INP and IPP were positively regressed to integrated chlorophyll concentration. Year-round f-ratio was 0.32, indicating two-thirds of the primary production in the ECS is attributed to nitrogen sources other than nitrate.