Nitrogen modulates phytoplankton growth in spring in the South China Sea

Abstract

The present study investigated the roles of nitrogen (N) in modulating phytoplankton growth in the South China Sea (SCS), an oligotrophic marginal sea in the western Pacific Ocean, by examining (1) the in situ nitrate and nitrite concentrations [NO 3+NO 2]; (2) the [ NO 3+ NO 2]/[ soluble reactive phosphorus ( SRP)] in the euphotic layer; (3) phytoplankton growth in response to nutrient enrichments; and (4) the spatial distribution of nitrate-based new production and primary production and the factors controlling their dynamics. N 2-fixation rates of the planktonic cyanobacteria, Trichodesmium sp. and the diatom-symbiont Richelia intracellularis, were estimated based on the abundance of the N 2-fixers and were compared to the nitrogen demands estimated based on the nitrate uptake rate (nitrate-based new production). The comparison evaluates the relative importance of two sources of new nitrogen, the atmosphere (N 2) vs. the deep waters (NO 3 −). The study was conducted on cruises in March 2000 and March 2001 when the northeastern monsoon was prevailing in the SCS. Both N and phosphorus (P) were scarce in the euphotic zone; surface [NO 3+NO 2] was in the range of 9– 309 nM and [SRP] ranged from 6 to 88 nM . Surface [NO 3+NO 2] was negatively ( p<0.01) correlated with stratification index of water column. The ratios of [NO 3+NO 2]/[SRP] were much smaller than the Redfield N/P Ratio of 16:1, suggesting a state of N-limitation. The water of the shallow shelf-slope was less stratified. Its surface nitrate concentration, chlorophyll a concentration, nitrate-based new production and total primary production were higher than those of the deep basin water. Simulated production experiments on the shelf and the basin revealed that nitrate-based new production ranged from 0.06 to 0.36 g C m −2 d −1 whereas primary production ranged from 0.12 to 0.63 g C m −2 d −1 . Both productions are positively ( p<0.01) correlated with the surface nitrate concentration. The results of the enrichment experiments showed that N alone limited phytoplankton growth. Enrichment with nitrate only or with nitrate plus phosphate resulted in a phytoplankton bloom as shown by the enhanced chlorophyll a concentration, chlorophyll a-specific primary and new productions. Similar to the control group (without nutrient addition), enrichment with only phosphate did not enhance algal growth. The nitrogen-fixing cyanobacteria, Trichodesmium sp. and R. intracellularis, were both scarcely distributed. Input to new production by N 2-fixation was at most 0.34 μg N m −3 h −1 , constituted less than 3% of the new production, even a generous estimate of the N 2-fixation rate was used in the estimation. This indicates that nitrate diffusing from the deep water was the main source of new nitrogen for phytoplankton growth in the SCS during the early spring. This finding is supported by the negative relationship ( p<0.01) between the stratification index of water column and the surface [NO 3+NO 2], and the positive relationship ( p<0.01) between [NO 3+NO 2] and the productivities. Our results show clearly that N limits phytoplankton growth in early spring in the SCS where N 2-fixation contributes little to new production in this oligotrophic system that iron deficiency is deemed unlikely.