| The increasing nutrient inputs may lead to a change of phytoplankton community structure. Therefore, to study the impacts of nitrogen and phosphorus ratio on marine phytoplankton is very important. Which nutrient, N or P, is the most limiting nutrient for phytoplankton growth in the oligotrophic South China sea has has no answer,this ques-tion has been on debate over many years.It need us to resolve this debate in the Nansha and Xisha Islands China sea by many studies on nutrient limitation.Growing marine eutrophication, red tides have become one of the global environ-mental problems. Nutrient limitation on phytoplankton have become a research hotspot in recent years. This paper summarizes the growth of marine phytoplankton nutrient limitation research methods, summarized abroad on seawater nitrogen, phosphorus, and other nutrients limiting phytoplankton growth findings.To investigate the effects of nitrate and phosphate on phytoplankton growth, the field experiments and enrichment incubation experiments were carried out in the waters of Nansha Islands in summer of 2013. The experimental areas located at two sites, S1 (N9°30’, E109°30’) and S2 (N4°30’, E109°30’), which was in the southwest continental shelf and near the James shoal area, respectively. The results showed that the chloro-phyll-a increased significantly (P<0.05) after adding single nitrate and mixed nitrate and phosphate. The chlorophyll-a concentration increased from 0.05 mg℉m-3 to 0.29 mg℉m-3 and 0.80 mg℉m-3 at S1 site after treated by adding single nitrate and mixed ni-trate and phosphate, respectively. Similarly, those increased from 0.09 mg℉m-3 to 0.79 mg℉m-3 and 1.02 mg℉m-3 at S2. It suggested that the enrichment of single nitrate or/and mixture of nitrate and phosphate could promote the growth of phytoplankton. However, the enrichment of phosphate should not promote the phytoplankton growth because the chlorophyll-a concentration showed no significant increase by adding single phosphate in both sites. Consequently, the restriction factor of phytoplankton growth at S1 was only nitrate concentration, but those at S2 were both nitrate and phosphate concentra-tions. Moreover, it suggested that the most adaptable ratio of N/P for phytoplankton growth should be between 5 to 25 due to the relative high concentration of chloro-phyll-a. Furthermore, the result of correlation analysis showed there was no significant relationship (P<0.05) between N/P ratio and chlorophyll a concentration, neither was N/P ratio and growth rate of phytoplankton, it suggested that the ratio should not influ-ence the phytoplankton growth in the waters of Nansha Islands in summer.To investigate the potential influence of nitrogen and phosphate on phytoplankton community structure., the field experiments and enrichment incubation experiments were carried out in the waters of Nansha Islands in summer of 2013. The experimental areas located at two sites, S1 (N9°30’, E109°30’) and S2 (N4°30’, E109°30’), which was in the southwest continental shelf and near the James shoal area, respectively. The re-sults showed that:(1) the number of phytoplankton species increased after enrichment; (2)With the increasing of Nutrient concentrations,Shannon-Wiener diversity index de-clined. (3)Before the enrichment incubation, Pyrrophyta were the dominant species,but after the nutrients addition were Bacillariophyta. Pyrrophyta has more competitive ad-vantage than Bacillariophyta in the oligotrophic region, resulted in the shift from Pyr-rophyta to Bacillariophyta. (4) Nutrient enrichment caused rapid growth of phytoplank-ton in a short time,but it inhibited phytoplankton growth when it exceeded a certain level and the inhibition was associated with the phytoplankton species.For example, Pyrrophyta. (5) With the increasing of the nutrients concentration in each group, the abundance of Pyrrophyta, Bacillariophyta in single N and single P groups had no little change, but the abundance of Pyrrophyta in mixed N and P groups significantly de-creased, the abundance of Bacillariophyta significantly increased. Mixed N and P lim-ited the phytoplankton growth together.To investigate the effects of nitrate and phosphate on phytoplankton growth, the field experiments and enrichment incubation experiments were carried out in the waters of Xisha Islands in winter of 2014. The experimental areas located at one sites, SO (N17°30.364’, E111°59.843’). The results showed that the chlorophyll-a increased significantly (P<0.05) after adding mixed nitrate and phosphate. The chlorophyll-a con-centration increased from 0.06 mg·m-3 tol.11mg·m-3 and1.71mg·m-3 at S0site after treated by adding 32 μmol·L-1 NO3--N+1 μmol·L-1 PO43--P and 4 μmol·L-1 NO3--N +0.25μmol·L-1 PO43--P mixed nitrate and phosphate, respectively. Similarly, those in-creased from 0.09 mg·m-3 to 0.79 mg·m-3 and 1.02 mg·m-3 at S2. It suggested that the enrichment of mixture of nitrate and phosphate could promote the growth of phyto-plankton. However, the enrichment of single nitrate and single phosphate should not promote the phytoplankton growth because the chlorophyll-a concentration showed no significant increase by adding single nitrate and single phosphate in SO sites. Conse-quently, the restriction factor of phytoplankton growth at SO were both nitrate and phosphate concentrations. Moreover, it suggested that the most adaptable ratio of N/P for phytoplankton growth should be between8 to 32 due to the relative high concentra-tion of chlorophyll-a. |
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