太湖西北湖区浮游植物和无机、有机氮的时空分布特征

以太湖重度蓝藻水华发生的西北湖区为研究对象,从河口至湖心区设置5个采样点,于2012年10月至2013年10月逐月采集表层水体样品,测定了水温、溶解氧和浮游细菌丰度,并分析了浮游植物群落结构的组成、溶解性无机氮(DIN)和有机氮(DON)浓度以及氮磷比.研究结果表明,太湖西北湖区浮游植物主要由蓝藻、硅藻、绿藻和隐藻组成.可能由于风、浪等混合作用使太湖西北湖区不同采样点之间蓝藻细胞密度没有显著差异.蓝藻生物量在浮游植物中所占比例最高为34%±15%,春季部分点位隐藻生物量高于50%,表明隐藻与蓝藻的相互竞争趋势显著.CCA排序图结果表明,DIN、DON浓度以及总氮∶总磷比(TN∶TP比)是影响西北湖区浮游植物优势属分布的重要环境因子.5个采样点铵态氮(NH_4~+-N)与DIN浓度具有显著差异,与DON浓度没有显著差异.夏季蓝藻水华暴发期间,可能由于蓝藻的吸收利用引起NH_4~+-N和硝态氮(NO_3~--N)浓度迅速降低.此外,由于NH_4~+-N浓度还可能受到沉积物NH_4~+-N释放的影响,因此,蓝藻细胞密度与NO_3~--N的相关系数和显著水平均高于NH_4~+-N.夏季TN∶TP比和DIN∶TP比降至最低,表明该湖区浮游植物,尤其是蓝藻的生长可能受到氮限制.蓝藻细胞密度与DON浓度呈显著负相关,表明在氮限制条件下,DON可能是蓝藻氮素利用的重要补充.

Temporal and spatial distributions of phytoplankton and inorganic and organic nitrogen in northwest region of Lake Taihu

Surface water samples of five sampling sites from the estuary to lake center were collected from October 2012 to October 2013 in the northwest region of Lake Taihu, where with severe cyanobacterial bloom. Water temperature, dissolved oxygen, and bacterial abundance were analyzed, as well as the phytoplankton community, dissolved inorganic nitrogen (DIN) and dissolved organic nitrogen (DON) and nitrogen to phosphorus ratios. The phytoplankton assemblage was mainly composed of Cyanophyta, Bacillariophyta, Chlorophyta and Cryptophyta. No significant difference in Cyanophyta cell density was observed, which may be explained by the combined effects of current flow and wind. Cyanophyta biomass contributed to the phytoplankton with the maximum value of 34%±15%, but Cryptophyta biomass was more than 50% in some sampling sites in spring, suggesting a competition between Cryptophyta and Cyanophyta. The CCA analysis suggested that the distribution of dominant genera was affected by dissolved inorganic and organic nitrogen concentrations, and total nitrogen to total phosphorus ratio.Significant differences in ammonium nitrogen(NH⁺₄-N) and DIN concentrations were observed among the sampling sites, no significant difference in DON was observed.Utilization by Cyanophyta probably caused the decline of concentrations of NH⁺₄-N and nitrate nitrogen(NO^-₃-N) during summer. Moreover, the dynamics of NH⁺₄-N was also affected by the release of NH⁺₄-N from sediment, thus, Cyanophyta cell density correlated more significantly with NO^-₃-N than NH⁺₄-N. TN:TP and DIN:TP decreased to the lowest values during Cyanophyta bloom, suggesting that phytoplankton, especially Cyanophyta may be limited by nitrogen. A significant negative correlation was observed between Cyanophyta cell density and DON, indicating that DON can also be utilized by Cyanophyta during nitrogen limitation.