凡纳滨对虾大水面高盐养殖水体叶绿素a的变化及与环境因子的关系OACSTPCD

Aquaculture in large water bodies has become an important culture mode of Litopenaeus vannamei in coastal waters.Fractionated chlorophyll-a(Chl-a)and environmental factors of the large water ponds with high salinity(54,n=3)and the control ponds(32,n=3)were investigated from May to July 2020 to explore the variations in Chl-a,phytoplankton particle size,and the response to environmental factors during the aquaculture season.Pearson correlation analysis was performed to analyze the relationship between the environmental factors and the size-fractionated Chl-a concentration.Partial redundancy analysis(RDA)was applied to assess the effects of environmental factors(including silicate,active phosphate,ammonia salt,nitrite,nitrate,water temperature,salinity,dissolved organic nitrogen,and dissolved organophosphorus)on total Chl-a,Chl-a of micro phytoplankton(micro Chl-a),Chl-a of nano phytoplankton(nano Chl-a),and Chl-a of pico phytoplankton(pico Chl-a).The following results were obtained: 1)Diurnal variation of Chl-a:Total Chl-a of the high-salinity group showed no significant diurnal variation(P＞0.05).Total Chl-a of the control group showed significant diurnal change in May and June(P＜0.05).The highest value of total Chl-a in May occurred at 15:00,while the highest value of that in June was at 08:00.For size-fractionated Chl-a,pico Chl-a in the high-salinity group showed significant diurnal variation in July(P＜0.05),with the highest value appearing at 12:00.Micro Chl-a in the control group showed significant diurnal changes in May,June,and July(P＜0.05),and nano Chl-a in the control group showed significant diurnal changes in June(P＜0.05). 2)Monthly changes of Chl-a:The lowest and highest values of total Chl-a occurred in June and July,respectively.Total Chl-a in July was significantly higher than that in May and June(P＜0.05).For size-fractionated Chl-a,the pico Chl-a and nano Chl-a of the high-salinity group in July were significantly higher than those in May and June(P＜0.05)and showed no significant difference between May and June(P＞0.05).Pico Chl-a,nano Chl-a,and micro Chl-a of the control group in July were significantly higher than those in May and June(P＜0.05)and showed no significant difference between May and June(P＞0.05). 3)Contribution of size-fractionated phytoplankton in high-salinity and control groups:The contribution of micro Chl-a,nano Chl-a,and pico Chl-a to total Chl-a in the high-salinity group were(15.64±0.16)％,(73.81±0.13)％,and(10.55±0.06)％,respectively.Nano Chl-a was dominant in May,June,and July.The contribution of pico Chl-a increased from 6.43％in May to 16.81％in July,and exceeded that of micro Chl-a.The contributions of micro Chl-a,nano Chl-a,and pico Chl-a to total Chl-a in the control group were(52.29±0.10)％,(41.82±0.10)％,and(5.59±0.01)％,respectively.Micro Chl-a concentration had a major advantage in May and June,accounting for 59.64％and 57.49％,respectively.Nano Chl-a concentration accounted for 35.46％and 36.90％,respectively.By July,nano Chl-a had a major advantage,contributing to 53.09％. 4)Pearson correlation analysis showed no significant correlation between the diurnal variation of Chl-a and the environmental factors of the high-salinity group in May and June(P＜0.05).Yet,the concentrations of nano Chl-a and total Chl-a were negatively correlated with the concentration of nitrate in July(P＜0.05).The concentrations of micro Chl-a and total Chl-a were positively correlated with those of silicate(P＜0.05).For the control group,Pearson correlation analysis showed a significant positive correlation between nano Chl-a and water temperature(P＜0.05).Total Chl-a and phosphate were negatively correlated in May(P＜0.05).There was a significant negative correlation between pico Chl-a and nitrate in July(P＜0.05). 5)For the high-salinity group,RDA revealed a significant positive correlation between Chl-a and water temperature,and the contribution of nano Chl-a increased with the increase in temperature.Total Chl-a was positively correlated with silicate and negatively correlated with phosphate,dissolved organic nitrogen,and dissolved organophosphorus in the high-salinity group.For the control group,RDA showed that total Chl-a was positively correlated with dissolved organic nitrogen and negatively correlated with silicate and nitrite.In general,Chl-a in high-salinity ponds has a small diurnal variation,and the phytoplankton particle size gradually decreased with cultivation,which may be caused by the increasing temperature and high organic nitrogen concentration.