渔业科学进展  2024, Vol. 45 Issue (1): 118-127  DOI: 10.19663/j.issn2095-9869.20221116001
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孙怡茹, 张继红, 吴文广, 杜彦秋, 孙威, 冯旭, 康秦梓, 孔杰. 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[J]. 渔业科学进展, 2024, 45(1): 118-127. DOI: 10.19663/j.issn2095-9869.20221116001.
SUN Yiru, ZHANG Jihong, WU Wenguang, DU Yanqiu, SUN Wei, FENG Xu, KANG Qinzi, KONG Jie. Characteristics and Influencing Factors of Size-Fractionated Chlorophyll-a in Litopenaeus vannamei Mariculture Ponds[J]. Progress in Fishery Sciences, 2024, 45(1): 118-127. DOI: 10.19663/j.issn2095-9869.20221116001.

基金项目

国家虾蟹产业技术体系(CARS-48)资助

作者简介

孙怡茹,E-mail: 3202127573@qq.com

通讯作者

张继红,研究员,E-mail: zhangjh@ysfri.ac.cn

文章历史

收稿日期:2022-11-16
收修改稿日期:2023-01-12
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
孙怡茹 1,3, 张继红 1,2, 吴文广 1, 杜彦秋 1, 孙威 1, 冯旭 1, 康秦梓 1, 孔杰 1     
1. 中国水产科学研究院黄海水产研究所 农业农村部海洋渔业与可持续发展重点实验室 山东 青岛 266071;
2. 海洋渔业科学与食物产出过程功能实验室 山东 青岛 266071;
3. 中国农业科学院研究生院 北京 100081
摘要:“渔盐一体化”是山东省滨州市凡纳滨对虾(Litopenaeus vannamei)盐田养殖的重要模式。为了解该模式下养殖水体浮游植物的叶绿素a (Chl-a)浓度、粒径结构的变化特征及主要影响因子,于2021年5—7月分别在养殖的初期、中期和收获期,测定高盐组(S=54)和对照组(S=32)养殖水体的分级Chl-a浓度[小型浮游植物(micro Chl-a)、微型浮游植物(nano Chl-a)、微微型浮游植物(pico Chl-a)、总Chl-a浓度]及相关环境参数的日变化和月变化。结果显示,日变化:对于总Chl-a浓度,高盐组无显著日变化(P>0.05),对照组在5月和6月存在显著的日差异(P<0.05)。对于浮游植物粒径结构,高盐组7月的pico Chl-a日变化显著(P<0.05);对照组7月的micro Chl-a和6月的nano Chl-a日变化显著(P<0.05)。月变化:两盐度组pico Chl-a、nano Chl-a和total Chl-a最低值和最高值都分别出现在6月和7月。且7月的总Chl-a显著高于5月和6月(P<0.05)。高盐组水体中nano Chl-a占主要优势,随着养殖的进行粒径结构特性出现了演替,其中,pico Chl-a对总Chl-a贡献率由5月的6.43%提高至7月的16.81%,超过了micro Chl-a的贡献率。对照组5月和6月以micro Chl-a占主要优势,分别占59.64%和57.49%,其次是nano Chl-a,分别占35.46%和36.90%,7月以nano Chl-a占主要优势,贡献率达53.09%。冗余分析(RDA)显示,Chl-a浓度与水温显著正相关,nano Chl-a的贡献率随温度升高而增加。高盐组总Chl-a浓度与硅酸盐浓度呈显著正相关,与磷酸盐、溶解有机氮、溶解有机磷浓度呈显著负相关;对照组总Chl-a与溶解有机氮显著正相关,与硅酸盐、亚硝酸盐浓度呈显著负相关。总体来讲,高盐组水体Chl-a浓度日变化较小,浮游植物粒级随养殖进行逐渐趋于小型化,可能与温度升高和较高的有机氮水平有关。
关键词凡纳滨对虾    叶绿素a    粒级结构    营养盐    海水养殖池塘    
Characteristics and Influencing Factors of Size-Fractionated Chlorophyll-a in Litopenaeus vannamei Mariculture Ponds
SUN Yiru 1,3, ZHANG Jihong 1,2, WU Wenguang 1, DU Yanqiu 1, SUN Wei 1, FENG Xu 1, KANG Qinzi 1, KONG Jie 1     
1. Key Laboratory of of Marine Fisheries and Sustainable Development, Ministry of Agriculture and Rural Affairs, Yellow Sea Fisheries Research Institute, Chinese Academy of Fishery Sciences, Qingdao 266071, China;
2. Laboratory for Marine Fisheries Science and Food Production Processes, Qingdao 266071, China;
3. Graduate school of Chinese Academy of Agricultural Sciences, Beijing 100081, China
Abstract: 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.
Key words: Litopenaeus vannamei    Chlorophyll-a    Size fraction    Nutrient    Mariculture pond    

凡纳滨对虾(Litopenaeus vannamei)具有营养丰富、养殖周期短、盐度适应范围广和抗病性强等特点(冯旭等, 2023),已成为目前国内外主要的对虾养殖品种。2021年我国凡纳滨对虾海水养殖产量达127.36万t,占甲壳类海水养殖总产量的68.66% (农业农村部渔业渔政管理局等, 2022)。近年来,在山东地区出现新兴的“渔盐一体化”养殖模式,将凡纳滨对虾养殖与盐业生产相结合,海水经养虾后再蒸发晒盐。这种“一水多用”的养殖模式整个生产过程实现了废水零排放,有效利用了海水资源(刘振鲁等, 2022)。通过控制养殖密度和投喂鲜活动物饵料等措施,高盐水体养殖的凡纳滨对虾肉质紧实,味道鲜美。但该养殖模式目前生产工艺滞后、饵料利用率低、水体环境不稳定等导致的高死亡率问题制约了产业可持续发展。因此,亟待了解养殖水环境的变化特性及其与养殖生产活动的关系,为大水面凡纳滨对虾高盐养殖提供技术支持。

浮游植物在水产养殖生态系统中具有不可替代的生态功能,除了作为养殖系统的生产者,同时也是对虾养殖前期的直接饵料和中后期的间接饵料,为浮游动物、无脊椎动物和对虾提供食物(Zebek et al, 2017)。浮游植物会对水体环境的变化做出迅速反应,因此可作为评价水体营养健康状况的生物学指标(阎喜武等, 1997; Bosak et al, 2012; 柴然等, 2020)。De la Rey等(2004)研究表明,当浮游植物的种类、数量和组成结构发生改变时,对水生态系统中其他生物也会产生影响,甚至可影响到整个生态系统的平衡。不同粒级的浮游植物因被摄食压力和生理功能的不同,对池塘食物链具有不同的生态学意义(Brewin et al, 2019)。目前,国内外对Chl-a的粒径结构和影响因素的研究多集中于海域(吴文广等, 2015; 孙越峰等, 2020; Delgadillo-Hinojosa et al, 2020; Ağirbas et al, 2022; Wei et al, 2022),而对大水面养殖水体中浮游植物Chl-a的变化特征报道较少(Montecino et al, 2000; Iriarte et al, 2004)。由于池塘水体面积相对较小,缓冲能力弱,其水体理化性质会因降雨、光照、水温、进水和排水等因素在一天内变化剧烈,浮游植物生物量也会随之变化,因此,研究池塘水体中分级Chl-a的日变化特性具有一定的意义。国内对加州鲈(Micropterus salmoides) (卫鹏等, 2022)、轮虫(Rotifer) (赵文等, 2004)、三疣梭子蟹(Portunus trituberculatus) (孙忠等, 2012)、刺参(Apostichopus japonicus)(张义伟等, 2009; 姜森颢等, 2014)和对虾(刘国才等, 2001)养殖池塘的分级Chl-a进行过报道,但对凡纳滨对虾高盐养殖水体分级Chl-a浓度的变化特性研究未见报道。

本研究选取滨州不同盐度的凡纳滨对虾大水面养殖池塘,在养殖的初期、中期和收获期测定水体的Chl-a浓度、粒径结构及其他生态环境因子,分析大水面对虾养殖水体中Chl-a的变化规律及其影响因素,以期为凡纳滨对虾大水面养殖的健康可持续发展提供参考。

1 材料与方法 1.1 研究池塘概况

实验点位于山东省滨州市,取样时间为2021年5、6、7月,即从放苗到收获的一个养殖周期。选取6口养殖池塘,于每月的26—30日进行取样。池塘分为高盐组(39.89~62.29)和对照组(29.40~34.60),每组3个平行。2个盐度组在“渔盐一体化”养殖模式下自然形成。实验虾池面积约为20 hm2,养殖池塘为泥底,水深在0.5~1.5 m之间。5月1日开始投放虾苗,7月15日开始收获,放苗密度约为30万尾/hm2,虾苗体长在1.2 cm以上。养殖初期投喂卤虫(Artemia)作为对虾的开口饵料,凡纳滨对虾长到6~7 cm后,只投喂对虾配合饲料,投喂量为对虾体重的5%~6%。

1.2 采样及测定方法 1.2.1 Chl-a样品的采集与测定

每次分别在08:00、12:00、15:00和17:00共4个时间点按五点取样法进行取样。使用有机玻璃采水器采集上层0.5 m海水样品各1 L,充分混匀,经100目的筛网过滤去除大型浮游生物。按照Cermeño等(2006)的方法,取500 mL采集并处理过的水样,经20 μm筛绢过滤,分离大于20 μm的小型浮游植物(micro Chl-a),再经过2 μm的玻璃纤维滤膜,分离2~20 μm的微型浮游植物(nano Chl-a),最后,通过0.45 μm的玻璃纤维滤膜,截留0.45~2 μm的微微型浮游植物(pico Chl-a)。滤膜置于–20 ℃冷冻保存,带回实验室后使用美国Turner-Designs Trilogy荧光仪测定Chl-a浓度(Agawin et al, 2000)。总Chl-a浓度为3个粒级Chl-a浓度的和。同时采取水样,用于营养盐含量的测定。

1.2.2 理化环境因子的测定

使用多参数水质分析仪(美国YSI 6600)测定池塘的水温、溶氧、盐度和pH。使用全自动营养盐分析仪(QuAAtro型,SEAL,德国)测定水体中的氮、磷、硅营养盐浓度。其中,硝酸盐(NO3-N)和总氮(TN)采用锌镉还原法,亚硝酸盐(NO2-N)采用盐酸萘乙二胺比色法,氨盐(NH4+-N)采用次溴酸钠氧化法、磷酸盐(PO43–-P)采用抗坏血酸还原磷钼蓝法,总磷(TP)采用钼酸铵分光光度法,硅酸盐(SiO32–-Si)采用硅钼黄法。溶解无机氮(DIN)浓度为NO3-N、NO2-N和NH4+-N浓度之和,溶解有机氮(DON)浓度为TN与DIN的差值。溶解有机磷(DOP)浓度为TP与PO43–-P的差值。

1.3 数据处理与统计分析

实验数据采用平均值±标准差(Mean±SD)表示,符合正态分布后采用R语言进行单因素方差分析(one-way ANOVA)和皮尔逊相关性分析,设置显著性水平为P<0.05。使用Canoco5软件进行冗余分析(redundancy analysis, RDA)。

2 结果与分析 2.1 池塘环境因子的变化特性

高盐组和对照组水体环境指标变化见图 1。高盐组和对照组的水温无显著性差异(P>0.05)。高盐组5月和6月的溶解氧浓度均显著低于对照组(P<0.05),6月溶解氧出现最低值,其平均浓度仅为(4.77±0.11) mg/L。高盐组的硅酸盐、硝酸盐、亚硝酸盐及DIN浓度均显著低于对照组(P<0.05)。活性磷酸盐浓度的峰值都出现在5月,高盐组浓度逐渐降低,在7月浓度为(0.34±0.04) μmol/L,而对照组6月最低。2个盐度组的氨盐都呈倒V型变化,6月份浓度最高。高盐组的DON浓度3个月均显著高于对照组(P<0.05),高盐组5月和6月DOP浓度显著高于对照组(P<0.05),7月2个盐度组无显著性差异(P>0.05)。

图 1 养殖水体环境因子的变化 Fig.1 Variation of pond environmental factors

温度、盐度和pH的日变化较小,无显著性差异(P>0.05)。对照组的活性硅酸盐浓度3个月的日变化均较显著(P<0.05),而高盐组波动较平稳,无显著性差异(P<0.05)。5月活性磷酸盐浓度日变化显著(P<0.05),6月只有高盐组差异显著(P<0.05)。两组3个月的氨盐和硝酸盐浓度均具有显著日变化(P<0.05)。对照组的亚硝酸盐浓度日变化差异显著(P<0.05),高盐组仅在7月波动较大,存在显著性差异(P<0.05)。

2.2 Chl-a浓度的变化

池塘总Chl-a浓度和分级叶绿素的日变化和月变化情况见图 2。高盐池塘3个月的总Chl-a浓度分别为11.11、7.13和35.60 μg/L,对照组的总Chl-a浓度分别为5.79、5.28和45.01 μg/L。2个盐度组总Chl-a浓度在7月存在显著性差异(P<0.05)。

图 2 实验池塘中Chl-a浓度的月份变化 Fig.2 Monthly variation of chlorophyll-a concentration in experimental ponds a:高盐组;b:对照组;*表示日变化差异显著,不同大写字母表示不同盐度间差异显著,不同小写字母表示不同月份间差异显著(P<0.05)。 a: High salt group; b: Control group; * indicates significant diurnal variation, different capital letters indicate significant differences between salinities, and different lowercase letters indicate significant differences between months (P < 0.05).

Chl-a浓度的日变化情况:高盐组总Chl-a浓度无显著日变化(P>0.05),而对照组的总Chl-a浓度在5月和6月均存在显著的日变化(P<0.05),5月的最高值出现在15:00点,6月的高值出现在08:00。对于分级叶绿素,在7月,高盐组的pico Chl-a浓度存在显著的日变化(P<0.05),高值出现在12:00;对照组的micro Chl-a浓度在5、6、7月均存在显著的日变化(P<0.05),6月对照组nano Chl-a浓度的日变化显著(P<0.05)。

Chl-a浓度的月变化情况:2个盐度组的总Chl-a浓度最低值和最高值都分别出现在6月和7月,且7月的总Chl-a浓度显著高于5月和6月(P<0.05)。对于分级叶绿素,高盐组的pico Chl-a和nano Chl-a浓度7月显著高于5月和6月(P<0.05),且5月和6月之间无显著性差异(P>0.05);对照组7月的pico Chl-a、nano Chl-a和micro Chl-a浓度均显著高于5月和6月(P<0.05),且5月和6月之间无显著性差异(P>0.05)。

2.3 分级Chl-a贡献率

高盐组和对照组池塘浮游植物不同粒径的贡献率如图 3所示。高盐组micro Chl-a、nano Chl-a和pico Chl-a对总Chl-a的贡献率分别为(15.64±0.16)%、(73.81±0.13)%和(10.55±0.06)%,5、6、7月均以nano Chl-a占主要优势,7月份pico Chl-a贡献率由5月的6.43%提高至7月的16.81%,并超过micro Chl-a的贡献率。

图 3 实验池塘不同粒级Chl-a的贡献率 Fig.3 The contribution of size-fractionated Chl-a of pico, nano and micro to total Chl-a in experimental pond a:高盐组;b:对照组 a: High salt group; b: Control group

对照组micro Chl-a、nano Chl-a和pico Chl-a对总Chl-a的贡献率分别为(52.29±0.10)%、(41.82± 0.10)%和(5.59±0.01)%。5月和6月micro Chl-a占主要优势,分别占59.64%和57.49%,其次是nano Chl-a,分别占35.46%和36.90%,7月nano Chl-a占主要优势,贡献率达53.09%。

2.4 Chl-a浓度变化与环境因子相关性分析 2.4.1 Chl-a浓度日变化与环境因子的关系

将环境因子(包括SiO32–-Si、PO43–-P、NH4+-N、NO3-N、NO2-N、水温、盐度、DON和DOP)与总Chl-a浓度和分级Chl-a浓度进行皮尔逊相关性分析。结果显示,对于高盐池塘,5月和6月,Chl-a浓度日变化与这9个环境因子均无显著性关系,仅在养殖的末期(7月),nano Chl-a和总Chl-a浓度与NO3-N浓度显著负相关,micro Chl-a和总Chl-a浓度与SiO32–-Si显著正相关(P<0.05)(表 1)。对照组,5月nano Chl-a与水温显著正相关,总Chl-a浓度和PO43–-P显著负相关;7月份pico Chl-a与NO3-N浓度显著负相关(P<0.05)(表 2)。

表 1 高盐组Chl-a浓度与环境因子的相关系数 Tab.1 Correlation coefficient between chlorophyll-a concentration and environmental factors in high salt group
表 2 对照组Chl-a浓度与环境因子的相关系数 Tab.2 Correlation coefficient between chlorophyll-a concentration and environmental factors in control group
2.4.2 Chl-a浓度月变化与环境因子的关系将总Chl-a、micro Chl-a、nano Chl-a、pico Chl-a作为响应

变量,9个环境因子(包括SiO32–-Si、PO43–-P、NH4+-N、NO3-N、NO2-N、水温、盐度、DON和DOP)作为解释变量进行RDA分析。当夹角为锐角时为正相关,当夹角大于90°时为负相关,夹角越小,相关性越强。RDA排序图显示(图 4),高盐组的pico Chl-a、nano Chl-a、总Chl-a与SiO32–-Si、水温显著正相关,与PO43–-P、盐度、DON、DOP显著负相关,micro Chl-a与NO3-N显著负相关。对照组所有粒径浮游植物Chl-a浓度与水温、DON显著正相关,与SiO32–-Si、NO2-N、盐度显著负相关。

图 4 Chl-a浓度与环境因子RDA排序图 Fig.4 RDA ordination of chlorophyll-a and environmental factors a:高盐组;b:对照组
S:盐度;T:温度;NO2-N:亚硝酸盐;NO3-N:硝酸盐;PO43–-P:活性磷酸盐;SiO32–-Si:活性硅酸盐;NH4+-N:氨盐;DOP:溶解有机磷;DON:溶解有机氮。
a: High salt group; b: Control group
S: Salinity; T: Temperature; NO2-N: Nitrite; NO3-N: Nitrate; PO43–-P: Active phosphate; SiO32–-Si: Active silicate; NH4+-N: Ammonia nitrogen nutrient; DOP: Dissolved organic phosphorus; DON: Dissolved organic nitrogen.
3 讨论

浮游植物的生长和繁殖通常受温度、营养盐和光照等环境因子的影响,另外,还受次级生长者(如浮游动物、滤食性贝类等)的摄食调控(Liu et al, 2003; Popovich et al, 2008)。在一定温度范围内,通常随着水温的升高,浮游植物的光合作用增强,生长速度加快。本研究发现,2种类型的水体,不论是总Chl-a浓度还是各分级Chl-a浓度(高盐组micro Chl-a除外),都与水温显著正相关(见图 4)。可见,水温是池塘中浮游植物生长的关键控制因子。

营养盐是浮游植物生长所必须的基础物质。浮游植物的生长不仅受营养盐绝对浓度的影响,而且,受不同营养盐比值即相对浓度的影响。虽然水体营养盐浓度较高,处于富营养化状态,但是从N、P、Si的比值来看,存在营养盐相对限制的情况。根据营养盐相对限制法计算N、P、Si之间的比例关系,若Si/P>22和N/P>22,则PO43–-P为浮游植物生长限制因子,若N/P<10和Si/N>1,则为DIN为限制因子,若Si/P<10和Si/N<1,则Si为限制因子(Justic et al, 1995)。据此来评估,5月和6月高盐组的浮游植物生长属于Si相对限制状态。冗余分析结果也显示,高盐组的pico Chl-a、nano Chl-a和总Chl-a浓度与SiO32–-Si呈显著正相关,这说明高盐组水体浮游植物生长受SiO32–-Si的限制。而对照组从养殖开始至收获,SiO32–-Si一直处于较高的浓度(12.95~26.47 μmol/L),显著高于高盐组(见图 1)。SiO32–-Si不是对照组浮游植物生长的限制因子,对照组水体浮游植物生长受DON含量的影响较大(Chl-a浓度与DON含量显著正相关,见图 4b)。是否是由于高盐限制了沉积物中SiO32–-Si的溶出释放导致了高盐组与对照组的SiO32–-Si含量显著不同,还有待进一步的研究。

本研究发现,2种类型水体Chl-a的粒径结构是不同的,在养殖初期(5月),高盐组以nano Chl-a为主[占(73.81±0.13)%],对照组以micro Chl-a为主[占(52.29±0.10)%],水体浮游植物粒级结构的变化可能与水体中NH4+-N含量的差异有关。高盐组因养殖前的肥水(投入有机肥),使得DON浓度显著高于对照

组。张建乐等(2020)研究显示,在DON含量较高的水体中,对有机氮营养物质利用能力强的微型及微微型浮游植物更有可能成为优势种,从而导致浮游植物群落结构发生变化,水体中的小型浮游植物向微型和微微型浮游植物演替。据报道,DON能直接被浮游植物利用12%~72% (Stepanauskas et al, 1999)。有机氮(如尿素)往往能刺激微型浮游植物的暴发,如近些年我国海域微小原甲藻(Prorocentrum minimum)和抑食金球藻(Aureococcus anophagefferens)赤潮与DON浓度过高有关(Mulholland et al, 2009; Gobler et al, 2012)。其次,SiO32–-Si是硅藻类浮游植物必须的营养元素。本研究镜检结果显示,小型浮游植物中硅藻占主导地位(待发表数据)。在5月和6月高盐组都出现SiO32–-Si限制,这将影响粒径大于20 μm的硅藻的生长。另外,高盐组浮游植物粒径小于对照组,也可能与投喂的卤虫摄食压力有关。通常卤虫在高盐水体中的摄食行为更为活跃(Sura et al, 2017)。虽然高盐组和对照组水体浮游植物的粒径结构不同,但本研究发现,随着养殖的进行(5—7月),2个实验组的浮游植物都有小型化的趋势(见图 2),这种变化特性可能与水温升高促进微型和微微型浮游植物生长有关。Mousing等(2014)研究发现,小粒径的浮游植物细胞在群落中的重要性随着温度的增加而增加,较大细胞的贡献率随温度升高而降低,产生这种现象与养分独立效应和养分共享效应有关(Moran et al, 2010)。夏滨等(2001)研究发现,冷水中较大粒级的浮游植物占比较高。也有研究表明,随着温度的升高,群落结构向较小细胞占主要优势转变(Sin et al, 2000; Coello-Camba et al, 2014)。

高盐组和对照组的产量分别为870和675 kg/hm2,产值分别为34 950和27 000元/hm2。从Chl-a浓度和粒径结构来看,高盐组的日变化较小(总Chl-a、micro Chl-a、nano Chl-a浓度在5—7月的3次调查中均无显著的日变化)(表 1),即高盐水体浮游植物相对稳定,有利于凡纳滨对虾的存活和生长。

浮游植物对水质调控具有重要作用,能够调节池塘水体透明度、酸碱度、溶解氧和营养盐水平(乔玲等, 2022)。细胞的粒径大小也是影响藻类新陈代谢的关键因素,有研究表明,不同细胞大小的藻类生长速率和对营养物质的吸收效率都不同,微型浮游植物在大多数水域中占主要优势是因为中等细胞大小的藻类代谢率最快(李胜男等, 2020)。所以,浮游植物群落粒级结构的变化在一定程度上也反映了水质营养的变化状况,可作为调节水环境的重要指示参数。

4 结论

综上所述,高盐组相较于对照组Chl-a浓度日变化较小,水体中浮游植物生物量相对稳定。且随着养殖进行,高盐组5、6、7月nano Chl-a占主要优势,对照组5月和6月micro Chl-a为主,7月nano Chl-a占主要优势。高盐组和对照组浮游植物粒级结构都具有小型化趋势,这一现象可能与较高的温度和DON浓度有关。

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