Text: Bread Clip Knowledge
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«——【Preamble】】 ——»
Phytoplankton is the most important primary producer in the marine ecosystem and an important support for marine aquaculture activities, providing food for marine organisms such as fish, shrimp and shellfish.
In particular, filter-feeding shellfish are the main food source and play an important role in regulating the level of eutrophication in water bodies.
The species composition and abundance of phytoplankton are affected by hydrological conditions, water environment conditions and other factors, and are very sensitive to environmental changes, which can sensitively and quickly reflect the current status and change trend of water environment quality, so they can be used as an important index for water environment quality evaluation.
It is of great significance to study the characteristics of phytoplankton community and its correlation with environmental factors in shellfish culture areas for shellfish farming and marine ecosystem protection.
Crassostreagigas has the advantages of fast growth, large size and low mortality during the breeding season, and in recent years, Crassostreagigas has become a hot spot for oyster farming in northern China.
In 2022, the area and output of triploid oysters in Fujian Province were 17,028.56 hm2 and 1,040,200 tons, respectively, accounting for 43.56% and 41.17% of the province's oyster culture area and output, respectively.
Lianjiang County, Fujian Province is the second largest aquatic county in the country, Huangqi Bay is located in Lianjiang County, Huangqi Peninsula between the Huangqi Mountain and Dinghaijiao, and there are triploid Fujian oyster breeding areas in the bay and outside the bay.
At present, there have been some reports on phytoplankton in shellfish culture areas such as oysters, scallops, mussels, and Philippine clams, but there are no relevant studies on the phytoplankton community characteristics in triploid oyster culture areas.
Through a 10-month survey of triploid oyster farming area in Huangqi Bay, the community characteristics of phytoplankton and their relationship with environmental factors were analyzed, which can provide a scientific basis for the healthy cultivation of triploid oysters and the protection of marine environment.
«——【Materials and Methods·】——»
Huangqi Bay is surrounded by mountains on three sides, the mouth of the bay is open to the southeast, the water area is about 200km2, about 12km from the west of the mouth of the bay is the mouth of the Ao River, and about 15km to the southwest is the mouth of the Minjiang River.
Six survey stations were set up in the triploid oyster culture area of Huangqi Bay, including three survey stations outside the bay (L1~L3 station) and inside the bay (L4~L6 station), and one control station (L0 station) was set up in the non-aquaculture area of the adjacent sea area.
The oyster seedlings in the outer and inner bay aquaculture areas were the same batch of triploid Fujian oyster shell seedlings, and the culture method was longline flat hanging string culture. The survey stations are shown in Figure 1.
The survey period was from December 2021 to September 2022, and one survey was carried out at the beginning of each month for triploid oyster culture areas (L1~L6 stations), and once for the control point (L0 station) in September 2022.
The surface seawater was collected by the plexiglass water collector, the water temperature, salinity, pH and dissolved oxygen were measured on site by WTWMulti3630 multi-parameter analyzer, and the samples of suspended solids, chemical oxygen demand, nutrients, phytoplankton and other samples were transported back to the laboratory for analysis, and the analysis methods of each project were carried out in accordance with the relevant data.
S is the total number of species in the sample; ni is the number of individuals of the ith species; N is the total number of individuals; Pi is the ratio of the number of individuals (ni) to the total number of individuals (N) of species i; ƒi is the occurrence rate, and when the dominance (Y) value ≥ 0.02, the species is the dominant species.
SPSS25 was used to compare the differences between the data of phytoplankton abundance and diversity index in different months in the cultivation area by one-way analysis of variance (ANOVA).
The independent samples T-test (α=0.05) was used to analyze the differences between groups in and out of the bay, such as marine environmental factors, phytoplankton abundance and diversity index.
Canocofor Windows 4.5 software was used to perform redundancy analysis (RDA) of phytoplankton dominant species and environmental factors, because the cell density of phytoplankton was quite different from that of environmental factors.
In the RDA and correlation analysis, the phytoplankton abundance was transformed by lg(X+1), and the results of RDA analysis were made by Canodraw software. The survey station map was created using Surfer18 software.
«——【Results and Analysis·】——»
During the survey, the monthly average water temperature fluctuation range was between 11.1~28.2 °C, the average water temperature was 19.4 °C, and the water temperature gradually decreased from December to February, and then the water temperature gradually increased, and the water temperature was the highest in August.
The monthly mean salinity fluctuated in the range of 26.1~33.3, with an average of 29.7, and the salinity of seawater was lower in June due to continuous rainfall, and the salinity outside the bay was significantly lower than that in the bay (P<0.05).
硝酸氮含量月均值波动范围为0.024～0.496mg/L，平均0.256mg/L，亚硝酸氮含量月均值波动范围为0.004～0.027mg/L，平均0.012mg/L。
The monthly average fluctuation range of ammonia nitrogen content was 0.012~0.060mg/L, and the average was 0.025mg/L. The monthly average fluctuation range of active phosphate content was 0.006~0.041mg/L, and the average was 0.024mg/L.
活性硅酸盐含量月均值波动范围为0.367～1.16mg/L，平均0.801mg/L。
From December to March, the nutrient content in seawater was relatively high, and began to decrease significantly in April, and the nutrient content in the sea area increased significantly in the rainy season when supplemented by terrestrial runoff (Table 1).
The water quality in the aquaculture area was better, the nutrient salt was richer, and the nitrate nitrogen content outside the bay was significantly higher than that in the bay (P<0.05), and there was no significant difference between the groups outside the bay and in the bay for the other indicators (P>0.05) The survey data in September 2022 showed that the water temperature at the control point was 27.7°C, the salinity was 33.4, and the pH value was 8.00.
The dissolved oxygen content was 5.67 mg/L, and the contents of nitrate, nitrite and ammonia were 0.052, 0.022 and 0.026 mg/L, respectively, which were similar to those in the aquaculture area outside the bay. The contents of active phosphate, active silicate, total nitrogen and total phosphorus were 0.024, 0.634, 0.202 and 0.038 mg/L, respectively.
The aquaculture area outside the bay was close to the aquaculture area in the bay (Fig. 2), and in general, there was no significant difference between the marine environmental factors at the control point and the aquaculture area.
According to the survey data in September 2022, the water temperature at the control point was 27.7°C, the salinity was 33.4, the pH value was 8.00, the dissolved oxygen content was 5.67mg/L, and the contents of nitrate, nitrite and ammonia were 0.052, 0.022 and 0.026mg/L, respectively.
These are similar to those of the out-of-bay aquaculture area; The contents of active phosphate, active silicate, total nitrogen and total phosphorus were 0.024, 0.634, 0.202 and 0.038 mg/L, respectively, which were slightly higher than those in the aquaculture area outside the bay and close to those in the aquaculture area in the bay (Fig. 2). In general, there was no significant difference between the marine environmental factors at the control sites and those in the culture areas.
Phytoplankton in the Huangqi Bay aquaculture area can be roughly divided into three groups: macrothermal group, warm water group, and warm temperature group, among which the macrotemperature group is the most common and has an absolute abundance advantage, and can be divided into different groups according to their different adaptability to salinity.
There are many species of phytoplankton in the cultivation area, such as Euglenasp., Melosiravarians, etc., among which Euglena appeared in various surveys, and became the dominant species in the bay culture area in the rainy season (May).
Mesodiniumrubrum also appeared in various surveys, and became the dominant species in December, February and March, which is an autotrophic marine ciliate that is widely considered to be phytoplankton and macrothermic because of its photosynthetic ability.
The phytoplankton species in the aquaculture area were abundant, and a total of 133 species (including varieties and variants) of 6 phyla, 65 genera and 6 phyla were identified. There were 101 species in 46 genera and 46 genera, accounting for 75.9% of the total species.
There were 28 species in 15 genera and dinoflagellates, accounting for 21.1% of the total species. There was one genus and one species for Cyanobacteria, Pyrophyta, Euglena and Protozoa, accounting for 0.75% of the total species, respectively.
During the survey, the dominant phytoplankton species were S. mesoribralis and Rhomboides deliformis, with Y values of 0.798 and 0.027, respectively. As can be seen from Table 2, the dominant phytoplankton species changed with different survey months.
Among them, S. mesoribralis was the dominant species in 10 surveys, and Rhomboida libilis was more dominant in July and September when the water temperature was higher, and it was also the dominant species in March and May.
From December to March, when the water temperature is low and the wind and waves are large, warm-temperature species (such as Streptoalis rotundum) and benthic diatoms (such as T. anthophyllum with troughs, Rhomboides macrophylla, etc.) can become the dominant species.
The dominant phytoplankton species in the out-bay and intrabay aquaculture areas were Strip. mesophytophytes (Y=0.827) and Rhomboides debilitus (Y=0.021), and the red tide of S. mesolyphate occurred in the waters outside the bay in April, with a Y value as high as 0.978.
The dominant species in the bay culture area were S. mesoribralis (Y=0.431), Rhomboides flibilis (Y=0.096) and C. spiralum (Y=0.021).
During the survey period, the number of phytoplankton species fluctuated between 41~85 species, with the lowest phytoplankton species in April and the most abundant species in August.
调查期间浮游植物丰度月均值波动范围在1.80×104～4.14×106cells/L之间，平均5.33×105cells/L。
The abundance of phytoplankton was low from December to March, and the peak occurred in April. From May to August, the abundance of phytoplankton was higher (1.02×105~3.96×105cells/L), and the abundance decreased slightly in September.
There were significant differences in phytoplankton abundance in the inner and outer bay areas in January, April, June and July (P<0.05), all of which were higher than those outside the bay, and there was no significant difference in the rest of the months (P>0.05) (Fig. 3).
During the survey period, the average abundance of phytoplankton outside the bay and within the bay was 9.89×105 and 7.75×104cells/L, respectively, which was significantly higher than that in the bay (P<0.05).
During the survey period, the monthly average fluctuation range of phytoplankton richness (d) in the breeding area was 0.80~2.46, and the average value was 1.90.
The monthly mean fluctuation range of diversity index (H′) was 0.50~3.54, and the average value was 2.69, and the monthly average fluctuation range of evenness (J) was 0.12~0.69, with an average value of 0.54 (Table 3).
Except for the phytoplankton indexes during the red tide in April, which were low, the indexes of the rest of the survey time were within the normal range, and the monthly average values of H′ in December, January, March, August and September were >3.
The diversity level of phytoplankton in the aquaculture area was good, and the community structure was stable, and the average values of phytoplankton d, H′ and J in the aquaculture area outside the bay were 2.00, 2.71 and 0.53, respectively.
The average values of d, H′ and J of phytoplankton in the bay aquaculture area were 1.80, 2.66 and 0.54, respectively, and the results of the T-test showed that there was no significant difference between the phytoplankton index groups outside the bay and the aquaculture area in the bay (P>0.05).
In September 2022, a total of 52 phytoplankton species were identified at the control site, with an abundance of 8.86×104cells/L, and the dominant species were S. mesoribata, S. tropicalis, Rhomboidus flexus, Scaphoid algae, S. crescentis, S. d's. danish, and Rhizophyllum brittle.
Compared with the culture area, phytoplankton species were more abundant, slightly more abundant, and the species composition was similar, and the first dominant species was S. mesoribensis (Table 4).
The d, H′ and J values of phytoplankton at the control point were 3.10, 3.24 and 0.57, respectively, and the d values were higher than those in the aquaculture area, and the other indices were close to those in the aquaculture area outside the bay.
Firstly, the dominant phytoplankton species in the breeding area (S. mesoribensis and Rhomboides delibilus) and the first dominant species (Karenella michi, A. verona, and R. rubrum) in the monthly survey were analyzed by DCA, and the results of DCA analysis showed that the length of the gradient axis (lengthsofgradient) was less than 3, so redundancy analysis (RDA) was chosen.
The RDA results showed that the correlation between phytoplankton dominant species and marine environmental factors was 0.949 and 0.865 in the first and second ranking axes, respectively, and the cumulative changes of phytoplankton dominant species and marine environmental factors in the first two ranking axes were 59.2% and 82.3%, respectively.
The MonteCarlo random permutation test showed that all canonical feature axes were significantly correlated with the dominant phytoplankton species (P<0.01), indicating that the ranking results were credible and could better reflect the relationship between phytoplankton and marine environmental factors.
The RDA ranking chart of phytoplankton dominant species and marine environmental factors (Fig. 4) showed that TP, AP, NO3--N, SS and T were the main influencing factors of the first ranking axis, and the correlation coefficients were 0.8624, 0.8565, 0.8322, 0.7724 and -0.7017, respectively, and the first ranking axis had the largest negative correlation with T and the largest positive correlation with TP.
The main influencing factors of the second ranking axis were S, COD, N/P, Si/N and Asi, and the correlation coefficients were -0.4909, 0.4230, -0.3481, 0.3467 and 0.3007, respectively.
The results showed that the main marine environmental factors affecting the abundance of dominant phytoplankton species were TP, AP, S and COD.
«——【Discussion·】——»
In June, the water temperature fluctuation range was 23.0~23.5 °C, and the salinity fluctuation range was 24.4~27.7, and the water temperature and salinity were more suitable for the growth of Karenella michael.
The results of RDA analysis showed that the abundance of Karenella michii was most closely related to nitrite nitrogen and COD, which was positively correlated with nitrite and COD, and negatively correlated with N/P, indicating that the lower N/P ratio was conducive to the growth of Karenella michi, which was consistent with the results of Hao Luo [35].
COD roughly reflects the level of organic matter in the water body, and phytoplankton is also an organic matter, so COD is positively correlated with the dominant species such as S. mesoribata, Rhomboides mesophyllum and Karenella michieri.
A. slotted and Lycophyllum rubrum became the first dominant species in February and December, respectively, and both of them mainly appeared in winter.
The results of RDA analysis showed that there was a positive correlation between A. slotchylae and A. rubrum and environmental factors such as total nitrogen, total phosphorus, nitrate, suspended solids, and active phosphate, and negative correlation with water temperature, Si/N, and Si/P ratios.
These two species are macrothermic species, and the reason for their high abundance in winter is mainly related to the high wind and waves in winter, in addition to the more suitable water temperature and the richer nutrient content in seawater.
The abundance of algae is positively correlated with the content of suspended solids due to the strong wind and waves in winter, and the sediment is suspended in seawater due to agitation.
«——【·Conclusion·】——»
A total of 133 species of phytoplankton in 6 phyla, 65 genera were identified during triploid oyster culture, including 101 species of Diatoms, accounting for 75.9% of the total species.
There are 28 species of dinoflagellates, 1 species each of cyanobacteria, goldenrods, euglenacea and protozoa. The most common and abundant phytoplankton groups were the broad-tempered and broad-salt taxa, and the dominant species were S. mesocostalis (Y=0.798) and Rhomboides flexurum (Y=0.027).
The fluctuation range of phytoplankton abundance was 1.80×104~4.14×106 cells/L, with an average of 5.33×105 cells/L, the abundance of phytoplankton was low from December to March, the peak appeared in April, the abundance of phytoplankton was higher from May to August, and the abundance decreased slightly in September.
The abundance of aquaculture areas outside the bay was significantly higher than that in the bay. The level of phytoplankton diversity was relatively high, and the community structure was relatively stable. The high dominance of S. mesolyans is related to its wide temperature and salt characteristics, strong physiological characteristics of photoadaptation, and abundant silicates in seawater.
The results of redundancy analysis showed that the main environmental factors affecting the abundance of dominant phytoplankton species were total phosphorus, active phosphate, salinity and chemical oxygen demand, and there was a positive correlation between S. mesoribralis and Rhomboides flexurae and environmental factors such as water temperature, Si/P, ammonia nitrogen, Si/N and salinity.
However, there was a negative correlation with active phosphate, total nitrogen, total phosphorus, active silicate, nitrate and so on, and the water temperature and active phosphate were the most affected by the water temperature and active phosphate, and the Si/P ratio was the most affected by the Si/P ratio.
The results showed that phosphorus content and salinity were the main environmental factors affecting the abundance of dominant phytoplankton species in estuarine bays where phosphorus was relatively limited.