| The mariculture industry has rapidly developed in recent decades due to population growth and the rising demand for seafood. Phytoplankton is not only an important indicator for assessing the carrying capacity of bivalve mariculture, but also a limiting factor for large-scale and high-density bivalve cultivation. According to the size of phytoplankton, it can be divided according to their size into picophytoplankton (< 2 μm), nanophytoplankton (2~20 μm) and microphytoplankton, (20~200 μm). Due to the low retention rate of picophytoplankton by filter-feeding bivalve is very low, using the total amount of phytoplankton available for aquaculture capacity assessment would result in an overestimation. Therefore, understanding the particle size composition and spatiotemporal distribution characteristics of phytoplankton in the target sea area can improve the accuracy of assessing bivalve culture capacity, and provide scientific guidance for marine bivalve aquaculture. Blue mussel Mytilus edulis and triploid oyster Crassostrea gigas are the most common bivalve species in Haizhou Bay, a typical bivalve mariculture area in China. In order to understand the size-fractionated phytoplankton and its environmental influencing factors, including water temperature, salinity, dissolved oxygen, pH, total nitrogen, total phosphorus, phosphate, silicate, nitrate, nitrite, ammonia nitrogen, dissolved inorganic nitrogen, N/P, N/Si and Si/P, both in bivalve mariculture areas (area 1, area 2) and non-mariculture areas (channel area, reference area). Phytoplankton biomass was investigated in March, July, September, October and December 2023 by measuring Chl-a of size-fractionated phytoplankton. Two-way variance analysis and redundancy analysis were used to analyze the effects of environmental factors on size-fractionated phytoplankton. The results showed: (1) The annual variation range of total Chl-a concentration in the investigated area was 0.86~18.49 μg/L, and the seasonal difference was significant (P < 0.05). The annual ranges of pico Chl-a, nano Chl-a and micro Chl-a concentrations were 0-0.9 μg/L, 0.13-6.12 μg/L and 0.35-15.3 μg/L, respectively, with significant seasonal differences (P < 0.01). But there was no significant in the concentration of Chl-a in March and December, with average values of (1.98±0.61) μg/L and (3.69±1.55) μg/L, respectively. In July and October, the average concentration of Chl-a in area 1 to (9.80±2.04) μg/L and (12.34±6.27) μg/L, that is much higher than those in other areas (P < 0.05), it may be due to the higher nutrient concentration in the coastal waters. In September, the Chl-a concentration in area 2 (1.47~1.94 μg/L) was significantly lower than in the non-bivalve mariculture areas (P < 0.01), simultaneously the concentration of nitrate and nitrite in area 2 was significantly higher than in the other areas. This was the rapid growth period of bivalves, and was presumed to be caused by bivalve feeding and excretion. (2) Phytoplankton communities exhibit notable spatiotemporal variation. In March, the phytoplankton communities of area 1 and the channel area were dominated by microphytoplankton, while area 2 and reference area were dominated by nanophytoplankton. In July, nanophytoplankton dominated in area 1 and channel area, while microphytoplankton dominated in area 2 and reference area. In September, microphytoplankton dominated in area 1, area 2 and channel area, while nanophytoplankton dominated in reference area. The proportion of picophytoplankton in reference area was significantly higher than that in other areas (P < 0.05). In October, the contribution rate of microphytoplankton increased gradually in all areas, and the value added in bivalve mariculture areas was significantly higher than that in non- bivalve mariculture areas (P < 0.05). In December, there was no significant difference in the contribution rate of particle size in different areas, but the contribution rate of microphytoplankton continued to increase across different areas, with an average value of 89.01%. The contribution rate of picophytoplankton was mainly affected by seasonal factors (P < 0.001), while the contribution rate of nanophytoplankton and microphytoplankton was significantly affected by seasonal, seasonal and regional interactions (P < 0.05), as found by two-way analysis of variance. (3) Seasonal and regional differences exist in the response of the particle size structure of phytoplankton to environmental factors in the survey areas. The redundancy analysis showed that the first two axes explained 79.31%, 86.94%, 88.35% and 99.09% of the species variation in area 1, area 2, channel area and reference area, respectively. There were seasonal and regional differences in the response of the particle size structure of phytoplankton to environmental factors in the four areas. There was a significant negative correlation between nanophytoplankton and N/Si in area 1. In area 2, picophytoplankton was significantly negatively correlated with total nitrogen, nanophytoplankton was significantly positively correlated with N/P. In channel area, the phytoplankton of three sizes were significantly negatively correlated with ammonia nitrogen. Microphytoplankton was positively correlated with DIN in reference area (P < 0.05). Based on the above discussion, it was found that the Chl-a concentration of phytoplankton in area 1 was mainly regulated by nutrients salts, while the Chl-a concentration of phytoplankton in area 2 was influenced by the effects of nutrients salts and cultured bivalve. For the entire survey area, seasonal changes in environmental conditions are the main cause of the variation of phytoplankton particle size structure. In addition, the presence of seasonal and regional interactions in nanophytoplankton and microphytoplankton suggests that bivalve farming may also have some effect on the size-fractionated phytoplankton. |
