| Phytoplankton, serving as crucial primary producers within marine ecosystems, exhibit rapid responsiveness to ecological shifts in aquatic environments, thereby playing a pivotal role in maintaining the health and stability of these systems. China, a leading aquaculture nation, boasts approximately 70 percent of global aquaculture output, with shellfish constituting a significant portion, accounting for 72 percent of total production. These phytoplankton serve as the primary food source for shellfish, which regulate their biomass through filter feeding mechanisms. Furthermore, the excretions from shellfish modify nutrient concentrations in the water, indirectly influencing the composition of phytoplankton communities and, consequently, impacting water quality and overall ecosystem health. By examining the intricate relationship between shellfish and phytoplankton and exploring the ramifications of shellfish farming activities on phytoplankton populations, we can anticipate and address the potential effects of marine environmental changes on aquaculture. This endeavor holds immense significance for assessing ecological carrying capacity and planning shellfish farming activities, thereby ensuring a harmonious balance between marine economic development and ecological preservation. Haizhou Bay, located between the southern part of the Shandong Peninsula and the northern part of Jiangsu Province, has shellfish and nori as its main farming species, with shellfish being the dominant species. There are few studies on the relationship between shellfish culture and phytoplankton communities in Haizhou Bay.
To explore the influence of mixed cultivation of oyster and mussel，as well as marine environmental factors on phytoplankton community structure, surveys of phytoplankton and environmental factors in Haizhou Bay were investigated in March, July, September, October and December 2023, respectively. The survey was divided into four areas: Area 1 and Area 2 as aquaculture areas, and the waterway area and offshore area as non- aquaculture areas. The shallow-water type III plankton network was used to vertically dragged from the seabed to the sea surface to collect phytoplankton biological samples. Environmental factors of both sea surface and bottom were investigated and the average value was calculated for data analysis. Two-way analysis of variance (Two-way ANONVA) was performed on environmental factors and phytoplankton communities for seasonal and regional changes. Canonical discriminant analysis (CDA) was used to analyze the similarity of phytoplankton community structure in different areas, and redundancy analysis (RDA) was conducted to study the relationship between dominant phytoplankton species and environmental factors.
This study found significant seasonal differences in temperature, salinity, pH, dissolved oxygen (DO), chemical oxygen demand (COD) and nutrient concentration in the survey area (P<0.001). The water temperature and salinity were affected by terrestrial inputs. During during July, September and December, the concentration of nutrient salts in Area 1 and Area 2 was higher than that in other regions. A total of 69 species of phytoplankton in 33 genera and 3 phyla were identified in the survey area, with bacillariophyta being the dominant group, accounting for 87% of the species. There were 14 dominant species appeared in the survey, including Chaetoceros lorenzianus, Chartoceros sp., Coscinodiscus grannii, Skeletonema costatum, etc, most of which belonged to bacillariophyta, with significant seasonal and regional variations. There were significant differences in phytoplankton abundance between seasons and regions (P<0.05), ranging from (1.40~739.11)×104cell/m3. After the red tide in September, the abundance of phytoplankton decreased compared to July and reached the highest value in October. Affected significantly by terrestrial inputs, the abundance in Area 1 was higher than that in other areas in all seasons, and the surveyed area generally showed a trend of higher abundance of nearshore phytoplankton compared to offshore areas. There were significant regional and seasonal differences in species diversity index (P<0.05). The survey conducted in September, following the occurrence of a red tide, found that the phytoplankton diversity index was higher in the aquaculture area than in the non-aquaculture area. Although some oysters died, the proportion of remaining shellfish in farming is still significant, and it is speculated that shellfish activities can increase the stability of the phytoplankton community to a certain extent. The result of CDA showed that the similarity of phytoplankton community structure between the aquaculture area（Area 1、Area 2）and the offshore area is low, and shellfish activities can influence the composition of the phytoplankton community structure. The waterway area, due to its proximity to the bay and slower water exchange, has a high similarity in phytoplankton community structure with Area 2. The results of RDA analysis showed that the abundance of dominant species of phytoplankton were affected by environmental factors such as temperature, pH, NO3-N and NO2-N, and the abundance of dominant species was positively correlated with nutrient concentration in July, September and October. In Area 1, environmental factors such as water temperature and salinity and nutrient concentration were greatly affected by terrestrial inputs, and the changes of phytoplankton community in this area may be affected by both geographical location and shellfish farming activities. This study preliminarily explored the relationship between phytoplankton and environmental factors in shellfish culture area, and can provide data for shellfish culture planning and aquaculture capacity assessment in Haizhou Bay. |
