Effects of seasonal variations and environmental factors on phytoplankton community structure and abundance in Beibu Gulf, China

The seasonal variation in phyla within the phytoplankton community is evident in the Beibu Gulf region, with Bacillariophyta being the most diverse group, accounting for 55% of genera in spring and slightly increasing to 57.9% in autumn. In contrast, Pyrrophyta exhibits a higher number of genera and taxa in spring at 43.3%, but this percentage declines to 40% in autumn. During both seasons, the phylum Cyanophyta had a limited occurrence, making up only 1.7% and 2.1% of the total species counts, but in the spring season, it emerged as the dominant phylum with a high cell density of 95.4%, surpassing other phyla such as Aphanocapsa elachista, Guinardia flaccida, Chaetoceros laciniosus, Trichodesmium thiebautii, and Thalassiosira subtilis. Conversely, in the autumn season, cyanophyta constituted merely 4% of the overall cell density. The relative abundance of Bacillariophyta species (Bacteriastrum sp. > Chaetoceros affinis > Chaetoceros lorenzianus > Thalassionema nitzschioides > Chaetoceros curvisetus) displayed a distinct trend, reaching its peak cell density of 94% during autumn. The significance of this study lies in its findings that different types of phytoplankton, such as dinoflagellates and diatoms, have distinct growth patterns in the northern Beibu Gulf. It also highlights the importance of nutrient levels, particularly nitrate, nitrite, silicate, and soluble reactive phosphorus, in influencing the composition of phytoplankton communities during the spring season. Additionally, the study reveals a negative relationship between ammonium concentration and water temperature during both spring and autumn. These findings suggest that changes in nutrient availability during seasonal shifts play a critical role in shaping phytoplankton dynamics in marine ecosystems. The observed negative relationship between ammonium concentration and water temperature raises questions about the potential impacts of climate change on phytoplankton dynamics in the Beibu Gulf. As temperatures continue to rise, it is possible that the nutrient-rich conditions in the spring season may be altered, leading to changes in the composition and abundance of phytoplankton populations. This could potentially have cascading effects on the overall ecosystem. Therefore, studying the interplay between temperature and nutrient availability is essential for accurately assessing the future health and productivity of the Beibu Gulf's phytoplankton communities.