A dataset on spatiotemporal variation of phytoplankton communities in the mangrove wetland of Shenzhen Bay

Shenzhen Bay, alternatively known as Deep Bay, is a typical subtropical coastal area that has been notably affected by human activities, such as nutrient enrichment and pollution^1,2,3. Positioned in a semi-enclosed and shallow setting, the bay is subject to sewage discharge from the burgeoning metropolis of Shenzhen, China. The bay’s entrance, found on the southwest shore, is connected to the Pearl River estuary, which brings in a significant amount of freshwater⁴. The Futian Mangrove Ecological Park (FMEP), situated within the mangrove zones of Shenzhen Bay, is instrumental in supplying organic matter and offering a habitat for aquatic organisms and bird species^5,6,7. Nevertheless, in recent decades, eutrophication has become a prominent threat to marine ecosystems^8,9, with the coastal waters bordering mangrove communities experiencing considerable anthropogenic pollutant influx, thereby endangering the health of mangrove ecosystems^10,11,12.

Phytoplankton is a vital component of mangrove ecosystems, with its population dynamics and community structure serving as the initial biological indicators of nutrient enrichment and environmental stressors^13,14. For instance, in the past nearly 30 years, due to the impact of eutrophication, Shenzhen Bay has experienced over 30 harmful algal blooms (HABs). In addition, the phytoplankton communities near the mangrove zone in Shenzhen Bay have shown long-term variations from 1994 to 2016^15,16. The bay has witnessed a recurring shift in dominant species, transitioning from dinoflagellates (Dinophyta) and cryptophytes (Cryptophyta) to diatoms (Bacillariophyta), followed by a period dominated by small dinoflagellates. Between 1994 and 2006, small flagellates and Chroomonas spp. were the prevalent species in the marine waters. In 2007 and 2009, phytoplankton blooms, including Skeletonema costatum and Thalassiosira spp., were the primary causative species. In 2010, the community was again primarily composed of small flagellates and Chroomonas spp., a trend that continued until 2015. Strikingly, in 2016, there was a significant decline in Chroomonas spp., while Thalassiosira spp. emerged as the dominant species. These changes in phytoplankton communities and dominant species are likely due to environmental factors and the joint pollution reduction efforts of the Hong Kong and Shenzhen governments¹⁶. Numerous studies have shown that nutrient enrichment significantly affects phytoplankton at the species and genus levels^17,18,19. However, no systematic surveys have been conducted to document the spatiotemporal variations in phytoplankton biodiversity and community structure within the FMEP mangrove ecosystem in recent years.

To enhance our comprehension and strengthen conservation actions for Shenzhen Bay’s mangrove ecosystem, it is crucial to analyse the temporal and spatial dynamics of the phytoplankton community. In the coastal waters adjacent to mangrove regions, harmful algal blooms have intermittently occurred. Phytoplankton growth can be hindered by light limitations due to water mixing and suspended sediments, as well as by ammonium toxicity and predation by zooplankton^4,16. These coastal zones are consistently subject to intense land-sea interactions. Economic development has led to elevated nutrient levels in most Chinese estuaries and coastal waters, which frequently result in bloom formations^20,21. Such blooms not only severely impact the mangrove ecosystem, including phytoplankton communities, but also diminish the environmental quality of the FMEP. Consequently, studying the variations in the phytoplankton community in these coastal waters is vital for the surveillance of potential algal blooms. At present, there is a dearth of research on phytoplankton in the coastal waters near FMEP, and the establishment of a comprehensive dataset in this area remains a gap that needs to be addressed.

In this research, we carried out seasonal sampling of phytoplankton and environmental factors from January 2019 to October 2021 in Shenzhen Bay, near the FMEP, located south of Futian District, Shenzhen, China (Fig. 1). Throughout the three-year study, we identified a total of 72 genera and 89 taxa of phytoplankton within the FMEP, spanning across 7 phyla: Bacillariophyta, Chlorophyta, Chrysophyta, Cryptophyta, Cyanophyta, Dinophyta, and Euglenophyta (Fig. 2, Table 1). Chlorophyta was the most diverse, comprising 28 genera and 34 taxa; Bacillariophyta followed with 19 genera and 28 taxa. Cyanobacteria constituted the third largest group, with 13 genera and 13 taxa. Other algal species were less abundant, summing up to 12 genera and 14 taxa. Spatially, the species number and cell density of phytoplankton exhibited consistent patterns across our sampling stations over the three-year period (Fig. 3A, B). However, the seasonal variations in species number and cell density of phytoplankton communities showed differences between years (Fig. 3C,D). We detected numerous potential bloom-forming species or harmful algal species, including Merismopedia and Pseudoanabaena in Cyanophyta; Navicula, Synedra, Cyclotella, Thalassiosira, Skeletonema, Pinnularia, and Nitzschia in Bacillariophyta; Peridiniopsis and Gymnodinium in Dinophyta; and Cryptomonas in Cryptophyta, among others (Fig. 4).

Study area and sampling stations (i.e., XH, DH, LHK, XFD, JCZ) in Futian Mangrove Ecological Park (FMEP), Shenzhen Bay from 2019 to 2021. The green part in the right figure represents the mangrove coverage area.

Sankey diagram visualizing the frequency of species in phytoplankton samples collected in FMEP, Shenzhen Bay from 2019 to 2021. The horizontal ordinate of the Sankey diagram at the bottom were taxonomic units at different taxonomy levels. The thickness of each branch presented the frequency of species in their relevant taxonomic units.

Spatiotemporal dynamics of phytoplankton communities in FMEP, Shenzhen Bay, 2019–2021. (A) Average species number across five sampling sites from 2019 to 2021. (B) Average cell density of total phytoplankton across five sampling sites from 2019 to 2021. (C) Average species number of every season from 2019 to 2021. (D) Average cell density of total phytoplankton every season from 2019 to 2021.

Heatmap analysis of phytoplankton samples from FMEP, Shenzhen Bay (2019–2021). (A) Top 20 dominant phytoplankton species based on average abundance, (B) Top 20 bloom-forming phytoplankton species based on average abundance. In the heatmap below, the x-axis represents the sample names, while the y-axis on the right indicates different phytoplankton groups. The colour scale reflects deviations from the mean value: negative for values below the mean and positive for values above. The heatmap values are represented by the colour scale in the top-right corner.

This comprehensive, multi-year monitoring dataset offers a wealth of valuable insights for studying the dynamics of phytoplankton community succession and the potential bloom-forming species or harmful algal species in the mangrove regions of Shenzhen Bay. It is an invaluable asset for assessing the impact of human activities, especially pollution, on the health and stability of these critical mangrove ecosystems.