Spatial patterns of phytoplankton communities in an International Seabed Authority licensed area (COMRA, Clarion-Clipperton Zone) in relation to upper ocean biogeochemistry

The Clarion-Clipperton Zone (CCZ) hosts one of the largest known oceanic nodule fields worldwide and is regulated by the International Seabed Authority. A baseline assessment of diversity and distribution patterns is essential for reliable predictions of disturbed ecosystem response scenarios for sustained commercial activities in the future. In the present study, the spatial patterns and diversity of phytoplankton communities were analyzed along with upper ocean biogeochemistry, in the licensed China Ocean Mineral Resources R&D Association (COMRA) contract area and the surrounding western CCZ between August 21 and October 8, 2017. Results indicated this was a typical low-nutrient low-chlorophyll a (Chl a ) environment, characterized by low levels of phytoplankton abundance and diversity. In total 112 species belonging to 4 phyla were recorded (>10 µm), with species counts including 82 diatoms, 27 dinoflagellates, 1 cyanobacteria and 2 chrysophyte. Dominant taxa in successive order of descending abundance and occurrence included Nizschia marina, Cyclotella stylorum, Dactyliosolen mediterraneus, Rhizosolenia setigera, Pseudo-nitzschia delicatissima, Thalassiothrix frauenfeldii, Synedra sp., Chaetoceros simplex and Pseudo-nitzschia circumpora. The depth-averaged abundance and Chl a concentrations were (265±233) cells/L and (0.27±0.30) µg/L, respectively. Diatoms accounted for 90.94% of the community with (241±223) cells/L, while dinoflagellates accounted for 5.67% and (15±13) cells/L. The distribution pattern exhibited the same trend as abundance, Chl a and species richness, showing subsurface maximum levels at around 100 m, with stations near 10°N having higher levels than in the north. Cluster analysis was performed in two assemblages, relating to geographic locations to the south and north of 12°N. The subsurface maximum of abundance, Chl a, species richness, dissolved oxygen and nitrite were generally corresponding to the presence of high salinity North Pacific Central Water at depths of 50–120 m. Higher availability of nitrate, phosphate and silicic acid in the subsurface may account for the shift in phytoplankton distribution, as shown by redundancy correspondence and spearman correlation analysis. Diel variation in an anchor station demonstrated prominent species succession without significant differences in oceanographic variables, among which diatoms succession resulted from the light limitation, while dinoflagellate diel variation mainly related to lateral transport of water masses. The observed patchiness in spatial phytoplankton distributional patterns was attributed to upper ocean environmental gradients in the CCZ. The baseline generated in this study could be analyzed using current conservation strategy programs associated with deep-sea mining.