Phytoplankton Composition during a Period of the Red Tide Bans in 2017 in Irong-Irong Bay, Western Samar, Philippines

Abstract

This study assessed the phytoplankton composition in Irong-Irong Bay, Western Samar, Philippines. Recurring red tide bans were imposed in this bay due to the elevated levels of saxitoxin that cause Paralytic Shellfish Poisoning primarily produced by Pyrodinium bahamense. Despite the presence of mariculture sites, Irong-Irong Bay has very little or nil data on its overall phytoplankton composition. Monthly field samplings were conducted to observe the phytoplankton community structure during an episode of persistent red tide bans from September–December 2017. Physicochemical parameters (e.g. temperature, salinity, transparency, pH, nutrient concentration) were measured. A total of 64 species of phytoplankton were observed and counted. The lowest mean cell density of phytoplankton was recorded in the month of November (3.1 x 104 cells/L), whereas December (37 x 104 cells/L) had the highest mean cell density but lowest values for species diversity (H’ = 0.942), richness (Dmn = 0.037), and evenness (J = 0.307). Pseudo-nitzschia dominated in the months of September (3.1 x 104 cells/L) and November (0.43 x 104 cells/L), whereas Skeletonema dominated in the months of October (9.9 x 104 cells/L) and December (29 x 104 cells/L). Pyrodinium bahamense was present in all sampling months with a total mean cell density of 0.08 x 104 cells/L but was never the dominant species. The low cell count relative to other Pyrodinium blooms may be accounted for by the presence of Noctiluca scintillans, its potential predator. Noctiluca scintillans was present in all sampling months. It peaked during the month of November with a mean cell density of 0.32 x 104 cells/L. A significant negative correlation was detected between the cell densities of these two dinoflagellates, providing evidence of the potential interactive relationship between these two species.