Nutrient conditions and the occurrence of a Karenia mikimotoi (Kareniaceae) bloom within East Johor Straits, Singapore

Abstract

Harmful algal bloom occurrences have been an increasing problem in Southeast Asia, including Singapore. Triggers and consequences associated with such events are difficult to understand because of highly variable environmental conditions. This study provides a description of bloom patterns within the East Johor Straits (EJS) environment between Peninsular Malaysia to the north and Singapore to the south. There were two sampling periods during 1) Oct 2015–Feb 2016; and 2) Oct–Nov 2017. Sampling on Feb 2016 was increased to a five-day sampling following a bloom of the dinoflagellate Karenia mikimotoi (with high densities of >8,000 cells mL−1). The study examined taxa abundances, nutrient content, as well as salinity and chlorophyll-a profiles. During both sampling periods, elevated raphidophyte densities preceded reduced-diatom conditions which, in turn, coincided with more abundant dinoflagellate densities. Nutrient measurements indicated that the environment was often eutrophic, with high inputs particularly of nitrogen (N) sources. The inorganic-nitrogen (N) and phosphate (P) content was also close to the 16N:1P Redfield’s ratio, suggesting that the EJS could readily switch between N- or P-limitation. The EJS was frequently also silicate (Si)-limited, which could limit diatom productivity in the future. Moreover, salinity was markedly reduced on Jan 2016 (around 20–25 PSU), but was higher (around 27 PSU) at the other sampling times. These patterns present the case that EJS blooms may involve an “open-niche” condition requiring diatom densities to be decreased. This outcome may be driven by environmental or biotic factors, but further monitoring is needed to establish these conclusions.