Response of phytoplankton assemblages to variations in environmental parameters in a subtropical bay (Chabahar Bay, Iran): Harmful algal blooms and coastal hypoxia

Abstract

The present work is a comprehensive study on the spatiotemporal variation of environmental parameters in relation to phytoplankton assemblages over a one-year period (March 2017–March 2018) at a semi-enclosed subtropical bay and adjacent coastal waters in the north of Gulf of Oman. The physical and biological parameters were mostly controlled by the southwest monsoon which usually lasts 4 months (June–September) in the region. The outstanding phenomena which highly affected the physicochemical parameters and biological activity were thermal stratification (established in the May–October period), and rising concentration of nutrients due to monsoon-driven upwelling in the late monsoon and post-monsoon periods. As a consequence, the hypoxic condition was established and the pH was also decreased to values smaller than 8.0. It seems that the nitrate concentration limits phytoplankton growth during most times of the year. However, the results showed that the phytoplankton population in the fall inter-monsoon period may also be controlled by silicate availability as its concentration declined to values as small as 0.8 μM in January. Chlorophyll a concentration showed a strong peak in January and a weaker peak in October. The former is due to the intense blooms of Noctiluca scintillans and the later was coincided with high nutrient concentrations and high phytoplankton density and diversity. Diatoms (93%) were dominant during the course of sampling and dinoflagellates (4.5%) were second in dominance. Exceptions were observed in March 2017 when the outbreak of dinoflagellates led to extensive water discoloration and in April 2017 during the massive bloom of Mesodinium rubrum. The most abundant genera were diatoms Pseudo-nitzschia (34.8%), Nitzschia (23.9%), Hemiaulus (20%), and Leptocylindrus (6.1%). While dinoflagellates blooms mostly occurred during high nutrients concentrations in late SW monsoon and inter-monsoon seasons; the bloom of potentially harmful diatoms occurred during the SW monsoon season.