Spatiotemporal Analysis of Harmful Algal Bloom (HAB) in the Strait of Hormuz and Identifying Abiotic Driving Forces

In the past two decades, frequent Harmful Algae Bloom (HAB) events have occurred in the southern coast of Iran, particularly in the Strait of Hormuz. Because of the resultant environmental consequences and economic losses, it is of high importance to identify the key abiotic forces of HAB and their critical ranges to mitigate HABs. Studies of algal blooms in the Strait of Hormuz over the past two decades show that Noctiluca has been the dominant algae. Regional studies show that nutrient concentrations are generally higher in the region in autumn than in winter and in winter than spring. Moreover, in early autumn upwelling in the east of the Strait of Hormuz and along the coasts of Iran causes the enrichment of coastal waters with nutrients, especially nitrate. On the other hand, the density gradient reduction between the Persian Gulf and Sea of Oman during the cold months, along with the loss of summer stratification due to air cooling in the region, leads to the mixing of water and nutrients in depth.This study aims at 1) identifying the appropriate remote sensing-based algorithms and numerical models for monitoring HABs, 2) recognizing the key abiotic factors in the formation and expansion of HABs, and 3) understanding the spatiotemporal variation of long-lasting HABs and their effective abiotic factors from 2003 to 2019. The findings of this study can provide useful insights into the design of an early alarm system of HAB to prevent its negative environmental and economic consequences.

In this study, variations of abiotic variables affecting the formation of algal blooms in the Strait of Hormuz including sea surface temperature and salinity, aerosol optical thickness (AOT), and wind speed were analyzed during the three HABs between 2003 and 2019, having a duration of longer than two months. Concentration of Chlorophyll a has been considered as the main indicator of algal bloom. To detect algal blooms in many studies, different thresholds of chlorophyll concentrations from 2.5 to 25 mg/m3, for different levels of warning, have been mentioned in the Literature. Based on the field data and values obtained from satellite imageries at the time of algal bloom, a threshold of 5 mg/m3 for chlorophyll concentration was selected to determine the algal bloom event in the Strait of Hormuz.Then, the performance of several numerical and remote sensing-based models for monitoring algal bloom and the aforementioned abiotic factors were assessed using Field data acquired from oceanographic, fisheries, geological and meteorological organizations. Several evaluation criteria including Wilmot agreement index (d), mean bias error (MBE), root mean square error (RMSE), and correlation coefficient (R) were used for validation of the models.After selecting the best models for each variables, their monthly maps between 2003 and 2019 were extracted. Then, the lumped (mean values in the study area) and spatial (between maps) correlation analysis between chlorophyll-a concentration and the other abiotic variables were performed using Pearson correlation method at 95%. In addition, correlation analysis was performed without and with considering a one-month delay. Subsequently, the anomalies of these variables during the three algal bloom events were examined compared to their long-term mean monthly values. Likewise, spatial changes in the chlorophyll concentration were obtained from the selected model and the areas affected by algal bloom in the Strait of Hormuz were identified.

Based on the results of the performance indices, OC3M, OCSST, HYCOM and MERRA2 models were selected as the optimal models for monitoring chlorophyll, sea surface temperature, salinity, and wind speed, respectively.In the first period, September 2008, the algal bloom was formed in the southwestern regions of the Oman Sea and through the east-west currents moved toward the northern shores of the Strait of Hormuz. By January 2009, it expanded and reached its peak in the interior part of the Persian Gulf. The second event occurred in January 2012, just lasted about two months. The northern regions of the Strait, especially Qeshm Island as well as the coasts of Oman and the UAE were the most affected areas by algal blooms. In the third algae bloom episode, the upsurge in algal concentration has started from the coastal areas of Oman and the UAE and quickly reached the central and western parts of the Strait of Hormuz in January 2015 and then reached its peak in the following month, especially in the southern parts of the Strait of Hormuz and UAE. Regarding the long-term pattern of changes in chlorophyll concentration in the area (figure 1), it can be said that from 2006 to 2015, almost every three years, an algal bloom event lasting more than a month can be observed in the central areas of the strait. Also, in most of the years during the study period (except for the number of years 2001, 2002 and 2016), the mean chlorophyll concentration approached the threshold of algal bloom from November to January. Moreover, the eastern areas are more at risk of increasing chlorophyll concentration than the western. This could be due to lower temperatures, lower salinity and more available nutrient in the Oman Seas compared to the Persian Gulf. The temporal variation of sea temperature and salinity in the west and east part of the Strait of Hurmuz showed a slight increase in the peak values in the western regions compared to the eastern and central areas. AOT is almost the same in all the three regions, while wind speed follows an irregular trend in three areas. These changes are more evident in the western regions.Due to the variations in the monthly mean of chlorophyll throughout the years 2003-2019 in the region, the lowest chlorophyll occurs in summer and increases with the beginning of autumn and lastly reaches its maximum in winter and then decreases until spring. This circulation pattern is common among subtropical waters. According to correlation analysis, while chlorophyll level has a significant direct correlation with salinity, it inversely correlates with sea surface temperature. In general, with falling temperature in autumn and winter, chlorophyll concentration rises in the Strait of Hormuz. Wind speed, which intensifies dust and the movement of sea currents, has a weak inverse relationship with chlorophyll. This means that in summer, when the highest degree of dust occurs in the region, the chlorophyll is negligible, and as the weather cools, the amount of dust in the region decreases. This seasonal pattern is known for aerosols for desert areas, including the Middle East.

The key results of this research are:1- The optimum models for monitoring chlorophyll, sea surface temperature, salinity and wind speed in the Strait of Hormuz are OC3M, OCSST, HYCOM, and MERRA2 models, respectively.2- Algae blooms are more likely to occur in the Strait of Hormuz between November and February.3- The central and northern regions of the Strait of Hormuz are more prone to algal blooms than other regions due to more desired abiotic conditions for algae growth. Similarly, algal blooms are more likely to occur in the eastern than the western part of the Strait of Hormuz.4- According to the study of the three algae bloom episodes having a duration of more than two months, the desired temperature and salinity ranges for algae growth are 22-27 ° C and 37.5-39 psu, respectively. Concurrent occurrence of these abiotic conditions highly increases the risk of algal blooms in the Strait of Hormuz.The increased nutrient discharge from urban and industrial effluents together with a rise in water temperature and salinity due to the development of desalination plants will increase the risk of algal blooms in the southern coasts of Iran. Using the promising potential of satellite data (with an appropriate resolution) and valid algorithms, as shown in this study, can pave the way for developing develop regional early warning systems of algae bloom.