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Abstract

Underwater habitats along the Karachi coast are dominated by monospecific strands of Sargassum. Macroalgal growth in these habitats mostly starts at the end of November during the post-monsoon period due to calm water conditions. However, stunted growth of Sargassum was observed after the NILOFAR cyclone. The current study was undertaken to highlight the impacts of climate change on Sargassum forests and overall community structure. Underwater observations were made from January to March from 2022 to 2024. NOAA online monthly mean sea surface temperature data were used for October and November 2014 to 2024. To detect any changes in community structure, the data was compared with previous studies. Stunted growth of Sargassum followed by mixed community structure was observed at 4 dive sites (HB 1, HB 2, Bu 1, and Bu 2). However, no Sargassum species was recorded at FB. The monthly mean sea surface temperature data for November remained close to 26.5°C. Comparing data with previous studies, it appeared that many species were not found in the current study. The possible causes of stunted growth might be due to rising sea surface temperatures that increase the frequency of cyclone formation and altered local wind patterns. Regardless of other factors, such as humadaty, low-pressure, a sea surface temperature above 26.5°C is necessary for cyclone formation. Sea surface temperature and wind speed are positively correlated with each other. As a result of cyclones and particularly an increase in wind speed, the propagules could not settle at the right time due to strong wave action. Stunted growth also reduced canopy cover. Reduction in canopy cover increased the growth of the perennial species due to the availability of sunlight. Sargassum has many benefits; however, its major roles are absorption of carbon dioxide and utilization of nutrients. As a considerable reduction in Sargassum biomass has occurred, it is possible that the concentration of carbon dioxide and phytoplankton growth will increase in the future, resulting an increase in warming and eutrophication. Sindh coast is highly vulnerable to climate change mainly due to shallow depth and weak upwelling (upwelling mostly occurs below 50 meters). Global warming also interacts with local stressors (urbanization, urban heat islands) and exerts negative impacts on marine communities. Karachi is the largest city of Pakistan. The severity of the urban heat island phenomenon in Karachi is progressively escalating. Future conservation efforts should prioritize close collaboration with government agencies and academia, while simultaneously enhancing climate change awareness and capacity building in the field of marine biology. Additionally, minimizing local warming effects require constructing eco-friendly residential sectors, controlling vehicular volatile organic compounds emissions, and properly managing future infrastructure development

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