Effects of the summer monsoon on the polychaete assemblages and benthic environment of three mangrove swamps along the Sindh coast, Pakistan

Abstract

Polychaetes are an important component of marine ecosystems. Monsoons are a common climate condition in the tropics and subtropics and are characterized by high temperatures, low annual temperature changes and high precipitation. Despite the potential importance of monsoons, little is known about their impact on the marine environment and fauna, including polychaete assemblages. We compared the polychaete species composition and abundance at Sandspit, Port Qasim, and Ketibunder mangrove swamps of the Sindh coast, Pakistan, collected during the premonsoon, monsoon, and postmonsoon seasons in 2020. We also measured six environmental parameters to correlate environmental and biological changes. A total of 17 species of polychaetes were collected. Except for the mean evenness index (J), Two-way ANOVA detected significant changes in the mean polychaete species number, abundance, diversity index (H’), abundance index (d) and dominance index (D), with lower values in the monsoon season than in the premonsoon and postmonsoon seasons at the three mangrove swamps. Two-way ANOVA also revealed significant changes in the six environmental parameters between the seasons and sampling sites. Our data suggested that the heavy rainfall resulting in reduced salinity was the main driver of reduced polychaete abundance and diversity during the monsoon season. The polychaete assemblages and benthic environment appeared to recover quickly after the monsoon at three mangrove swamps, indicating their adaptation to such annual dramatic fluctuations in environmental conditions.