Macrobenthos diversity and its relationship with sediment characteristics in Garanduwa Lagoon, Sri Lanka

Abstract

Macrobenthos play a vital role in benthic communities of aquatic ecosystems, inhabiting sediments beneath the water column. They are widely regarded as reliable indicators of environmental conditions. This study provides the first comprehensive assessment of macrobenthic diversity in the Garanduwa Lagoon and its relationship with sediment characteristics. The study location, Garanduwa Lagoon (5◦56’14.31”N, 80◦29’01.33”E), is located in the Southern Province of Sri Lanka, an area experiencing increasing anthropogenic pressures from surrounding settlements. Sediment samples were collected using an Ekman Grab from twelve randomly selected sites within the lagoon in December 2022. In the laboratory, sediment physicochemical parameters, pH, salinity, conductivity, organic matter content, and particle size, were analysed. Wet sieving was used to extract macrobenthic organisms, which were identified to the lowest possible taxonomic level. Macrobenthic diversity was quantified using the Shannon-Wiener index, while species richness and density were calculated for each site. Correlation analysis was conducted to assess the influence of sediment characteristics on macrobenthic diversity. A total of 759 individuals representing 19 macrobenthic families were recorded. Shannon-Wiener diversity values ranged from 0.25 to 1.46 across sites. Species richness varied between 1 and 5, with the family Littorinidae being the most abundant. The lowest abundances were recorded in the families Tellinidae, Mytilidae, and Potamididae. Sediment pH ranged from 3.2 to 7.6. Low salt water intrusion lowered the salinity (9.5±0.237 ppt) and conductivity values (19.8±0.318 mS) of the lagoon sediments, while the highest organic matter content (50.80%) was recorded near the mangrove vegetation due to plant detritus derived from surrounding vegetation. Statistical analysis revealed that sediment pH, salinity, conductivity, and organic matter content significantly influenced macrobenthic diversity (p<0.05; r between± 0.50 and ±1). Principal Component Analysis highlighted that salinity, conductivity, and organic matter content were the only sediment parameters positively correlated with a few grain size classes: very coarse sand, coarse sand, and, silt and clay. Notably, sediment pH was positively correlated with diversity, while the other parameters showed negative correlations. These findings highlight the sensitivity of lagoonal microbenthic communities to sediment conditions and their potential as indicators of environmental change.