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.
