Year-round occurrence of crustacean holoplankton in Mae Klong River Mouth as indicators of natural food availability in the Gulf of Thailand

Climate change has been a cause of significant global environmental variation, with rising temperatures observed in many regions.Temperature is a critical factor affecting organisms, including crustacean holoplanktons, a key food sources in aquatic ecosystem, particularly in nursery grounds for aquatic animals.Estuarine areas, such as river mouths, are especially important as they experience considerably physiochemical fluctuations.This study was conducted year-round at two sites: Don Hoi Lod (DH) and Mae Klong River (MK) in Samut Songkhram Province, Thailand.Water samples (approximately 30 m 3 each) were collected at a 30 cm depth by 200 m sieve size of plankton net.The samples were preserved and identified in laboratory.The density of crustacean holoplankton was estimated and expressed by individuals per m 3 .Field measurements were recorded.The relationship between crustacean holoplankton and environmental factors were analyzed using canonical correspondence analysis.The results indicated that salinity and temperature exhibited similar pattern between the DH and the MK sites which were high in the dry season and low in the southwest and northwest monsoons.However, these physical factors varied significantly across different seasons.Two main crustacean taxa were identified: class Copepoda and order Cladocera (class Branchiopoda).The copepod groups included order Calnoida Cycoida, Harpecticoida and the nauplius stage.Species richness was higher at the DH site than the MK.Seasonal change had a strong influence on holoplankton between the two sites.Additionally, the overall population of crustacean holoplankton decreased significantly with rising temperatures.These findings suggested that seasonal changes and increasing temperature could impact the biodiversity of crustacean holoplankton in estuarine area.