Coastal Blue Holes in a Large and Shallow Tropical Estuary: Geomorphometry and Temporal Variability of the Physicochemical Properties

Blue holes are part of complex water systems formed by tropical wetlands, karst aquifers, and different coastal environments. In the southeastern Yucatán Peninsula, three submerged blue holes (i.e., Lool ja’ , Ch’och-ja’ , and Taam-ja’ ) are located in the shallow tropical estuary of Chetumal Bay. This study examines the morphological features of the three blue holes and explores the seasonal variability of their physicochemical parameters through quarterly field measurements taken between 2021 and 2022. Additionally, this study represents the first documentation of the Lool-ja’ blue hole, which has the largest surface area and volume of the karst structures analyzed. Temperature and salinity profiles along the water column were measured together with surface pH and DO at the blue hole center and within its vicinity (~ 500 m apart). Time series of water temperature, conductivity, and water level were also measured between June and December 2021. Morphological features of the blue holes were explored through georeferenced echo-sounding and 3D-modelling. Seasonal variations were observed on the mixed water layer depth (MLD). During the rainy (dry) season, the surficial mixed water mass became deeper (shallower). Variability in the MLD was proportional to the area occupied by the mouth of the blue hole ( r = 0.813). Further homogenized layers accompanied by density interfaces were observed at different depths inside the blue holes. Finally, temperature and salinity diagrams revealed the possible interseasonal exchange of water in the Lool-ja’ and Ch’och-ja’ blue holes with possible intrusion from marine sources.