Dynamics of absorption properties of CDOM and its composition in Likas estuary, North Borneo, Malaysia

Abstract

Chromophoric Dissolved Organic Matter (CDOM) is a vital water constituent in aquatic ecosystems that contributes to water colour, affects light penetration, and impacts primary production. This study aims to determine the spatial and monsoonal variability of CDOM absorption properties in the Likas estuary, characterise the source of CDOM, and investigate the correlations between CDOM absorption properties and salinity. Likas estuary is a small estuary located in Kota Kinabalu city on the west coast of Sabah, facing the South China Sea. A mangrove ecosystem surrounds it with manufactured structures such as residential areas and public facilities. Surface water samples were collected at 19 stations: upstream of rivers to the river mouth and coastal area during spring tides every month, from June 2018 to July 2019, for 14-months. The distribution of a_CDOM(440) in the study area is predictable as a signature in a coastal area with a decreasing gradient from the upstream towards coastal water (0.29 ± 0.19 m⁻¹ to 1.05 ± 0.39 m⁻¹). There are increasing spatial patterns of spectral slopes S_275-295 and S_R. However, S_350-400 and S_300-600 declined spatial gradients from the upstream to coastal water. Thus, S_300-600 indicates a linear relationship between a_CDOM(440), which unconventional results in coastal water. We suspect this is due to a small coverage of the study site with a distance of 0.5 m intervals of each station. This could be why the S_300-600 had constant values throughout the study area (with no statistical difference between stations). In addition, S_300-600 was merely varied in the stations located at the river mouth and coastal water. Based on the spectral slope ratio (S_R), most of the stations located in the Darau, Inanam, and Bangka-Bangka rivers had S_R values less than 1. Hence, CDOM in these stations is a terrestrial-dominated source. Therefore, from our observations during the study period, monsoonal variation could alter the source of CDOM in the study area.