Conducting in situ observations to measure microplastic concentrations in the abiotic compartment is essential for enhancing our understanding of plastic pollution along the land-sea continuum. While numerous protocols have been devised to investigate microplastic (MP) pollution in oceans and rivers, mainly in surface waters and sediments, many may not be suitable for assessing MP contamination in coastal transition waters such as estuaries, where MPs can exhibit high mobility within the water column. Assessing MPs in tidal systems requires several samplings throughout the tidal cycle, which is challenging due to short sampling periods and the presence of suspended sediments. Sampling MPs in these environments is therefore challenging using nets (risk of clogging), bottles (limited water quantity), or pumps (potential sample contamination). To address this challenge, a new instrument, the Ascension profiler, has been introduced for in-situ water pumping and filtering, avoiding sample contamination. This device offers the flexibility of using filters with different mesh sizes and enables the filtration of large water volumes to ensure representative sampling even at high water depths. In our study, we propose an innovative in-situ sampling strategy protocol that accounts for the typical time scales of estuarine variability as well as the longitudinal salinity gradient using Ascension. We present all the preliminary tests made with the instrument to set up a protocol (water volume, pumping duration, filter size) adapted to the conditions of the macrotidal hyperturbid Gironde estuary. Furthermore, this new sampling method and protocol hold promise for application in other oceanic, coastal, and freshwater environments.< Réduire