Simulation of encounter rates between zooplankton organisms and microplastics in a tropical estuary

Coastal estuarine systems may hold a large number of microplastic particles, which predators often mistake for prey. This study estimated the encounter rates between microplastics (alleged prey) and zooplankton having different feeding modes, trophic positions, swimming velocities, and perception distances, under calm and turbulent conditions, and during two seasons. Surface water samples were taken at 10/12 sites of the Sontecomapan lagoon, southern Gulf of Mexico, to quantify microplastic concentration. Zooplankton organisms considered were copepods, chaetognaths, and luciferids, common organisms in the lagoon. In June, at surface waters and during calm conditions, mean encounter rates were 1.5, 2450, and 980 particles per individual per hour, that is, for copepods, chaetognaths, and luciferids, respectively. When the wind blows (0.8 m s-1) encounter rates were 1.2, 1.4, and 2.6 times higher than in calm conditions. In October, mean encounter rates under calm conditions were 0.2, 355, and 142 particles per individual per hour, for copepods, chaetognaths, and luciferids; these values increase 1.3, 1.6, and 3.3 times when the wind blows (1.12 m s-1). The major number of encounters in June was due to a higher concentration of microplastics, despite the lower turbulent velocity. Regarding their trophic position, we propose that secondary consumers (chaetognaths and luciferids) are more affected because they could eat microplastics via contaminated prey or accidentally ingest them owing to confusion in the motion signals, especially under turbulent conditions. Another consequence of encounters could be the entanglement of microplastics in the body of the animals, especially in those with complex morphology, such as crustaceans. Encounters between zooplankton and microplastics do not always result in ingestion or entanglement, but the encounters are the first step in the case of occurrence.