Seasonality of pluviosity and saline intrusion drive meiofauna and nematodes on an Amazon freshwater-oligohaline beach

Abstract Salinity in estuarine environments is one of the most important influencing factors for meiofauna. The effects of seasonal variation in rainfall and saline intrusion on meiofauna/nematodes diversity and abundance on a mesotidal beach located in the mixing zone of an Amazon estuary were analyzed. Samplings occurred bimonthly over one year at four stations in the intertidal zone. Density variation in meiofauna community had a direct relationship with increased rainfall and decreased salinity. Oligochaeta had the highest density during the months with the highest rainfall and lowest salinity. The highest total Nematoda richness value, as well as richness of brackish water genera value, was registered in September and gradually decreased until March, a month with the highest rainfall. Enoplolaimus, a typical predator, had the highest densities in the months with the highest rainfall, probably due to the food availability (Oligochaeta). The highest Nematoda density at the station closest to the high-tide mark occurred due to detritus accumulation. On the other hand, the lower densities at the middle stations are due to the higher desiccation that occurs in these regions compared to the station closer to the low-tide mark. Meiofauna was strongly affected by seasonal changes in rain and salinity, both at the level of large groups and at the level of Nematoda genera. In addition, seasonal changes in salinity clearly affected organisms indirectly through their prey-predator interactions.