Effects of seasonal fluctuation of amazon river discharge on the spatial and temporal changes of meiofauna and nematodes in the amazonian coast

Abstract

In the Amazonian coast seasonal changes of environmental characteristics are the main structuring factors of coastal benthic communities. Spatial and temporal variations in meiofauna and nematofauna were studied during rainy and dry seasons in the coastal zone of the Biological Reserve of Piratuba Lake (Bazil Amazonian coast). At each site (seven sites) five samples were taken in vegetated (inside the mangrove – 100 m from the border) and non-vegetated (outside of the mangrove – 100 m from the border) areas. Each sample was sliced in 0–2 cm, 2–5 cm and 5–10 cm deep layers. Water surface temperature and salinity were determined. Meiofauna was composed of 15 taxa: Nematoda, Copepoda, Ostracoda, Amphipoda, Cladocera, Isopoda, Acari, Polychaeta, Gastropoda, Turbellaria, Rotifera, Loricifera, Nemertea, Kinorhyncha, and Insect larvae. Nematoda (39 genera in 14 families) was the dominant group, accounting for 70% of the total individuals. Daptonema, Neochromadora, Hypodondolaimus, and Dichromadora were dominant. Only salinity significantly changed between seasons and was significantly higher in the dry season. Meiofauna and nematofauna density and richness were lower than those in other mangroves worldwide. The significant changes in meiofauna and nematofauna between seasons and depths were related to higher salinity and food availability during the dry season and more food and oxygen available close to the surface (0–2 cm depth). Meiofauna did not show significant differences between vegetated and non-vegetated areas; however, higher density and richness were generally recorded in the vegetated areas, where food is more abundant, and the substrate is more stable. Non-selective deposit-feeding nematodes were dominant in all areas, seasons and depths. During the rainy season, the second most abundant feeding group was predators/omnivores as a response to an increase in organic detritus, whereas epigrowth feeders were the second most abundant group during dry season due to increased phytoplankton and phytobenthic production.