Environmental characteristics
All the environmental factors studied differed significantly
among sampling occasions and estuarine zones, except for
sediment sorting. There was a clear salinity gradient from the
upper to the lower estuary on both sampling occasions. Bottom
sediments were composed of well-sorted fine sand or well-sorted
very fine sands.
Coarser sediments were observed in the upper estuary and
during the dry season. The amount of organic matter in the
sediments was significantly higher during the rainy season. In the
dry season, organic matter slowly decreased from the upper to the
lower estuary. In the rainy season, maximum amounts of organic
matter were observed in the middle estuary (Table 1 and Table 2).
Benthic fauna
Along the estuary and between seasons, there were pronounced
changes in species composition. In the dry season, in the upper
estuary Namalycastis terrestris, Nephtys fluviatilis, and
Tubificidae were dominant. Sigambra grubii, Mediomastus sp.,
and Peloscolex sp. dominated in the middle estuary; while in the
lower estuary Nephtys simoni, Boccardiella ligerica, and Bivalvia
were the most abundant taxa. In the rainy season, N. terrestris and
N. fluviatilis were again the most abundant taxa in the upper
estuary. In the middle estuary, Mediomastus, Ostracoda, and
Sigambra grubii were dominant. In the lower estuary, Armandia
sp. and Aricidea philbinae became the most abundant taxa.