Dissolved Humic Substances in a Coastal Lagoon of the Nile Delta: Distribution, Inputs and Behaviour

Dissolved humic substances (HS) were determined in the surface water of Lake Edku (Egypt), a coastal lagoon of the Nile delta. Their sources, distribution, seasonal variability and behaviour are assessed. Marked seasonal variations were observed, with a minimum, (average 1•93 mg l-1) during winter and a maximum during summer (average 4•33 mg l-1). The lake was geographically subdivided into eastern, central and western basins. The central basin showed the highest concentrations of HS in summer and autumn which is attributed to intensive development of macrophytes and to enhanced phytoplankton primary production. In the eastern basin, high turbidity and water turbulence downstream from the drains limit primary production, while in the western basin, water turbulence, sandy bottom, elevated salinity and decrease in nutrients due to mixing with seawater limit the growth of macrophytes as well as the phytoplankton productivity. The total humic substances budget in lake Edku water was calculated. Primary production is estimated to contribute about 5150•106 g year-1 while the drains contribute 4328•106 g year-1. About 65% of the whole dissolved humic substances (allochtonous and autochtonous) input are lost (trapped, transformed, degraded) in the lake before being delivered to the sea.