Water transparency and health of coastal salt marshes: simple enclosure experiments on nutrient dynamics

1. In many coastal salt marshes. water transparency is decreased by the resuspension of sediments. We analyse the influence of suspended not-photosynthetic matter on the regulation of phosphorus (P) dynamics in the water column., compared with the role of phytoplankton and sediments. 2. Field surveys and manipulative experiments of P enrichment in light and dark enclosures, open and closed at their bottom, were performed in the brackish Lake Alimini Grande (Otranto, LE, Italy). 3. Water transparency in Alimini Grande was inversely related to seston organic mass. 98.8% of which was composed of dead organic matter and heterotrophic organisms. 4. P enrichment experiments showed an increase of the phytoplankton biomass with time from the beginning of the manipulation (4.9 times) in the light enclosures. but also an exponential decrease of P concentration in all the enclosures, even in the dark one closed at the bottom. 5. The rate of P removal from the water column was not significantly affected by phytoplankton activity, even though P concentration was always lower in light than in dark enclosures, or by the activity of sediments. 6. Dead suspended organic matter with associated micro-organisms seemed to remove nutrients very efficiently from the water column in Lake Alimini Grande, contributing significantly to control the potential risk of lake eutrophication. Copyright (C) 2001 John Wiley & Sons, Ltd.