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 res uspension of sediments. We analyse the influence of suspended not-photosynt hetic matter on the regulation of phosphorus (P) dynamics in the water colu mn., 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 bra ckish Lake Alimini Grande (Otranto, LE, Italy). 3. Water transparency in Alimini Grande was inversely related to seston org anic mass. 98.8% of which was composed of dead organic matter and heterotro phic 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 affect ed 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 Gra nde, contributing significantly to control the potential risk of lake eutro phication. Copyright (C) 2001 John Wiley & Sons, Ltd.