NUTRIENT FLUXES AND GROWTH OF CLADOPHORA-SERICEA IN A SHALLOW DANISH BAY

Cladophora spp. form stable floating algal mats in the archipelago of southern Fyn, Denmark (2 m water) in the spring and summer months. There was a significant diel variation in the concentration of NH4+, NO3-, PO43-, O2 and in pH and temperature in the mats. These changes were observed in either a closed chamber positioned in the sediment or by in situ manifold sampling. Changes, particularly in NH4+, were used to estimate net mat nitrogen uptake rates. Much recycling of nitrogen occurred in the mat and there was evidence of NH4+ concentration peaks, even during the day. Phosphate had a similar pattern. The highest NH4+ uptake rates were in the bottom of the mat, probably due to nutrient supply from the sediment. A large release of NH4+ at the onset of dark, followed by low uptake rates in the dark, was evidence for the release of loosely bound, rather than intracellular NH4+. Diel and monthly uptake rates (DELTANH4+ h-1) showed that growth may have been light-limited in the spring and N-limited in the summer. High algal C:N ratios in the summer were consistent with this hypothesis. A budget for June showed the importance of recycling of N within the mat (2.57 mmol m-2 d-1) compared with NH4+ input from the sediment (0.83 mmol M-2 d-1), lateral water transport (0.31 mmol m-2 d-1) and the deposition from the atmosphere (0.40 mmol m-2 d-1). C. sericea out-competes phytoplankton, resulting in clear water. Growth of C. sericea resulted in O2 deficit under the mat due to respiration of the mat and sediment. Sinking and export of the mat at the end of July could cause local problems.