Biofiltering efficiency in removal of dissolved nutrients by three species of estuarine macroalgae cultivated with sea bass (Dicentrarchus labrax) waste waters 1.: Phosphate

The potential of three estuarine macroalgae (Ulva rotundata, Enteromorpa intestinalis and Gracilaria gracilis) as biofilters for phosphate in effluents of a sea bass (Dicentrarchus labrax) cultivation tank was studied. These seaweeds thrive in Cadiz Bay and were also selected because of their economic potential, so that environmental and economic advantages may be achieved by future integrated aquaculture practices in the local fish farms. The study was designed to investigate the functioning of P nutrition of the selected species. Maximum velocity of phosphate uptake (2.86 mumol PO4 g(-1) dry wt h(-1)) was found in U. rotundata. This species also showed the highest affinity for this nutrient. At low flow rates (< 2 volumes d(-1)), the three species efficiently filtered the phosphate dissolved in the waste water, with a minimum efficiency of 60.7% in U. rotundata. Net phosphate uptake rate was significantly affected by the water flow, being greatest at the highest rate assayed (2 volumes d(-1)). The marked decrease in tissue P shown by the three species during a flow-through experiment suggested that growth was P limited. However, due to the increase in biomass, total P biomass increased in the cultures. A significant correlation was found between growth rates and the net P biomass gained in the cultures. A three-stage design under low water flow (0.5 volumes d(-1)) showed that the highest growth rates (up to 0.14 d(-1)) and integrated phosphate uptake rates (up to 5.8 μmol PO43- g(-1) dry wt d(-1)) were found in E. intestinalis in the first stage, with decreasing rates in the following ones. As a result, phosphate become limiting and low increments or even losses of total P biomass in these stages were found suggesting that phosphate was excreted from the algae. The results show the potential ability of the three species to reduce substantially, at low water flow, the phosphate concentration in waste waters from a D. labrax cultivation tank, and thus the quality of effluents from intensive aquaculture practices.