Studies on the biofiltration capacity of Gracilariopsis longissima: From microscale to macroscale

Abstract

The potential of the red agarophyte Gracilariopsis longissima as biofilter for phosphate and ammonium in effluents outflowing intensive marine fish cultures was assessed at different scales. Previous studies showed that both laboratory (microscale level) and outdoor cultivation (mesoscale level) were feasible, with a maximum sustainable yield of 270 g fresh wt m − 2 day − 1 approximately, at a biomass higher than that predicted in a logistic model, a deviation attributable to an improvement of the culture conditions during the monitoring period. At a mesoscale level, a 34-h cycle suggested that the nitrification rate on the seaweed fronds showed diel fluctuations, with rates peaking early in the morning, when ammonium uptake rates were negligible. Mean nitrification rates were similar to ammonium uptake rates, suggesting that nitrifyers outcompete G. longissima for the use of ammonium; especially when mean biofiltering efficiencies were less than 15% during the 34-h period. G. longissima thrives naturally in different earthen ponds of a fish farm in Cádiz Bay Natural Park, Southern Spain, especially in the outflowing reservoir earthen ponds, where biomass reached values up to 278 g dry wt m − 2 during the spring. A field cultivation system for G. longissima (macroscale level) was designed to find the best scenario in terms of earthen pond, season or current conditions. The best cultivation method was the growth of vegetative cuttings on suspended braided nylon ropes. The highest growth rates (up to 6% day − 1 ) and biomass (up to 10 g fresh wt cm − 1 rope) were obtained in ponds receiving outflow waters, suggesting a nutrient effect. The net P production reached 24.9 μg P cm − 1 rope day −1 and was also higher on braided nylon suspended ropes placed at the outflowing reservoir earthen ponds. A similar result was found regarding net N production. However, in this case, mean production (≈ 170 μg N cm − 1 rope day − 1 ) was similar in the different earthen ponds and channels. The increase in P and N biomass suggested that G. longissima was biofiltering efficiently nutrient wastes from the fish farm. The results pointed out the high potential ability of G. longissima to biofilter waste waters from a fish farm, encouraging a large scale cultivation of this species. Future practices using this macroalgae may be implemented in local fish farms, resulting in both environmental and economic advantages.