Spatio-temporal variation in organic nitrogen and carbon in sediments associated with tropical submerged-cage aquaculture

The diminution of marine fish populations due to over-fishing has stimulated the increase of mariculture activities, including in cages located near the coast. However, these activities may be detrimental influence to marine sediments near the culture sites in coastal sites. The first open-ocean mariculture operation began during 2002 south of Culebra Island, Puerto Rico to culture the fish Rachycentron canadum and Lutjanus analis. The purpose of this study was to evaluate the concentrations of total organic nitrogen (TON), total carbon (TC), and organic matter (OM) in marine sediments near the mariculture site to determine the spatial and temporal dynamics of these nutrients during the first culture period. Results indicate significant differences in the concentration of TON between the cage and control sites. The L. analis cage had a higher mean concentration of TON (0.442 mg N/g) than the R. canadum cage (0.380 mg N/g) and control site (0.300 mg N/g). TC and OM mean concentrations were not significantly different; however, mean TC concentrations had fluctuations similar to those of the mean TON concentrations. TON and TC mean concentrations were significantly different over time, with an increase in the mean TON (0.66 mg N/g) and TC (199 mg C/g) concentrations from April 2003 to August 2003, with a peak in June 2003, which agreed with the increase in the mean monthly feed input at the culture site (12,947 kg) and the increase of wastes because the fish had reached a commercial weight (4.5 kg). Harvesting began in June 2003, so numbers of fish decreased during subsequent months. Organic matter decreased during June, but peaked during October 2003. Although the increase of the nutrient concentration is relatively low compared with other studies, data represent only the first year of mariculture activity. As the company increases the number of cages, this site should be monitored to determine possible increases in nutrient concentrations in the sediments.