Efeitos diretos e indiretos da pesca na reciclagem de nutrientes em rios pantaneiros

Abstract

Ecologically, consumers can play an important role as nutrient recyclers in the environment where they live, as they directly remineralize nutrients through excretion, or indirectly through assimilation and incorporation of organic matter from the diet into tissues. Professional fishing emerges as a possible threat to the dynamics of nutrients in aquatic environments, as it is reflected in the harvesting of fish biomass and the consequent reduction in their ecological contribution. In this study, we investigated how the removal of biomass through fishing influences the nutrient supply in the aquatic system and how omnivorous, carnivorous, detritivorous and piscivorous diets contribute to nutrient input in the same system. For this, we used EMBRAPA Pantanal fish stock data between the years 2012 to 2016 and calculated the excretion rates of nitrogen and phosphorus through a universal model for the prediction of N and P that considers biomass, diet, temperature as determining factors for excretion rates; elemental body composition was estimated through carbon, nitrogen and phosphorus present in fish tissue. We saw that about 817t of biomass was fished over the years, corresponding to a loss of about 87t of body C, 24t of body N and 4.56t of body P; at the level of excretion, fish no longer contribute about 4702 kg/month of nitrogen and 180 kg/month of phosphorus to the system. Seven of the thirteen species studied together accounted for approximately 93% of the contribution to nitrogen excretion and 92% to phosphorus excretion to the environment. Carnivorous had higher population excretion of N (approximately 1860 kg/month) and omnivores had higher excretion of P (97 kg/month) over the years analyzed; in terms of body nutrients, the guild of carnivores had impressive numbers, with around 10,000kg of nitrogen and approximately 2,000kg of phosphorus in their tissue. Thus, a possible extinction or drastic reduction of fish, through fishing, can significantly alter the N:P dynamics and the identity of a possible limiting nutrient in the environment