Se descubrió la causa de la mortandad de peces en la laguna del Parque General Paz. Un estudio de la Universidad Nacional de San Martín constató una alta concentración de nitrógeno y fósforo en el espejo de agua, responsable del rápido desarrollo de diatomeas. Éstas se fijaron a las branquias de las mojarritas, que murieron por asfixia. Qué se recomienda para restaurar el equilibrio ambiental.

Por Hugo Campos *
hugomcampos@yahoo.com.ar

A raíz de la gran mortandad de mojarritas ocurrida en la laguna del Parque General Paz, desde el Museo Histórico “Cornelio de Saavedra” se pidió la intervención de la Agencia de Protección Ambiental (APA) con el fin de identificar sus causas. Al mismo tiempo se dio aviso a la Junta Comunal 12 para que también interviniera. Desde esta entidad enviaron a una licenciada en paisajismo, quien recomendó la necesidad de tomar muestras de los peces para hacerles la necropsia pertinente y poder dar con las causas. Por su lado la APA hizo una toma de muestra del agua, la cual arrojó un resultado preliminar de buena calidad.
El periódico El Barrio se hizo eco de esta problemática publicando una nota en su edición del mes de junio, bajo el título “Extraña mortandad de peces en el Parque General Paz”. Ante la incertidumbre por la continuidad del fenómeno se recurrió al Grupo de Limnología del Instituto de Investigación e Ingeniería Ambiental (3iA) de la Universidad Nacional de San Martín (UNSaM), a cargo de la Dra. Gabriela Mataloni y equipo. Con una rápida respuesta, se acercaron a tomar muestras de agua y colectar ejemplares de peces muertos para su análisis.
En el informe se constató la altísima concentración de nutrientes, tanto de nitrógeno como fósforo, responsables del rápido desarrollo de microalgas. Cuando la presencia de nutrientes, en este caso el fósforo, es alta se dice que el cuerpo de agua está “eutroficado”. Debido a la altísima concentración de este elemento el estado pasa a ser “hipereutrófico”, el cual suele ser un rasgo habitual de muchos estanques artificiales urbanos.
La “hiper riqueza” de nutrientes, sumada a un período de calor prolongado e infrecuente para el otoño, provocó un crecimiento explosivo de microalgas, entre ellas diatomeas. Dentro de este grupo hubo una especie en particular perteneciente al género Fragilaria, que fue la principal responsable de la mortandad. Ésta se produjo al fijarse en las branquias de los peces, que reaccionaron primero a través de inflamación y finalmente secretando muchísima mucosidad, la cual no les permitió realizar correctamente el intercambio de oxígeno disuelto en el agua y derivó en su posterior fallecimiento por asfixia. Se destaca que las especies de diatomeas presentes en el agua no implican ningún riesgo para la salud de otro tipo de animales humanos y no humanos.

La laguna del Parque General Paz, detrás del Museo Saavedra.

Este desenlace se da por una cadena de eventos desafortunados que comenzaron con la presencia de las carpas “Koi” (Ciprinus carpio), las cuales de por sí son exóticas e invasoras. Al remover constantemente el fondo, donde el nitrógeno -en forma de nitratos y nitritos- y el fósforo pueden quedar inmovilizados, hacen que éstos estén disponibles para las microalgas como el caso de las diatomeas. También contribuyó a esta mortandad la “puesta en valor” de la laguna, consistente en remover las plantas acuáticas existentes para incluir los chorros de agua y las luces de colores. Esas plantas extraían y fijaban el excedente de nutrientes del agua, evitando estos crecimientos explosivos, además de proveer refugio a los peces autóctonos más pequeños.
Entre las conclusiones del informe de la UNSaM, se destacan los siguientes ítems:

1) En base a antecedentes bibliográficos sobre la capacidad de algunas especies de diatomeas de causar mortandad masiva de peces por daño a las branquias, concluimos que en este caso Fragilariaspp ha sido responsable de un fenómeno similar.

2) Se recomienda realizar muestreos del agua del estanque con una frecuencia al menos bimestral, no sólo de parámetros físico-químicos en general sino también de su estado trófico a través del análisis de fósforo total, transparencia y clorofila. Además, el estudio de la estructura y dinámica del fitoplancton contribuirá a detectar
tempranamente floraciones nocivas.

3) A fin de evitar floraciones de microalgas, se sugiere que, de sembrar peces, éstos no sean ciprínidos (por ejemplo carpas) para no aumentar la carga de nutrientes en el cuerpo de agua. Asimismo, mantener poblaciones de plantas acuáticas y cosechar partes de las mismas en forma periódica es una forma ambientalmente amigable de remover nutrientes del sistema.

De estas conclusiones se desprende la necesidad de un plan de restauración ambiental para este cuerpo de agua, donde la prioridad debe ser informar a la población de la imperiosa necesidad de extraer y relocalizar los ejemplares de carpas “Koi”, para luego poder continuar con la siembra de plantas superiores que limiten la concentración de fósforo y nitrógeno, aún si esto implicara la desactivación de los chorros de agua. Finalmente, se recomienda la reintroducción de peces autóctonos que le devolverán la vida a la laguna y además brindarán, entre otros servicios ambientales, el de controlar las larvas de mosquitos, incluidas la del Aedes aegypti.
Por otro lado, esta situación debería servirnos de alerta y muestra de los primeros síntomas del calentamiento global que podrían ocasionar, con mayor frecuencia, este tipo de sucesos. Se trata de una evidencia del impacto humano sobre la atmósfera, que ya no es algo lejano sino que lo estamos viviendo.

Lic. en Gestión Ambiental.

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