Anoxia in a coastal bay: case study of a seasonal event

Bahía Concepción (Gulf of California) was studied to describe some extremely low dissolved-oxygen values at the bottom of the bay. Surveys included measurements of vertical distribution of temperature, dissolved oxygen, nitrate, nitrite, phosphates and hydrogen sulfide. Thermal stratification from early spring to autumn and a well-mixed water column during the winter were present. Dissolved-oxygen concentration was homogeneous in the water column from winter to spring (5-7 mg L-1). From summer to autumn, bottom dissolved oxygen ( 20 m) decreased to reach conditions of hypoxia (1-2 mg L-1) or anoxia; nitrate, nitrite, and phosphates increased (up to 13, 1.7 and 2.2 *M) followed by a last short period of a deep hydrogen sulfide layer (up to 3.1 mg L-1). A back calculation to estimate the input of organic matter in the deep layer showed that 18 g C m-2y-1 needed to be trapped in the bottom to induce such an oxygen depletion. This period is thought to be unfavorable for bottom infauna in this bay.<br>Se midió seis variables físico-químicas en Bahía Concepción (Golfo de California, 1993 y 1994) para explicar el origen de valores muy bajos de oxígeno disuelto en el fondo. La estructura hidrográfica mostró una fuerte estratificación térmica vertical del inicio de la primavera hasta el otoño, mientras que durante el invierno se observa una columna de agua homogénea. Las concentraciones de oxígeno disuelto siguen el mismo patrón: homogéneas de invierno a primavera (5-7 mg L -1), mientras que del verano al otoño disminuyen en la capa de fondo ( 20-m) hasta alcanzar condiciones de hipoxia (1-2 mg L-1) o anoxia. Las concentraciones de nitrato, nitrito y ortofosfatos también aumentan en la capa de fondo (13, 1.7 y 2.2 *M respectivamente), seguido de un corto período de formación de sulfuro de hidrógeno (hasta 3.1 mg L-1). Por cálculos indirectos se establece que 18 g C m-2 a-1 de materia orgánica son atrapados en el fondo provocando la severa disminución del oxígeno disuelto. Este período se considera crítico para la fauna bentónica de esta bahía

Artisanal fisheries in La Paz Bay and adjacent oceanic area (Gulf of California, Mexico)

We analyzed the artisanal fishery capture data for La Paz Bay (Baja California Sur, Mexico) from 1998 to 2005. Capture data were related to the reproductive cycle of each species. Complete fishing records for 18 species of commercial importance belonging to eight families were identified. The most frequently captured species in La Paz Bay was the Pacific red snapper Lutjanus peru, with 43% of the total capture. This species was followed in importance by the spotted rose snapper Lutjanus guttatus, other groupers (Serranidae), and the jacks (Carangidae), with 18.8%, 15%, and 7.3% of the capture, respectively. Other commercial species captured in smaller amounts included the yellowfin tuna Thunnus albacares, the California flounder (or California halibut) Paralichthys californicus, the flathead mullet Mugil cephalus, the goliath grouper Epinephelus itajara, and different species of mojarra (Gerreidae). With few exceptions, catches outside the bay were higher than in the inner bay. All species were caught throughout the year regardless of their reproductive season. We recommend the implementation of fishery regulations for La Paz Bay, especially in the case of the Pacific red snapper, since it is the most exploited. Currently, overexploitation and possible local shortage of species may occur at any time and without warning in the near future. Aside from fishery management, an attractive option for increasing the supply of marine resources in the area is to use the inner bay for marine fish farming.

Influencia del drenaje de estanques de cultivo de camarón sobre las características físicas y químicas de las aguas marinas adyacentes

Monhly sampling was performed during August 1993 to October 1996, inside and outside three tidal ponds used for shrimp culture. Our aim was to identify if the culture conditions could influence the surrounding waters and induce some adverse impact. Temperature, dissolved oxygen, salinity, sulfate, nitrate, nitrite, inorganic and organic phosphorous variables were measured. The sampling period comprised three stages of pounds development: a) Initial stage (0% of water exchange, no shrimp culture) characterized by high salinity (80%) and sulfate (6 g l-1) values, and high nitrite variability; b) Intermediate stage (30% of water exchange, shrimps fed with pelleted food), where a bance between external and internal variables was reached. It was obtained a yield of 450 kg ha-1. c) Final stage (itroducing paddlewheel aerators and fertilizing the ponds with superphosphate and urea), showing an increase in dissolved oxygen, nitrates and inorganic phosphorous variability, either inside and outside the ponds, and increase in organic phosphorous inside the ponds. During this final stage a yield of 1500 kg ha-1 was obtained. Te organic matter input by the pelleted food, induced microalge growth and shrimp metabolism during the final stage, was efficiently oxidized inside the ponds. In such a way only inorganic N and P were observed outside the ponds matching the internal variability. The management of these ponds at this final stage does not induce a negative environmental impact on sorrounding waters, and assures an attractive shrimp harvesting.De agosto 1993 a octubre 1996 se realizaron muestreos mensuales en tres estanques para cultivo de camarón y en el medio marino adyacente, con el objetivo de identificar el impacto ambiental de los estanques en el medio exterior. Se obtuvieron datos de temperatura, oxígeno disuelto, salinidad, sulfatos, nitritos, nitratos, ortofosfatos y fósforo orgánico. El muestreo se extendió durante tres fases del desarrollo y operación de los estanques: a) Fase inicial (sin intercambio de agua y sin camarones en cultivo), identificadad por altos valores de salinidad (80%) y de sulfatos (6 g l-1), y una gran variación de los nitritos dentro de los estanwues; b) Fase intermedia (con un intercambio de agua del 30% y camarones alimentados con peletizados), caracterizada por un equilibrio entre el interior y el exterior de los estanques; y c) Fase final (con intercambio de agua del 30%, organismos alimentados con pletizados, fertilización de estanques y aereación mecánica), identificada por un aumento en la variación del oxígeno disuelto, nitratos y ortofosfatos dentro y fuera de los estanques, y por un aumento del fósforo orgánico en el interior de estos. En la segunda fase del cultivo se obtuvo un rendimiento de 450 kg ha-1 y en la fase final se elevó hasta 1500 kg ha-1. La materia orgánica introducida a los estanques durante la fase final (adimento peletiado, producción de microalgas y excretas del metabolismo de camarón) fue adecuadamente mineralizada, por lo que solamente se aportaron al exterior iones inorgánicps. Bajo estas condiciones de manejo se producen atractivos rendimientos y los estanques de cultivo no ocasionan un impacto ambiental negativo inmediato en las aguas adyacentes