Ecosystem metabolism dynamics and environmental drivers in Mediterranean confined coastal lagoons

Abstract

Aquatic metabolism is an important descriptor of ecosystem functioning. The metabolism of ponds and confined coastal lagoons has been poorly studied in comparison to other aquatic systems, in which the metabolic dynamics are better understood. In this study, we described the ecosystem metabolism of two confined Mediterranean coastal lagoons located in La Pletera salt marsh (NE Iberian Peninsula), which is dominated by flooding-confinement patterns. We estimated the metabolic rates by applying Bayesian models to three years of high-frequency open water oxygen data. Our aim was to test if nutrients and other environmental variables (the temperature, conductivity, light and water level) that registered as important drivers of metabolism in the literature were the primary drivers of metabolic variation in confined coastal water bodies. We observed clear seasonal patterns in the metabolic rates, with extremely high oxygen variability during the summer season ranging from supersaturation (saturations > 200% were recorded) to anoxia (<5%). Despite the high rates of production registered during the summer, periods of anoxia could prevail for several days during that season. Thus, although the aerobic production and respiration were quite balanced in the lagoons during the study period, these lagoons are probably more heterotrophic since their anaerobic respiration has not been estimated. Because the studied lagoons are rich in nutrients, we expected a low response in the metabolic rates to nutrient increases, since the physiological response of primary producers to nutrient loading is usually low in nutrient-saturated ecosystems; our results supported this hypothesis. The temperature was the primary driver, highlighting the importance of seasonality in these highly productive ecosystems. Our results also showed an uncoupling between the metabolic rates, which were higher in the summer, and the standardized ones, after removing the temperature and irradiance effects, which were higher in the winter and negatively related to the conductivity. This finding suggests that potential productivity (standardized rates) is more sensitive to winter inputs and, in contrast, the actual productivity is more related to summer concentration processes due to confinement.