Long-Term Dynamic in Nutrients, Chlorophyll a, and Water Quality Parameters in a Coastal Lagoon During a Process of Eutrophication for Decades, a Sudden Break and a Relatively Rapid Recovery

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Coastal lagoons are considered among the marine habitats with the highest biological productivity, and support a great variety of human activities and pressures that make them especially vulnerable to trophic imbalances. While dystrophic crises are common in many lagoons, others like the Mar Menor show homeostatic mechanisms, high resilience, and clear waters. This paper analyses the water column descriptors dynamic during the last 22 years in this coastal lagoon, in the context of a eutrophication process produced by an increase in nutrient inputs, mainly derived from agriculture. Despite water column nitrate concentration increased by one order of magnitude, the lagoon maintained homeostatic regulation for two decades, keeping the water transparency and relatively low levels of nutrients and chlorophyll a (Prebreak phase), followed by a sudden change of state in 2016 with an abrupt increase in average nutrients and chlorophyll a concentration and loss of water transparency (Break phase), and a relatively rapid recovery after the reduction of nutrient discharges (Recovery phase). The activation of the regulation mechanisms seems to manifest through an ammonium production in the water column, as a consequence of the activity in the trophic web. The low correlation between chlorophyll a and nutrients concentration, mainly at small spatio-temporal scales, is in disagreement with eutrophication traditional models, and suggests a rapid response of primary producers to nutrient inputs and a zooplankton control in the short-term, which in turn is controlled by the rest of the trophic web components. Homeostatic properties that in the Mar Menor lagoon have provided resistance to eutrophication are based on several mechanisms: channeling its production toward the benthic system (maintaining high biomasses of primary producers, filter feeders, and detritivores), a top-down control of the pelagic trophic web exerted by ichthyoplankton and jellyfish, and exporting surplus production outside the system. Resilience of the system would be based on the high turnover in the species composition related to the restricted connectivity to the sea, the spatio-temporal variability of the environmental conditions, and the multiplicity of spatial–temporal scales involved in lagoon processes. TRIX index was sensitive to the trophic and water quality changes. However, in our study, its current score does not allow to anticipate or alert the eutrophication risk and the trophic breakpoint of the system.
Las lagunas costeras son ecosistemas altamente productivos y también muy vulnerables ya que soportan una fuerte presión antrópica. Por ambas razones, en muchas lagunas, las aguas turbias y las crisis distróficas son muy comunes. Sin embargo, otras como el Mar Menor, manteniendo una importante producción pesquera, muestran mecanismos homeostáticos muy robustos que les permiten mantener aguas transparentes. El análisis del proceso de eutrofización que ha sufrido el Mar Menor, a lo largo de los últimos 22 años tras los cambios en la actividad agrícola en su cuenca de drenaje permite comprender el funcionamiento de estos mecanismos. En una primera fase (Prebreak), a pesar del incremento de los nitratos en un orden de magnitud, la laguna mantuvo la regulación homeostática durante dos décadas, aguas transparentes y niveles relativamente bajos de nutrientes y clorofila a. Tras este periodo, en 2016, tuvo lugar un cambio repentino de estado (fase de Break), con un incremento en la concentración de nutrientes y clorofila, perdiendo la transparencia del agua y la mayor parte de las praderas bentónicas a más de 2 m de profundidad. Tras la reducción de las descargas de nutrientes se produjo una recuperación relativamente rápida (fase de Recovery). Varios mecanismos están detrás de estas propiedades homeostáticas: canalización de la producción hacia el sistema bentónico (manteniendo biomasas altas de productores primarios, filtradores y detritívoros), un control top-down de la red trófica pelágica ejercida por el ictioplancton y las medusas, y la exportación de excedentes de producción fuera del sistema o su retención en los sedimentos.