Nutrient-phytoplankton production relationships in a groundwater-influenced tropical coastal lagoon

Abstract

Interactions between nutrient and biotic components of phytoplankton in Celestún lagoon, a groundwater-influenced tropical coastal lagoon were studied. Gradients of nutrients, chlorophyll a, phaeopigments, production and respiration rates were estimated from March 1990 to March 1991. The inner zone of the lagoon was strongly affected by groundwater discharges with high concentrations of nitrate (35μmol) and soluble reactive silica (400μmol). The seaward zone was characterized by high salinity (<30o/oo) and low concentration of nutrients. The middle zone of the lagoon was characterized by intermediate values of salinity and high concentrations of soluble reactive phosphorus (>9μmol) and ammonia (15μmol). Seasonal chlorophyll a variations showed a gradient of high concentrations in the middle region (20–28mgm−3) to low concentrations at both ends of the lagoon during the rainy season (5–8mgm−3). Net primary production ranged from 0.1 to 2.2gCm−3day−1, and showed a similar seasonal pattern in all stations, with a peak in the early rainy season and smaller secondary peak during the ‘nortes’ season. Multivariate analysis was carried out in order to describe different stages of nutrient-phytoplankton coupling. These were related to groundwater discharges and climatic patterns. The relationships between nutrient and phytoplankton showed a coupling between mineralization-import of nutrient patterns and respiration-production of phytoplankton. The dissolved inorganic nitrogen to soluble reactive phosphorus ratio in Celestún lagoon varied seasonally from 70:1 during the dry and rainy seasons, to <5:1 during the ‘nortes’ season. This suggested a potential shift in nutrient limitation for phytoplankton growth.