Hydrological Versus Biological Drivers of Nutrient and Carbon Dioxide Dynamics in a Coastal Lagoon

Abstract

Coastal lagoons are dynamic aquatic systems that are susceptible to eutrophication due to long residence times and high inputs of nutrients. We hypothesise that groundwater-derived nutrients make a significant contribution to primary production, eutrophication and carbon cycling in these systems. Here, we report results from seasonal (pre-bloom, bloom and post bloom), nutrient, pCO2 and 222Rn (a natural groundwater tracer) surveys in a coastal lagoon that frequently experiences macroalgae blooms (Avoca Lagoon, Australia). Groundwater inputs of DIN and DIP deliver the nutrient load equivalent to the entire lagoon inventory in 1.6 to 3.5 days and 1.7 and 6 days respectively, indicating groundwater was a major source of inorganic nutrients to the system. Lagoon pCO2 displayed significant spatial and temporal variability, with the average pCO2 shifting from 1717 μatm during pre-bloom, to 137 μatm during the bloom and 3056 μatm post bloom. Radon-222 displayed a significant positive relationship with dissolved inorganic nitrogen (DIN) and pCO2 during the pre-bloom period. This suggests a hydrological dominance of surface water DIN and pCO2 during the pre-bloom period. During the bloom period, DIN displayed a positive relationship with pCO2 and negative relationship with dissolved oxygen, indicating a strong biological control over the nutrient pool. Phosphate did not appear to be controlled by either groundwater inputs, or ecosystem metabolism throughout the study. While groundwater discharge stimulated primary production through nutrient inputs (thus reducing surface water CO2), it also directly delivered significant quantities of CO2 to surface waters. The net effect of groundwater inputs of nutrients and dissolved carbon into the lagoon was a stimulation of CO2 fluxes to the atmosphere.