Effects of physical forcing on short-term plankton dynamics in a narrow coral reef lagoon (Ouano, New Caledonia): a two-week high-frequency study

Abstract

The response of plankton communities to short-term climatic events was studied in a narrow coral reef lagoon representative of the channel-type lagoons of the high islands of the tropical Pacific. Phyto- and zooplankton data (abundance and taxonomic composition) were sampled at high frequencies for 2 weeks in April–May 2017 under various tidal conditions and combined with environmental and physical measurements and modeling hydrodynamic products. The short-term external events (wind, waves and rain) that occurred caused changes in the circulation patterns and increased nutrient concentrations in the lagoon. The plankton reacted quickly to this enrichment, with an increase in chlorophyll a and the rapid development of a microphytoplankton community dominated by diatoms, and of opportunistic herbivorous zooplankton (tintinnids, appendicularians). Zooplankton composition and distribution were strongly modulated by the circulation patterns through advection and mixing. Our results show that the tidal and diel components of the temporal variability of planktonic groups were blurred by sporadic event components (i.e. wind, wave and rainfall events). Environmental and biological responses to these external physical forcings occurred at lagoon scale in this channel-like lagoon, unlike what is observed in larger lagoons (such as the southern lagoons of New Caledonia) where spatial variability is much higher.