Population dynamics and environmental conditions affecting Trichodesmium spp. (filamentous cyanobacteria) blooms in the south–west lagoon of New Caledonia

The present description of Trichodesmium spp. population dynamics and associated environmental variables is the first one using a very short sampling interval (2–3 days). Such a strategy allows a fine description of changes involving the population density and characteristics. It also lends itself to interpreting those changes from past environmental conditions. During the two surveys, 2 and 5 month-long, T. erythraeum dominated the Trichodesmium population, with temporary occurrence of T. thiebautii and five blooms could be described. These events appeared at temperatures > 24 °C and followed, in all cases, nitrate, soluble reactive phosphorus and chlorophyll a enrichments, with a 3–7 day time lag. Low wind speed (< 4 m s− 1) was not a prerequisite for Trichodesmium bloom developments as long as temperatures exceeded 26 °C. As abundance increased during the bloom, so did the number of filaments (trichomes) in colonies and their buoyancy, leading to a clear positive biomass gradient from the bottom to the surface. A simple model, using variable growth rates, showed trichome ascent would be responsible for 87–99% of concentrations at 0.5 m, with Trichodesmium net growth rates ranging from 0.11 to 0.38 d− 1. Finally, rapid trichome density declines could be ascribed to nutrient depletion and massive surface death following ascent.