While Trichodesmium erythraeum is prevalent in the semi-closed SW lagoon of New Caledonia, T. thiebautii is dominant in the more open SE lagoon. This led to the comparison of the two species from the results of two high-frequency surveys focused on T. erythraeum (Rodier and Le Borgne, 2008) and T. thiebautii (the present paper). (1) Environmental conditions and triggers of the blooms are the same: calm weather and/or temperature >26 degrees C, and temporary nutrient inputs are required for both; (2) growth rates under favorable conditions are similar (0.14-0.27 d(-1)) but (3) T. thiebautii has lower net ascent rates along the water column during blooming events, due to distinct buoyancy capacities and the resulting vertical distributions; (4) carbon and dinitrogen fixation rates are not significantly different and contribution of Trichodesmium spp. represent less than 35% and 5% of the total phytoplankton carbon and nitrogen requirements, respectively. It is concluded the two species can be mixed in ecological studies, except for their vertical distributions during the bloom periods.
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