Long term characterization of Trichodesmium erythraeum blooms in Gabès Gulf (Tunisia)

The present paper reports on a twenty six year monitoring of the diazotrophic cyanobacteria, Trichodesmium erythraeum in the Gulf of Gabès associated with environmental parameters and meteorological variables. Trichodesmium erythraeum blooms were not recurrent all years and were observed on average 2.11 times per year over the period between 1988 and 2013. Blooms were associated with temperature exceeding 24 °C and wind speed generally less than 5 m s−1. Trichodesmium erythraeum reached very high densities fluctuating between 0.12×106 and 720×106 trichomes dm−3. The wind speed during dust events and the number of dust days per year were highly correlated to Trichodesmium abundances. Two wind regimes during dust events were identified. The South -South East direction crossing the Tunisian desert generated the most intensive blooms. High dissolved inorganic nitrogen concentrations (2−14.6 µM) and orthophosphate concentrations (0.05-2.79 µM) were observed during bloom events leading to high N/P ratio well above the Redfield ratio and fluctuating linearly as function of Trichodesmium abundance. The anomalous N/P ratio could result from Trichodesmium erythraeum biological N2 fixation and/or the contribution of atmospheric deposition.