Fouilland, E., Mostajir, B., Levasseur, M., Roy, S., Vidussi, F., de Mora, S., et al. (2016). Effect of mixing on the structure of a natural plankton community: a mesocosm study. Vie et Milieu, 66(3-4), 251–259.
Résumé: A plankton community (< 202 μm) from the St. Lawrence Estuary was isolated
in four outdoor mesocosms with SLOW and fast mixing regimes. Variations in the concentrations
of nutrients, chlorophyll a (Chl a), nitrogen transport rates and plankton species composition
were monitored over a 10 day period. The vertical mixing times (Tm) for the slow and
fast mixing regimes were 180 and 60 min, corresponding to a vertical eddy diffusivity (Kv) of
2.34 and 7.03 cm2 s–1, respectively. The different mixing regimes had a strong effect on the
physiology of the phytoplankton and the specific structure of the plankton assemblage. The
Slow mixing regime stimulated the development of a mixed community of flagellates, small
diatoms and proto-metazooplankton while the fast mixing regime triggered the development
of a large diatom-dominated community with lower abundances of proto-metazooplankton. At
the end of the 10 day experiment, the Chl a concentrations were 50 % higher in the mesocosms
with the fast mixing regime than in those with the slow mixing regime. These results indicate
that, under low nutrient conditions, higher turbulence gives a competitive advantage to diatoms
and decreases the zooplankton grazing pressure, resulting in net positive growth. Extrapolation
of these results to natural systems suggests that a wind-driven mixing event may increase
the net phytoplankton biomass production of a stratified water column, even if there is no external
input of nutrients.
Laanaia, N., Vaquer, A., Fiandrino, A., Genovesi, B., Pastoureaud, A., Cecchi, P., Collos, Y. (2013). Wind and temperature controls on Alexandrium blooms (2000-2007) in Thau Lagoon (Western Mediterranean). Harmful Algae, 28, 31–36.
Résumé: Since 1998, blooms of Alexandrium catenella/tamarense in the lagoon of Thau developed regularly each autumn, reaching a maximum of several millions cells per liter in 2004. By contrast, spring blooms occurred only twice (in 2000 and 2007). During these periods, sea surface temperatures (SST) and the wind patterns appear to impact the bloom occurrences much more than the apparent limiting resources such as inorganic nutrients. The analysis of SST and wind from April to June and September to November (from 2000 to 2007) indicates first that there has to be an initial wind stress in order to resuspend the cysts buried in the sediment. Blooms then occur after a period of weak winds (<4 m s(-1)) and of stable SST close to 20 degrees C (+/- 2 degrees C). Those conditions appear to be most favorable for germination of Alexandrium cysts and its ensuing vegetative growth. This period of stability (a few days to a few weeks) allows the development of the inoculum from the cyst's germination, its cohesion because of reduced hydrodynamics, and development of vegetative cells that are sensitive to agitation. Strong winds during 1-2 day periods can interrupt the bloom dynamics by dispersing (advection due to southeasterly winds) and/or eliminating (turbulence due to northwesterly winds) the vegetative cells. In the spring, under the same conditions of optimal SST, strong wind episodes dominate and those, as well as biological factors very likely lead to a lower occurrence of blooms relative to the fall situation. (C) 2013 Elsevier B.V. All rights reserved.