Abstract
In oceanic communities, picophytoplankton often dominates phytoplankton biomass and productivity. Diel variations in picophytoplankton abundance and growth have been well documented. In the current study, we used flow cytometry to assess the short-term variations (3 h) of the abundance of the most dominant picophytoplankton, Synechococcus spp. and picoeukaryotes, in the coastal regions of northeastern Taiwan. To explore the change in growth and mortality rate in the daytime and over 24 h incubation, we performed a two-point modified dilution experiment for measuring growth, viral lysis, and nanoflagellate grazing rate. In this study, the growth rates of picoeukaryotes were 0.21 and 0.06 h−1, and those of Synechococcus spp. were 0.15 and 0.06 h−1 for daytime and 24 h incubation, respectively, and the values were higher at significant levels in the daytime than those for 24 h incubation. These growth rate values of picoeukaryote and Synechococcus spp. after incubation for 24 h were approximately underestimated at 71% and 55%, respectively. This finding suggests that estimates based on 24 h sampling may not accurately reflect the true growth rate of these populations on ecologically relevant timescales.
Highlights
Picophytoplanktons (0.2–2 μm), comprising picocyanobacteria and photosynthetic picoeukaryotes, are important components of aquatic ecosystems
We found that the total abundance of Synechococcus spp. and picoeukaryotes had a pattern with an increased abundance during the day, and reached a peak at nearly 22:00 pm, followed by a gradual decrease during the nighttime (Figure 1)
Studies of small-scale variability in picophytoplankton abundance have found that temporary imbalances between growth and mortality rates throughout a day generate important daily variation in aquatic environments
Summary
Picophytoplanktons (0.2–2 μm), comprising picocyanobacteria and photosynthetic picoeukaryotes, are important components of aquatic ecosystems. Picophytoplankton contribute to approximately 60–80% of the total marine primary productivity [1], suggesting that picophytoplankton has crucial roles in primary productivity in oligotrophic regions. The primary factors that affect picophytoplankton are temperature, nutrient availability, and irradiance level. The daily light–dark alternation is undoubtedly a very important external stimulus. Picophytoplankton is highly synchronized to the light–dark alternation [2,3] and the timing of division varies among species, groups, and even strains [2]. The diel variations in picophytoplankton abundance, growth, and division have been well documented [2,4,5]
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