Abstract
Understanding the annual cycle of Microcystis is essential for managing the blooms of this toxic cyanobacterium. The current work investigated the sedimentation of microcystin-producing Microcystis spp. in three reservoirs from Central Spain during the summer and autumn of 2006 and 2007. We confirmed remarkable settling fluxes during and after blooms ranging 106–109 cells m−2 d−1, which might represent 0.1%–7.6% of the organic matter settled. A comprehensive analysis of the Valmayor reservoir showed average Microcystis settling rates (0.04 d−1) and velocities (0.7 m d−1) that resembled toxin settling in the same reservoir and were above most reported elsewhere. M. aeruginosa settling rate was significantly higher than that of M. novacekii and M. flos-aquae. Despite the fact that colony sizes did not differ significantly in their average settling rates, we observed extremely high and low rates in large colonies (>5000 cells) and a greater influence of a drop in temperature on small colonies (<1000 cells). We found a 4–14 fold decrease in microcystin cell quota in settling Microcystis of the Cogotas and Valmayor reservoirs compared with pelagic populations, and the hypothetical causes of this are discussed. Our study provides novel data on Microcystis settling patterns in Mediterranean Europe and highlights the need for including morphological, chemotypical and physiological criteria to address the sedimentation of complex Microcystis populations.
Highlights
The colonial genus Microcystis (Chroococcales) is one of the most common bloom-forming cyanobacteria in freshwater bodies and a major producer of the hepatotoxins microcystins (MCs) worldwide [1,2], posing a great concern for water quality management.Understanding the annual life-cycle of Microcystis is essential for developing water management strategies to delay or minimize the blooms of this harmful cyanobacterium [3]
0.5%–1% of the organic matter in traps came from Microcystis-dominated phytoplankton that had settled in Lake Mendota, USA, which is in the range of the 0.1%–2.8% we found for Valmayor, but is clearly below the 3.8%–7.6% estimated for the Cogotas reservoir
Maximum Microcystis settling velocities in Valmayor (0.7–0.9 m d−1) were markedly higher than those reported in most previous studies, such as the maximum of 0.24 m d−1 and 0.25 m d−1 estimated in lakes Kasumigaura, Japan [17] and Volkerak, the Netherlands [6], but were in the range of the maximum 0.11 d−1 observed in Lake Mendota, USA [16] or the 0.16 d−1 reported in Rostherne Mere, UK [18] at particular moments
Summary
The colonial genus Microcystis (Chroococcales) is one of the most common bloom-forming cyanobacteria in freshwater bodies and a major producer of the hepatotoxins microcystins (MCs) worldwide [1,2], posing a great concern for water quality management. M. aeruginosa, the best known Microcystis spp., has a meroplanktonic behavior with an annual life-cycle consisting of 4 stages for the temperate regions [4,5,6]: pelagic growth that occurs mostly in summer; sedimentation of the pelagic population to the bottom sediments in autumn; overwintering as benthic or small pelagic populations; and reinvasion of the water column (recruitment) in spring, returning to the beginning of the cycle This cycle has been subject of a number of field studies beginning from the early 1980s to the present [4,5,7,8,9,10], including the development of some predictive mathematical models [6,11]. The following aims were investigated: (1) determine the quantitative importance of the sedimentation process in the loss of pelagic Microcystis populations; (2) establish the spatiotemporal patterns of the sedimentation processes, evaluating the influence of environmental factors (temperature and inorganic matter content) and/or colony morphology (morphospecies and colony size) on such patterns; and (3) monitor the shifts in the MC cell quotas of settling
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