Abstract
AbstractUsing frequent and long‐term measurements (eight times per day, 2011 to present) from a geostationary satellite sensor (Geostationary Ocean Color Imager, GOCI), this study investigates diurnal changes of cyanobacterium Microcystis aeruginosa blooms (near‐surface high concentrations or surface scums) in Taihu Lake, from which vertical migration patterns could be inferred. After proper atmospheric correction, a cyanobacterial index algorithm is used to quantify equivalent surface cyanobacterial density (σ, 0–100%) at both pixel and synoptic scales from each cloud‐free image, followed by analysis of diurnal changing patterns of σ at both scales. Three typical diurnal changing patterns are identified from all images, which show distinctive and different seasonality from the long‐term statistics. Spatial distributions of the “hotspot” regions where diurnal changes are most often observed have also been established. While the seasonality of the three patterns appears to be a result of seasonality in both temperature and light availability, large blooms only occur 1 d after major wind events. Based on several lines of evidence, we hypothesize that the diurnal changes of such observed surface bloom patterns are likely a result of vertical migration rather than horizontal dissipation/aggregation of cyanobacteria. The mean migration speeds inferred from either a simple model or a radiative transfer model (< 0.03 cm s−1 or < 1 m h−1) are consistent with those reported earlier from laboratory measurements for certain cyanobacteria colony sizes. Complete understanding of the three types of diurnal patterns and direct validation of the hypothesis, however, require further investigations from field measurements.
Highlights
These oxygen-containing gas bubbles can modulate the colony movement as increased oxygen can facilitate the rise of colonies, and for the same reason decreased oxygen can make the colonies sink
Most observations of cyanobacterial vertical migration have been made from laboratory experiments under controlled conditions (Ganf 1974; Dervaux et al 2015; Medrano et al 2016b; Qin et al 2016), as similar observations are difficult to make in natural environments (Ibelings et al 1991)
This definition follows that of Reynolds and Walsby (1975) where changes in cyanobacteria buoyancy are the main drivers of the bloom formation, and this definition is different from the growth-based definition where the term “bloom” refers phytoplankton biomass increases due to growth
Summary
These oxygen-containing gas bubbles can modulate the colony movement as increased oxygen can facilitate the rise of colonies, and for the same reason decreased oxygen (through consumption by respiration or when bubbles break and escape from the colonies) can make the colonies sink. Microcystis cells and colonies are known to be able to form scums or high-concentration near-surface layers due to vertical migration of cyanobacteria (Reynolds and Walsby 1975; Zohary and Robarts 1990) and such scums or near-surface blooms can be observed from satellite remote sensing without sophisticated algorithms. Their vertical migration may be observed indirectly from short-term (e.g., diurnal) changes of these surface blooms, where such changes can be quantified with measurements from the Geostationary Ocean Color Imager (GOCI, Ryu et al 2012) that provides eight images a day. With 1h frequency, it measures the TOA radiance in the eight spectral bands eight times a day from 08:00 h to 15:00 h local time over a large region covering 2500 km 3 2500 km centered at 368N 1308E
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