Sequoia Scientific LISST Instruments in Benguela Upwelling System
In May of 2022 the French research vessel Tara sailed northward along the Western coast of Africa as part of the multi-year Microbiomes mission. Several instruments from Sequoia Scientific were installed on Tara during transit between Cape Town, South Africa and Walvis Bay, Namibia. The coastal region of Southwest Africa is a unique area of the world’s oceans, and of particular interest to biological oceanographers as one of the most productive and biodiverse ocean regions in the world. The Benguela current flows northward along the coast, resulting in export of surface water offshore. This draws cold nutrient rich water to the surface which results in high primary production that in turn supports a rich fishery. The chlorophyll rich waters near the coast are often visible from remote sensing satellites.
Figure 1. Enhanced MODIS image showing chlorophyll concentration in the surface ocean off the coast of South Africa and Namibia (source: https://earthobservatory.nasa.gov/images/91198/finding-life-in-the-benguela-current)
The research vessel Tara was equipped with five different Sequoia Scientific Instruments:
LISST-200X (Particle Size and Concentration)
LISST-Tau (Beam Transmissometer)
LISST-Holo2 (Particle Size and Imaging)
LISST-Horizon (Particle Size and Multi-Angle Scattering)
Hyper-bb (Hyperspectral Backscattering)
The instruments were installed in an underway flowthrough system, where seawater from beneath the ship’s hull was pumped through the instruments. Additional flowthrough instruments included a Sea-Bird Scientific ac-s (spectral absorption and attenuation), Seapoint SUVF (CDOM fluorometer), and a Sea-Bird TSG (temperature and salinity). These instruments sampled the surface water continuously throughout the 30-day transit from Cape Town, South Africa to Walvis Bay, Namibia. Several in-shore to off-shore transects were conducted along the Namibian coast. These transects spanned from the cold biologically productive inshore waters to the warmer clearer waters offshore. The Sequoia instrumentation installed onboard Tara was able to track these changes in the surface water. Shown below is transect conducted just south of Walvis Bay, Namibia. The upwelling of cold water near the coast is evident in the temperature transect and in the optical transmission measured by the LISST-Tau. The LISST-Tau shows particle rich water nearshore that gradually decreasing offshore.
Figure 2. Location of inshore/offshore transect conducted during Tara Microbiomes mission. Temperature, beam attenuation, and chlorophyll concentration reveal a nearshore phytoplankton bloom supported by nutrient-rich upwelled water.
The particle rich waters near shore were high in chlorophyll, indicating the particles were chlorophyll containing phytoplankton. As opposed to sand blown into the sea from the Namib desert, which is also common in this area. The backscatter from the Hyper-bb measured nearshore (right) showed a significant dip in the backscattering spectrum near 670nm, corresponding to an absorption peak of chlorophyll-a, another indication of the upwelling-fueled phytoplankton bloom.
Figure 3. Particulate backscattering coefficient measured by Hyper-bb instrument in a nearshore phytoplankton bloom. Red line shows the median of all measurements.
The LISST-Holo2 installed in the ships flowthrough system collected particle images of the phytoplankton bloom. The LISST-Holo2 revealed the presence of both individual and chain diatoms in large concentrations. Additionally, the LISST-Holo2 showed almost a complete absence of large zooplankton (e.g. copepods, crustacean larvae). Images from the LISST-Holo2 show a variety of particles with sizes varying mostly between 10 and 100 microns.
Figure 4. Subset of reconstructed particle images collected by a LISST-Holo2 in the nearshore phytoplankton bloom. Images show the particle composition to be mostly individual and chain diatoms ranging in size from approximately 10 to 100 microns.
The evolution of the phytoplankton bloom along the transect is evident in the particle size distribution (PSD). The LISST-200X reported a decrease in the mean particle size and concentration as function of distance offshore. The decrease in mean size was likely caused by predation and/or export of plankton from the surface water. The PSD shows primarily small particles remaining in the water column at the end of the transect offshore.
Figure 5. Mean particle size and particle size distributions measured by LISST-200X. Mean particle size gradually decreased as function of distance offshore. Overall particle concentration decreased sharply.
The Sequoia instruments onboard the research vessel Tara were able to provide unique insight into this region of the ocean, providing a detailed characterization of ocean particles in this region, , including forward scatter, backscatter, particle size, concentration, and imagery. Continuous 24-hour operation of the instruments was aided by specially designed flowthrough chambers that are available for each instrument. Notably, the LISST-Horizon and Hyper-bb instruments installed on Tara operated nearly continuously for 2 years of the Tara microbiomes mission. The data shown here represents only a small window into the vast dataset collected during the mission.