Biological oceanography Ecology Harmful Algal Bloom Remote Sensing

Cyanobacteria invasions… from space?

Article: Kahru, M., R. Elmgren, E. Di Lorenzo, O. Savchuk. 2018. Unexplained interannual oscillations of cyanobacterial blooms in the Baltic Sea. Nature: Scientific Reports.  8:6365. doi:10.1038/s41598-018-24829-7.

Ah, spring! Buds on trees, blossoms on flowering bushes – the bright green hue that overtakes the scenery, alerting us to the beauty of new beginnings. 

The first time I saw a satellite image of a North Atlantic spring phytoplankton bloom (a massive bloom of microscopic ocean plants), the vast bright swirls appeared to me how the first hint of spring feels – evoking feelings of anticipation for new life, the promise of warmth. The ocean, like the budding trees and flowers, is very much alive and blooming.

Phytoplankton blooms occur in regions of the ocean where conditions are just right for abundant photosynthesis – that is, plenty of sunlight and nutrients (such as in coastal environments and near ocean fronts). The phytoplankton in blooms provide oxygen for every other breath we take, and help the ocean absorb and export carbon to the deep. These organisms are primary producers, meaning they reside at the lowest level of the marine food chain – thus, their life cycles determine the health of entire marine ecosystems. Unfortunately, blooms can have both positive and negative effects; harmful algal blooms occur when an overgrowth of algae covers the sea surface in scummy, green, sometimes foul-odored, toxic algae.

Cyanobacteria: the good, the bad, and the algae

Cyanobacteria are a bit different from other phytoplankton – they’re photosynthetic, but they’re also bacteria. Bacteria (often called microbes) can live within humans, in the soil, or pretty much anywhere, and are vital to the functioning of virtually every living thing. In fact, a symbiotic (mutually beneficial) relationship between cells and cyanobacteria long ago initiated the evolution of the parts of a plant cell that allow them to perform photosynthesis (specifically, the mitochondria and chloroplast). Cyanobacteria came to visit, and ended up staying – in all plants – permanently.

Figure 1: Cyanobacteria bloom in the Baltic Sea taken with satellite imagery in July 2005 (from Kahru et al. 2018).

Their blooms can often be seen from satellites, overtaking entire regions in blue-green filaments and rings (hence the ‘cyan’ in cyanobacteria, see Figure 1). What makes cyanobacteria so interesting (besides the fact that they helped plant life evolve) is that some possess the ability to “fix” nitrogen in the water when it is in limited supply – in other words, they make it biologically available to other phytoplankton for consumption in low-nitrogen environments, such as the subtropical mid-ocean regions, which can then initiate even more biological productivity.

On the other hand, some species of cyanobacteria – such as the ones in this study – can starve other organisms of light and oxygen, and produce harmful toxins that are dangerous to other organisms, humans, and the environment. Therefore, it’s important to understand the environmental conditions that control the variability of these types of cyanobacteria at the sea surface; their magnitude and occurrence can provide vital information on both nutrient cycling and ecosystem health.

The Baltic Sea – a haven for cyanobacteria

The Baltic Sea is a semi-enclosed, highly productive marine environment with powerful interactions between different levels of the food chain that make cyanobacteria an important component of the biological community. Some studies have used fossil records as evidence for cyanobacteria having been a significant part of this ecosystem for many thousands of years; Bianchi et al. (2000) suggest that their blooms are a natural phenomenon occurring regardless of anthropogenic factors. However, these blooms have been known to cause low oxygen conditions below the blooms that are harmful to marine life (Funkey et al. 2014). Thus, learning the causes of bloom variability in environments like the Baltic Sea is essential to understanding how they will impact nutrients and ecosystem vitality.

How are cyanobacteria measured from space?!

This study uses 39 years of ocean surface color satellite data from 1979 to 2017 to explore the variability of Baltic Sea cyanobacteria blooms that cover as much as around 77,000 square miles (about the surface area of South Dakota) of the sea surface every year during their bloom season (July and August). Satellites detect ocean color by sensing the reflectance of the pigment of light emitted from the sea surface, which is easily detectable during large blooms.

Different phytoplankton emit a range of pigments from wavelengths of 400 to 700 nanometers (nm). This study uses satellite measurements of the “red band,” or the wavelength of the color red – around 670 nm – to detect turbidity. Turbidity, which is essentially a measure of the cloudiness of the water, provides information on the magnitude of cyanobacteria blooms by detecting high amounts of reflectance, or backscatter (reflection of light from particles back into space) – which is strong when countless cyanobacteria are floating at the surface during a bloom. While other processes can affect turbidity, the months of July and August are dominated by cyanobacteria – especially the genus Nodularia, which tends to accumulate near the surface most often, making it easily detectable by satellite (Kahru & Elmgren, 2014). Because measurements are limited to periods of clear skies and orbital coverage over the Baltic Sea, the detected cyanobacteria are normalized by the total number of observations averaged over July and August for each year – they call this the “frequency of cyanobacteria accumulations” (FCA).

Unexplained variability

Figure 2: Time series of normalized cyanobacteria (or “FCA”) from 1979 to 2017 for three regions of the Baltic Sea (from Kahru et al. 2018).

From 1979 to 2017, both decadal and interannual variability of cyanobacteria were observed. While the decadal variability correlated with environmental factors (such as temperature, sunlight, wind, and nutrients), an approximately three-year signal appears in all three study regions of the Baltic with no apparent explanation of environmental forcing – see Figure 2.

How could such a stable signal be so difficult to pinpoint for causes? The researcher suggests that this variability is likely biological rather than environmental. These biological influences have yet to be explored, but could include a combination of predator-prey interactions, relationships between cyanobacteria of different ages, or the effects of marine viruses – all of which could be influenced in some way by environmental factors. Pretty complicated stuff.

Looking forward…

This study highlights the complexity of biology in the ocean, and the many factors that can influence its variability and global effect. Little is known about marine viruses and their interactions with cyanobacteria (see our post on cyanobacteria-virus interactions here). Equally little is known about how climate change will affect not only the environmental factors that cause these populations to vary, but interactions in the marine food web that cascade up to our food supply (see our posts on climate change and cyanobacteria here and here). Studies like these, which use satellite imagery to measure the abundance of microscopic plants at the sea surface, will continue to help us detect important large-scale marine population dynamics and identify the causes for what we observe.


Other References

Carolina P. Funkey, Daniel J. Conley, Nina S. Reuss, Christoph Humborg, Tom Jilbert, and Caroline P. Slomp. 2014. Hypoxia Sustains Cyanobacteria Blooms in the Baltic Sea. Environmental Science & Technology, 48 (5), pp. 2598-2602. doi: 10.1021/es404395a.

Bianchi, T. S., E. Engelhaupt, P. Westman, T. Andren, C. Rolff, R. Elmgren. 2000. Cyanobacterial blooms in the Baltic Sea: Natural or human-induced? Limnol. Oceanogr., 45 (3), pp. 716-726.

Kahru, M. and R. Elmgren. 2014. Multidecadal time series of satellite-detected accumulations of cyanobacteria in the Baltic Sea. Biogeosciences, 11, pp. 3619-3633. doi:10.5194/bg-11-3619-2014.

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