Phytoplankton are Earth’s life raft!
Near the end of this Walde Sailing video they show a red substance in water.
This is commonly known as red tide. Red tide is likely an iron-rich phytoplankton bloom. When nitrogen and iron are added in combination from sediment run-off or pollution to waters then chlorophyll a concentrations can increase 40-fold leading to diatom proliferation, and reduced community diversity.
Nutient addition like this can also lead to coral bleaching and die off.
After blooming these organisms die and sink in the water column where microbes consume them and deplete the oxygen resulting in dead zones with little sea life. A dead zone the size of Scotland in the Gulf of Oman was recently discovered by robots exploring the Arabian sea, previously unknown because the area wasn’t safe for humans to do the sampling.
Still waters and a lack of mixing with air exacerbates these dead zones especially at lower depths.
Scientists also recently concluded that the last massive extinction event in earths history was the result of anoxia.
Oxygen is the byproduct of phytoplankton and they are responsible for the bulk of atmospheric oxygen when the cycle is regulated. We don’t want them dieing off in these explosive life raft blooms!
We can minimise anoxia in our oceans while still benefitting from phytoplankon oxygen production by reducing nutrient run-off in sediment from land with techniques like simple one-rock high filter dams seeded with plants to grow in the sediment and act as biofilters. This applies not only to our oceans but also inland waters, where even our water supply is at risk from sedimentation and a reduction in water volume in fresh water reserviors we get out drinking and irrigation waters from.