A closer look at 5 decades of marine observations throws light on the trigger for plankton blooms.
The ocean has been described as the flywheel of the climate system, absorbing enormous amounts of heat during summer and releasing it back during the winter. This has a damping influence on temperature differences seasonally and from day to night. The air-sea exchange of heat is also related to turbulence in the upper ocean, which in turn controls plankton dynamics. Now scientists from PML have uncovered a previously unrecognised link between the heat flux and the timing of the spring plankton bloom.
Spring blooms occur when the abundance of microscopic algae increases dramatically over a short period of time. Such is the extent of these blooms that they can easily be spotted by sensors carried on satellites orbiting the Earth.
For half a century or more, it has been thought that bloom timing was controlled by the top thermal layer of the ocean shallowing above a critical depth, giving the perfect mixture of light and nutrients to fuel bloom growth. However, research in the past 5 years has shown that the spring bloom can precede this critical depth being reached by several weeks.
So what really controls their timing?
PML scientists used long time-series datasets collected in the western English Channel to determine when the sea began to absorb rather than emit heat, for each year over a 50 year time-frame. The day of the year this occurred allowed the biological samples to have a common time-frame of reference. The noisy biological abundance data for bacteria, phytoplankton and zooplankton could effectively be time-shifted each year to “day zero” being when the net heat flux (NHF) became positive, allowing comparisons from year to year to be made.
What became clear to the researchers was that when the NHF switches from negative to positive the total phytoplankton abundance increases rapidly. It also became clear that the NHF switch is a critical control in determining the length of the plankton growing season, phytoplankton species succession and collapse, and also the bio-diversity patterns of phytoplankton and zooplankton. Interestingly, the smallest bacterial organisms studied were not controlled by the switch between negative and positive NHF. Rather they are strongly controlled by the magnitude of the NHF. Bacterial diversity is a maximum in highly turbulent, negative NHF conditions (winter solstice) and a minimum in less turbulent, positive NHF conditions (summer solstice).
These results may give a tantalising unifying link between blooms, species succession, bio-diversity and different life-strategies.
PML's Tim Smyth, lead author on the study, points out: “The importance of this discovery is that the NHF can be determined by purely physical factors all of which are obtainable from satellite or in-situ observations. That means we can use it globally to predict with some accuracy when plankton blooms are likely to form and fade, their succession and bio-diversity patterns. This will prove to be very useful in understanding how a changing climate may alter the wider ocean ecosystem.”