Today we start again, and we are going to answer to questions from Aja’s last article. So, the theme is the Deep Chlorophyll Maxima with the focus on constant changes given by the complicated process of climate change occurring nowadays. 

As a reminder, the Deep Chlorophyll Maxima is a deep layer of the ocean (different depth among different oceans) in which high chlorophyll (mainly from phytoplankton) concentration is present because enough light and nutrients are available from the different layers of the water column. Other factors determining the distribution of a species to a specific depth are also growth rate and sinking velocity.

To understand the ecological role of microalgae we have to underline that these organisms are the base of the marine food web; supporting the fish stock, they are responsible for the biogeochemical cycle of carbon and many other elements in the sea. We have seen that a lot of processes are responsible for the depth of the layer and for the distribution and composition of phytoplankton communities in it. Surely climate change, being a multifactorial process, can be considered one of these having an influence, more or less, on all the other processes. 

Climate change (warming) predictions describe a shift of phytoplankton distribution in the upper layer of the sea surface. This is because the reduction of mixing processes given by the rise of temperature increase the strength and duration of the stratification of water masses. Specifically: 

• In the tropics and at mid-latitudes the decrease of water mixing reduces the nutrient supply from below. So, these water masses can be considered nutrient-limited in a climate warming perspective. 
• At high latitude, without climate warming and melting of the sea-ice, the intense water mixing generally carries the phytoplankton hundreds of meters into the darkness. In a climate warming perspective, instead, the lower intensity of the water mixing pushes the phytoplankton only in the upper layer. This is why we can consider this a light-limited system. 

The reduced vertical mixing may induce oscillation in the phytoplankton of the Deep Chlorophyll Maxima because of the difference in timescale between sinking flux of microalgae and upward flux of nutrients. This destabilization causes a reduction in total oceanic primary production (higher production only in the upper layer) and a modification in species composition and element reutilization. 

The higher primary production in the upper layers of sea water during a phytoplanktonic bloom causes also the increase of hypoxic zones because of the higher afflux of dead phytoplankton at the end of the bloom. This oxygen depleted condition may have an effect on biological system of a lot of species, considering both their behavior (e.g. migration) and physiological responses determining regional changes in species distribution and abundance. 

That’s why these events are not only limited to the sea but can affect also our life. For example, the modification in species abundance and distribution influences the fishery yields and so the economic system of fishery and our diet.

For this reason the safeguard of the ocean is not just mere environmentalism but is closely related to human uses and finances. 

Maria Bruno

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Huisman, J., Pham Thi, N. N., Karl, D. M., & Sommeijer, B. (2006). Reduced mixing generates oscillations and chaos in the oceanic deep chlorophyll maximum. Nature, 439(7074), 322–325. https://doi.org/10.1038/nature04245

Liccardo, A., Fierro, A., Iudicone, D., Bouruet-Aubertot, P., & Dubroca, L. (2013). Response of the deep chlorophyll maximum to fluctuations in vertical mixing intensity. Progress in Oceanography, 109, 33–46. https://doi.org/10.1016/j.pocean.2012.09.004