Abstract
It is widely accepted that the Mediterranean is an oligotrophic sea where winter mixing favors the proliferation of diatoms and high values of zooplanktonic biomass, mainly associated with the growth of copepods. Stratified conditions from mid-spring to late autumn are dominated by the picophytoplanktonic groups and the increment of cladoceran abundances. This general picture has important exceptions. A regionalization of the Mediterranean Sea can be established, distinguishing oligotrophic and mesotrophic areas and different blooming periods. The RADMED monitoring program covers a large area from the southwestern limit of the Mediterranean to the Catalan Sea. The analysis of phyto and zooplankton time series extending from 1992 to 2016 in some cases, and from 2007 to 2016 in others, have shown that the Spanish Mediterranean waters have differentiated areas and trophic regimes as a result of the existence of several fertilizing mechanisms which include winter mixing, tidal mixing in the Strait of Gibraltar, cyclonic circulation cells and frontal systems. The present work describes these different mechanisms acting on the Spanish Mediterranean waters, and also the potentiality of monitoring programs for providing statistics suitable for operational activities or the initialization/validation of ecological models.
Highlights
The oceans play a key role in the current climate change scenario
Intense cold and dry winds during winter produce the mixing of the upper water column supplying nutrients to the photic layer and producing a homogeneous vertical distribution of nutrients and phytoplankton abundance [47,48]
Our long-term seasonal patterns show that diatom abundances increase during winter or spring and this phytoplankton bloom is followed by a zooplankton bloom in spring-summer, which would agree with the increase of zooplanktonic abundances during summer previously reported [49]
Summary
The oceans play a key role in the current climate change scenario. From a physical point of view, they have stored more than 90% of the heat absorbed by the Earth since the mid twentieth century [1]and deep water formation processes are capable of transferring large amounts of CO2 to the deep ocean because of the higher solubility of gases in cold winter waters [2]. From a biological point of view, wind-driven upwelling processes, mixing of the upper water column caused by winter stormy activity, and deep water convection inject nutrients to the photic layer increasing the primary production. A fraction of this new production eventually reaches the deep waters being transformed into CO2 and nutrients by the action of nitrifying bacteria. In this way, the so called solubility and biological pumps contribute to the sequestration of CO2 [3] absorbing about 30% of the human CO2 emissions during past decades [4,5].
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