Photo: Paula Salgado after defending her doctoral thesis (Autor: Paula Salgado)

Esporles, May 10, 2019. Paula Salgado has defended her doctoral thesis supervised by the doctor Gotzon Basterretxea Oyarzabal, from IMEDEA (CSIC-UIB). The event took place on May 10 at the University of Balearic Islands.

This PhD thesis aims to assess the spatial and temporal patterns of phytoplankton variability and primary production in the surface waters of the Mediterranean Sea. The study is based on the use of satellite ocean color data acquired during the period 1998-2015. Complementary datasets (e.g. sea surface temperature, climatic indices, meteorological data, and atmospheric dust) are also used to address the different specific studies included in this work.

A first study is aimed to analyze the contribution of the seasonal and non-seasonal components of phytoplankton variability at basin scale. Then, seasonality is analyzed from a phenological perspective i.e. by parameterizing the seasonal cycles using a pixel-by-pixel threshold-based algorithm. We observe that seasonality dominates variability representing up to 80% of total chlorophyll (Chl) variance in oceanic areas, whereas in shelf-sea regions high frequency variations may be dominant representing up to 49% of total Chl variance. Seasonal variations are typically characterized by a phytoplankton growing period occurring in spring and spanning on average 170 days in the western basin and 150 days in the eastern basin. Also, a positive trend in Chl biomass and an increase in the amplitude and duration of the phytoplankton growing period is observed. Changes in Chl concentration in the eastern (and more oligotrophic) basin are generally low. In this basin, the Chl peak has declined and the growing period duration has also decreased. The trends in phytoplankton Chl and phenology, estimated in this study during the 1998-2014 period, do not reveal significant overall decline/increase in Chl concentration or earlier/delayed timings of the seasonal peak on average over the entire Mediterranean Sea basin. However, large regional variations are detected, suggesting that the response of phytoplankton to environmental and climate forcing may be complex and regionally driven.

A second study addresses the regional patterns of interannual variability in satellite-derived Chl. A neural network classification, based on the Self-Organizing Maps (SOM) analysis in the time domain, is used to reveal regions of similar temporal variability of Chl in the Mediterranean Sea. Characteristic temporal patterns extracted by the SOM analysis show different scales of variation that can be related to already identified oceanographic features and biogeochemical variability in the Mediterranean Sea. Clear differences are observed between regions located in the western basin and Adriatic Sea, where rivers, winter mixing and winds are known to drive variations in primary production at regional scale and regions located in the eastern basin, represented by a large and rather homogeneous region. The study suggests that North Atlantic Oscillation (NAO) has more influence in the Chl variations occurring in regions located in the western basin whereas El Niño Southern Oscillation index (ENSO) exhibits higher impact on the central Mediterranean and eastern basin during its positive phase. Both NAO and ENSO show non-stationary coherence with Mediterranean Chl. The analysis also reveals a sharp regime shift occurring in 2004–2007, when NAO changed from positive to negative values. This shift particularly affected the winter phytoplankton biomass and it is indicative of climate driven ecosystem-level changes in the Mediterranean Sea. Our results stablish a regional connection between interannual phytoplankton variability exhibited in different regions of the Mediterranean Sea and climate variations.

A third study contained in this thesis focusses on satellite derived primary production rates in the coastal waters of the Mediterranean Sea. We observe that 20% of the overall PP in the basin comes from shelf regions (i.e. < 200m) and from that, most of it (~80%) is regenerated production. Almost 50% of this production occurs in the coastal waters of the eastern basin, whereas the western and Adriatic shelf contribute with 28 and 24% of total coastal production. High regional scale variability is observed ranging from > 350 gC m-2 yr-1 in the most productive areas, generally associated with major river discharges, to highly unproductive provinces (< 50 p gC m-2 yr-1) in the south eastern Mediterranean. The long-term PP variability was dominated by interannual variations that were inversely correlated in with sea-surface temperature and more loosely with NAO and Mediterranean Oscillation Index (MOI) climatic indices. Regionally, most coastal areas presented either non-significant or weakly declining PP trends whereas positive PP trends were observed in the Adriatic region. We classified the coastal waters into 18 alongshore regions with mean PP magnitudes varying fivefold. Two groups of coastal waters were identified; regions showing low cross-shore variability, mainly located in narrow width shelf areas, and regions showing strong cross-shore gradients, observed in wider regions with river discharges.

Approximately 50% of the global primary production takes place in the ocean and, consequently, marine phytoplankton play a fundamental role in global carbon fluxes. Studying the spatial and temporal variability of phytoplankton, as well as estimating factors that determine this variability, is essential to understand the dynamics, productivity and biogeochemical cycles of the ocean, and to anticipate the effects of climate change on the marine environment. It is expected that this PhD thesis can contribute to a better understanding of these changes in the Mediterranean Sea.

Source: IMEDEA (UIB-CSIC)