Water temperature drives phytoplankton blooms in coastal waters.

Francesca Vidussi,David Parin,Thomas Trombetta,Monique Simier,Adrianna Ianora,Sébastien Mas,Behzad Mostajir

doi:10.1371/journal.pone.0214933

Abstract

Phytoplankton blooms are an important, widespread phenomenon in open oceans, coastal waters and freshwaters, supporting food webs and essential ecosystem services. Blooms are even more important in exploited coastal waters for maintaining high resource production. However, the environmental factors driving blooms in shallow productive coastal waters are still unclear, making it difficult to assess how environmental fluctuations influence bloom phenology and productivity. To gain insights into bloom phenology, Chl a fluorescence and meteorological and hydrological parameters were monitored at high-frequency (15 min) and nutrient concentrations and phytoplankton abundance and diversity, were monitored weekly in a typical Mediterranean shallow coastal system (Thau Lagoon). This study was carried out from winter to late spring in two successive years with different climatic conditions: 2014/2015 was typical, but the winter of 2015/2016 was the warmest on record. Rising water temperature was the main driver of phytoplankton blooms. However, blooms were sometimes correlated with winds and sometimes correlated with salinity, suggesting nutrients were supplied by water transport via winds, saltier seawater intake, rain and water flow events. This finding indicates the joint role of these factors in determining the success of phytoplankton blooms. Furthermore, interannual variability showed that winter water temperature was higher in 2016 than in 2015, resulting in lower phytoplankton biomass accumulation in the following spring. Moreover, the phytoplankton abundances and diversity also changed: cyanobacteria (< 1 μm), picoeukaryotes (< 1 μm) and nanoeukaryotes (3–6 μm) increased to the detriment of larger phytoplankton such as diatoms. Water temperature is a key factor affecting phytoplankton bloom dynamics in shallow productive coastal waters and could become crucial with future global warming by modifying bloom phenology and changing phytoplankton community structure, in turn affecting the entire food web and ecosystem services.

Highlights

Ocean phytoplankton generate almost half of global primary production [1], making it one of the supporting pillars of marine ecosystems, controlling both diversity and functioning
The water temperature was positively correlated with chlorophyll a (Chl a) fluorescence in 2015 (ρ = 0.25, p-value < 0.01) but not in 2016
Based on the time-lag correlations, water temperature played a significant role in determining Chl a dynamics, especially the onset of blooms (Table 2)

Summary

Introduction

Ocean phytoplankton generate almost half of global primary production [1], making it one of the supporting pillars of marine ecosystems, controlling both diversity and functioning. Phytoplankton in temperate and subpolar regions are characterized by spring blooms, a seasonal phenomenon with rapid phytoplankton biomass accumulation due to a high net phytoplankton growth rate [2] This peak biomass of primary producers in the spring supports the marine food web through carbon transfer to higher trophic levels from zooplankton to fishes. The critical depth hypothesis is a bottom-up model based on abiotic drivers and proposes that a bloom starts when there is sufficient solar radiation and the surface mixing layer becomes shallower This change induces stratification [4,5,6,7], allowing phytoplankton to remain in the surface layer, such that their growth rates overcome their losses (i.e., mostly by zooplankton grazing). These models are based on actively mixed layers that may occur in turbulence windows (the critical turbulence hypothesis [8]) or under deep convection shutdown (the convection shutdown hypothesis [7]), allowing the phytoplankton to remain in the surface layer long enough to benefit from favorable light conditions and start a bloom

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