Seasonal and interannual variations in coccolithophore abundance off Terceira Island, Azores (Central North Atlantic)

Áurea Narciso,Francesca Gallo,André Valente,Mário Cachão,Lluïsa Cros,Eduardo B Azevedo,Joana Barcelos E Ramos

doi:10.1016/j.csr.2016.01.019

Abstract

In order to characterize the natural coccolithophore community occurring offshore Azores and to determine their annual and interannual patterns, monthly samples were collected, from September 2010 to December 2014, in the photic zone off Terceira Island.The present study revealed a clear seasonal distribution and a considerable interannual variability of the living coccolithophore community. The highest coccolithophore abundances were observed during spring and winter months, especially due to the smaller species Emiliania huxleyi and Gephyrocapsa ericsonii. In fact, the highest biomass period was registered during April 2011, associated with enhanced abundance of the overcalcified morphotype of E. huxleyi, which was possibly influenced by subpolar waters and subsequent upwelling conditions. The highest abundances of Gephyrocapsa muellerae were recorded during June 2011 and 2014, indicating that this species characterizes the transition between the period of maximum productivity and the subsequent smoother environmental conditions, the first and the later stages of the phytoplankton succession described by Margalef, respectively. During summer to early fall, a gradual decrease of the overall coccolithophore abundance was observed, while the species richness (Margalef diversity index) increased. A subtropical coccolithophore assemblage mainly composed by Umbellosphaera tenuis, Syracosphaera spp., Discosphaera tubifera, Rhabdosphaera clavigera and Coronosphaera mediterranea indicated the presence of surface warmer waters accompanied by reduced mixing and low nutrients concentration. During late fall to winter, the coccolithophore abundance increased again with a concomitant reduction in species diversity. This is potentially linked to low sea surface temperatures, moderate nutrients concentration and surface mixed layer deepening. During 2011, colder and productive waters led to an increase in the total coccolithophore abundances. On contrary, during 2012, characterized by milder environmental conditions, an increase in the diversity of the community was prevalent. The noticeably coccolithophore response to variations in the main physical variables and on main traditional nutrients corroborates the importance of this group as proxy of marine environmental conditions in the past.

Highlights

Coccolithophores are pelagic unicellular algae, members of the haptophyte class Prymnesiophyceae Hibberd, distinguished by the ability to produce calcite platelets called coccoliths, which surround the living cell and form an exoskeleton called coccosphere (e.g. Winter and Siesser, 1994; Young, 1994)
An overall warming occurred from June to October, with surface temperatures between 16 and 24 °C, followed by a surface water cooling to a minimum of 14.5 °C and subsequent mixing and enhanced nutrient supply in the photic zone
This study revealed that the coccolithophore community noticeably responds to variations in main physical temperature and salinities properties of the water column, mainly due to seasonality, as well as on the chemical variations on main traditional nutrients including silicates

Summary

Introduction

Coccolithophores are pelagic unicellular algae, members of the haptophyte class Prymnesiophyceae Hibberd, distinguished by the ability to produce calcite platelets called coccoliths, which surround the living cell and form an exoskeleton called coccosphere (e.g. Winter and Siesser, 1994; Young, 1994). Produce dimethyl sulfide (DMS), which stimulates cloud formation, actively participating in the climate system (Berger et al, 1989; Westbroek et al, 1993; Rost and Riebesell, 2004). Since this calcifying group has potential to fossilize as calcareous nannoplankton, it becomes a (paleo)environmental proxy directly dependent on temperature, salinity, and nutrients and light availability (e.g., McIntyre and Bé, 1967; Giraudeau et al, 1993; Winter and Siesser, 1994). Coccolithophores usually dominate the phytoplankton biodiversity in the oligotrophic central gyres of the oceans (e.g. McIntyre and Bé, 1967; Winter et al, 1994; Ziveri et al, 2004), being markedly present at temperate (e.g. Giraudeau and Bailey, 1995; Cachão et al, 2000) and high latitude eutrophic regimes (e.g. Andruleit, 1997; Baumann et al, 2000)

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