Responses of phytoplankton communities from the Subtropical North Atlantic Gyre to increasing CO2 concentrations and consequent carbonate chemistry changes in the ocean - Azores (ROPICO2)

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Nutrient-specific responses of a phytoplankton community: a case study of the North Atlantic Gyre, Azores

Publication . Ramos, J. Barcelos e; Schulz, Kai G.; Voss, Maren; Narciso, Áurea; Müller, Marius N.; Reis, Francisco V.; Cachão, Mário; Azevedo, Eduardo B.

Nutrient concentrations are unevenly distributed in the oceans, influencing the abundance and composition of phytoplankton communities. Even so, the dominant driving factors responsible for variability between phytoplank ton communities are still unclear. In the North Atlantic Gyre, the Azores present a good opportunity to study phytoplankton communities of oligotrophic areas that experience nutrient pulses. We followed the development of an enclosed natural phytoplankton community occurring off the coast of Terceira (Azores) and tested the effects of single (nitrate, phosphate, silicate and a mix of the trace metals Fe, Co, Cu, Mo, Zn and Mn) and combined nutri ent enrichments on phytoplankton abundance, particulate organic matter (POM) build-up, nutrient drawdown and community composition. Towards the end of the microcosm-based incubation, biomass developed dramatically (430-fold) when all the nutrients considered were added simultaneously. Importantly, the community composition at the end of the incubation was dependent on the combination of nutrients supplied, with diatoms dominating most of the treatments; coccolithophores under Phosphate + Trace Metals; and organisms with characteristics of a nitro gen fixer such as low δ15N under full nutrient enrichment. These results indicate group-specific nutrient require ments and limitations occurring near the Azores with a few taxa dominating the groups’ response to nutrient pulse

2017Journal article

Open access

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

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

In order to characterize the natural coccolithophore community occurring offshore Azores and to de-termine their annual and interannual patterns, monthly samples were collected, from September 2010 toDecember 2014, in the photic zone off Terceira Island.The present study revealed a clear seasonal distribution and a considerable interannual variability of theliving coccolithophore community. The highest coccolithophore abundances were observed during spring andwinter months, especially due to the smaller speciesEmiliania huxleyiandGephyrocapsa ericsonii. In fact, thehighest biomass period was registered during April 2011, associated with enhanced abundance of the over-calcified morphotype ofE. huxleyi,whichwaspossiblyinfluenced by subpolar waters and subsequent up-welling conditions. The highest abundances ofGephyrocapsa muelleraewere recorded during June 2011 and2014, indicating that this species characterizes the transition between the period of maximum productivity andthe subsequent smoother environmental conditions, thefirst and the later stages of the phytoplankton suc-cession described by Margalef, respectively. During summer to early fall, a gradual decrease of the overallcoccolithophore abundance was observed, while the species richness (Margalef diversity index) increased. Asubtropical coccolithophoreassemblage mainly composed byUmbellosphaera tenuis,Syracosphaeraspp.,Dis-cosphaera tubifera,Rhabdosphaera clavigeraandCoronosphaera mediterraneaindicated the presence of surfacewarmer waters accompanied by reduced mixing and low nutrients concentration. During late fall to winter, thecoccolithophore abundance increased again with a concomitant reduction in species diversity. This is poten-tially linked to low sea surface temperatures, moderate nutrients concentration and surface mixed layerdeepening. During 2011, colder and productive waters led to an increase in the total coccolithophore abun-dances. On contrary, during 2012, characterized by milder environmental conditions, an increase in the di-versity of the community was prevalent. The noticeably coccolithophore response to variations in the mainphysical variables and on main traditional nutrients corroborates the importance of this group as proxy ofmarine environmental conditions in the past.&2016 Elsevier Ltd. All rights reserved.1. IntroductionCoccolithophores are pelagic unicellular algae, members of thehaptophyte class Prymnesiophyceae Hibberd, distinguished by theability to produce calcite platelets called coccoliths, which surroundthe living cell and form an exoskeleton called coccosphere (e.g. Winterand Siesser, 1994; Young, 1994). Coccolithophores, known since thelate Jurassic (Hay, 2004), have an ocean wide distribution and re-present the most productive calcifying organisms on Earth (Honjo,1996). They also affect the ocean-atmosphere carbon dioxide exchangeand produce dimethyl sulfide (DMS), which stimulates cloud forma-tion, thus actively participating in the climate system (Berger et al.,1989;Westbroek et al., 1993;Rost and Riebesell, 2004). Since thiscalcifying group has potential to fossilize as calcareous nannoplankton,it becomes a (paleo)environmental proxy directly dependent ontemperature, salinity, and nutrients and light availability (e.g.,McIntyreand Bé, 1967;Giraudeau et al., 1993; Winter and Siesser, 1994).Coccolithophores usually dominate the phytoplankton biodiversity inthe oligotrophic central gyres of the oceans (e.g. McIntyre and Bé, 1967;Winter et al., 1994;Ziveri et al., 2004), being also markedly present attemperate (e.g.Giraudeau and Bailey, 1995;Cachão et al., 2000) and highlatitude eutrophic regimes (e.g.Andruleit, 1997;Baumann et al., 2000).Although the geographic distribution of coccolithophores hasbeen well studied over the last four decades, its seasonal dynamicsContents lists available atScienceDirectjournal homepage:www.elsevier.com/locate/csrContinental Shelf Researchhttp://dx.doi.org/10.1016/j.csr.2016.01.0190278-4343/&2016 Elsevier Ltd. All rights reserved.nCorresponding author. Present address: CIIMAR-Madeira, Caminho da Penteada105, Funchal, Portugal.E-mail address:aureanarciso@uac.pt(Á. Narciso).Continental Shelf Research 117 (2016) 43–56

2016Journal article

Open access