Microphytobenthic community composition and primary production at gas and thermal vents in the Aeolian Islands (Tyrrhenian Sea, Italy)

https://doi.org/10.1016/j.marenvres.2016.04.009Get rights and content

Highlights

  • Total abundance was positively influenced by gas emission and high temperature.

  • Vents were characterised by low community richness and diversity.

  • Photosynthetic activity was stimulated by gas emission and high temperature.

  • Tychopelagic diatoms species were absent at the vents due to high hydrodynamics.

  • Morphological deformities and more silicified diatom frustules were detected.

Abstract

Sediment samplings were performed to investigate the microphytobenthic community and photosynthetic activity adaptations to gas emissions and higher temperature in the Aeolian Islands during a three-year period (2012–2014). Higher microphytobenthic densities were recorded at the vent stations and values were even more pronounced in relation with high temperature. The gross primary production estimates strongly depended on microphytobenthic abundance values reaching up to 45.79 ± 6.14 mgC m−2 h−1. High abundances were coupled with low community richness and diversity. Motile diatom living forms were predominant at all stations and the greatest differences among vent and reference stations were detected on the account of the tychopelagic forms. Morphological deformities and heavily silicified diatom frustules were also observed. A significant influence of the gas emission and high temperature on the phototrophic community was highlighted suggesting the Aeolian Islands as a good natural laboratory for studies on high CO2 and global warming effects.

Introduction

The Aeolian archipelago (Tyrrhenian Sea) is characterised by CO2 and temperature diverse conditions and offers a good natural study area for research on shallow benthic (Maugeri et al., 2009) and pelagic systems (Karuza et al., 2012). This type of study areas are gaining importance as the CO2 concentrations are unsustainably rising in the atmosphere and since 1970 the cumulative emissions of CO2 increased by 40%. About 60% of these anthropogenic CO2 emissions have been removed by sinks (ocean and vegetation uptake) and stored in natural carbon cycle reservoirs. The ocean alone has absorbed about 30% of the emitted anthropogenic CO2 causing ocean acidification that, based on models, will range from 0.06 to 0.32 in decrease of pH at the end of the 21st century (IPCC, 2014).

Firstly, the acidification studies were focused mainly at the calcifying organisms because of the direct effect of lower pH on their ability to maintain the external calcium carbonate skeletons (Orr et al., 2005). More attention to the effects of acidification on non-calcinated organisms followed as more complex shifts in marine ecosystem composition and function were highlighted (Fabry et al., 2008, Hall-Spencer et al., 2008, Pörtner, 2008, Riebesell, 2004). The interest towards CO2 vent areas is even higher, because they can be used as natural laboratories to study the impact of CO2 leakage from Carbon Capture and Storage (CCS) systems, a key technology for the disposal of CO2 derived from power plants and other industrial sources (Lewicki et al., 2007). The possible effects of such CO2 releases were already studied on viruses and prokaryotes (Karuza et al., 2012, Rastelli et al., 2015, Tait et al., 2015), phytoplankton (Fu et al., 2007), zooplankton (Halsband and Kurihara, 2013) and also zoobenthos (Basallote et al., 2012, Kita et al., 2013, McConville et al., 2013, Murray et al., 2013). However, even though the microphytobenthos (MPB) are useful bioindicators, responding to the conditions at the sampling site, ubiquitous and easy to sample (Desrosiers et al., 2013) the research conducted on the microalgae in relation with different pCO2 is very limited. Regarding this particular aspect, only a few studies on the MPB communities have been published (Dias et al., 2010, Johnson et al., 2013, Johnson et al., 2015, Raghukumar et al., 2008, Roleda et al., 2015).

Studies considering some functional aspects are even more limited (Wenzhöfer et al., 2000) at the CO2 vent sites. Even though primary production estimates measured in situ with the 14C technique are commonly used to have an overview of the benthic trophic state (Cibic et al., 2012, Krause-Jensen et al., 2012, Rubino et al., 2015), we found no such studies on vents to this date. Therefore, in this study we upgrade the information gained from data on MPB with the main photoautotrophic pathway, the primary production.

Laboratory and mesocosm studies of diatom growth and photosynthesis under elevated pCO2 showed very diverse responses (Gao et al., 2012). Predominately the effect on growth was stimulative (Kim et al., 2006, King et al., 2011, Low-Decarie et al., 2011, Yang and Gao, 2012) or not significant (James et al., 2014, Roleda et al., 2015) but in some cases a negative effect on growth was shown (Ihnken et al., 2011, Low-Decarie et al., 2011, Torstensson et al., 2012). Other effects were reported, like decreased silicification of diatom frustules (Mejía et al., 2013) and selection of larger diatom genera (Johnson et al., 2013) under higher pCO2, both very important responses, as diatoms represent the world's largest contributors to biosilicification. On the other hand, the dominance of smaller sized diatoms has been recorded under higher environmental temperature (Falkowski and Oliver, 2007, Winder et al., 2009).

The majority of the previous studies on the combined effects of pCO2 and temperature on any benthic or pelagic community were conducted in vitro or in mesocosms (Gao et al., 2012). In this field study we assessed the effects of high CO2 and temperature on the microphytobenthic community in the sediments of the Aeolian Islands. In particular, the aims of this study were: i) to investigate to what extent the emission of gas alone and in combination with the high temperature affects the microphytobenthic abundance and its community composition; ii) to detect a possible change in the primary production rate in relation with the gas emission and high temperature; iii) to identify other differences among microphytobenthic assemblages such as different cell size, morphological deformities or diverse degrees of silicification of the diatom frustule.

Therefore, our study gives new insights into the overall pattern of microphytobenthic community response to the extreme environment of shallow hydrothermal vents.

Section snippets

Study area

The Aeolian archipelago (Tyrrhenian Sea, Italy) is a ring-shaped volcanic arc, composed of seven islands and 10 seamounts, associated with the Peloritanian–Calabrian orogenic belt. Panarea is the smallest (3.3 km2) of the islands and it represents the emergent part of a wide stratovolcano that is more than 2000 m high and 20 km long where the subduction-related volcanic activity is still present (Tassi et al., 2009). In the early 1980's researchers began to conduct gas geochemistry surveys of

Abiotic parameters

At the bottom, seawater temperature did not vary remarkably among stations or sampling periods, showing similar values during samplings in June 2012, May 2013 and 2014 (19.0 ± 0.1, 18.0 ± 0.4 and 17.5 ± 0.5 °C, respectively) with only October 2012 reaching 23.4 ± 0.0 °C. Salinity was even less variable with an average (over stations and sampling periods) value of 37.9 ± 0.3. The %PAR was quite high, always above 22% with the average irradiance at all stations equal to 619 μE m−2 s−1.

Grain-size

Effect of grain-size and light availability on the microphytobenthic community

In this study area the grain size, particularly the sand fraction, was quite variable among stations and sampling periods. This is mainly due to the strong hydrodynamics of this area, especially at the smaller scale, responsible for the continuous changing of the sediment bottom morphology. Even though depth, grain size and light availability have been previously demonstrated as important selection factors (Cibic et al., 2007, Miles and Sundbäck, 2000), they did not show a significant role

Conclusions

Our study showed a significant influence of the gas vents with high CO2 concentration, and even more a selective response to the combined effect of high CO2 and temperature at the hydrothermal vents, on the MPB community composition and total abundance. This influence was also mirrored in increased PP rates, especially at the hydrothermal vent. Here, even though the MPB diversity was lowered, the adapted species, especially those belonging to the genus Navicula, were capable to survive in very

Acknowledgements

This work has received funding from the European Community's Seventh Framework Program (FP7/2007-2013) under grant agreement n° 265847 (“Sub-seabed CO2 Storage: Impact on Marine Ecosystems” – ECO2). We are grateful to Cinzia Comici and Annalisa Franzo for the samplings and to Federica Cerino for her help in the methodological part of this study and the review of this paper.

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