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| 26 Feb 2024
Planktic foraminifera assemblage composition and flux dynamics inferred from an annual sediment trap record in the Central Mediterranean Sea
Abstract. The Sicily Strait, located in the central Mediterranean Sea, represents a key point for the regional oceanographic circulation as it is considered the sill that separates the western and eastern basins. Therefore, it is considered a unique zone regarding the well-documented west-to-east Mediterranean productivity gradient. Here we document the planktic foraminifera assemblages retrieved by the C01 sediment trap between November 2013 and October 2014. 19 samples from the sediment trap deployed at a water depth of around 400 m have been used. More than 3700 individuals and 15 different species have been identified. Globorotalia inflata, Globorotalia truncatulinoides, Globigerina bulloides, Globigerinoides ruber and ruber (pink) were the five main species identified, accounting for more than 85 % of the total foraminifera.
The total planktic foraminifera flux mean value was 630 shells m-2 d-1, with a minimum value of 45 shells m-2 d-1displayed during late autumn 2013 and a maximum of 1890 shells m-2 d-1 reached during spring 2014. Most of the species fluxes followed a similar pattern. This is likely the result of the regional oceanographic configuration and the marked seasonality in the surface circulation. During spring and winter, the Atlantic waters dominate the surface circulation, bringing cool and nutrient enriched waters. This results in a planktic foraminifera flux increase and a dominance of western basin taxa. During summer and autumn, the circulation is dominated by the eastern warm and oligotrophic Levantine water, which results in planktic foraminifera flux decrease and the dominance of easter basin species. Our comparison with satellite derived SST and chlorophyll-a data showed that G. inflata was associated with cool and nutrient rich conditions, while both G. ruber morphotypes were associated with warm and oligotrophic conditions. However, no trends were identified for G. truncatulinoides or G. bulloides.
In addition, a comparison of the Sicily Strait data with other Mediterranean time series located in the Alboran Sea, Gulf of Lions and the Levantine basin was carried out. Our data indicated that the annualized planktic foraminifera flux was lower than in the westernmost Alboran Sea but higher than in the easternmost Levantine basin. However, the Sicily Strait species diversity was the highest among the compared zones, highlighting the influence of the different basins and its transitional aspect from a planktic foraminifera population perspective.
Finally, we compared the sediment trap planktic foraminifera assemblage with the assemblages from seabed sediment located in the vicinity of the Sicily Strait. Our results showed that the sediment trap population significantly differed from the assemblages in the seabed sediment. The deep-dwelling species dominated the sediment trap samples, while eutrophic and oligotrophic species were significantly more abundant in the core-tops, highlighting a potential effect of the recent Mediterranean environmental change, such as warming and a potential shift in the oceanographical conditions on the planktic foraminifera population.
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RC1: 'Comment on egusphere-2023-3101', Anonymous Referee #1, 15 Apr 2024
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Publisher’s note: this comment was edited on 25 April 2024. The following text is not identical to the original comment, but the adjustments were minor without effect on the scientific meaning
Béjard et al. utilized one year of sediment trap data from the Sicily Strait to characterize the surface water planktonic foraminiferal ecology. They subsequently compared their findings with sediment trap data from both the eastern and western Mediterranean basins, demonstrating enhanced biodiversity in the central Mediterranean. Additionally, they compared their single sediment trap sample with core-top sediments from the vicinity of their site, concluding that there has been a shift towards cooler, deeper dwelling communities since the late Holocene as an adaptation to ongoing warming.
While I acknowledge the authors’ characterization of the Mediterranean as a 'miniature ocean' and recognize the significance of publishing foraminifera datasets from this relatively understudied sea, I maintain skepticism regarding the reliability of the results as an ecological signal. This skepticism arises primarily due to the presence of benthic specimens in the sediment trap, which allows for the interpretation of results influenced by hydrodynamics. In essence, the results represent a mixed signal of both ecology and hydrodynamics. For instance, the increased occurrence of deep-dwelling specimens may be attributed to the preferential settling of heavier specimens and the winnowing of lighter ones at the trap site. Similarly, the resemblance of the assemblage to that of the eastward core top sample could be a result of sediment winnowing from the predominantly westward flowing LIW water mass (200 m to 400 m) at the intermediate trap collection depth.
More specifically, it has been shown that planktonic foraminifera calcify in accordance with their habitat depth, with species dwelling at deeper levels producing heavier shells compared to those inhabiting surface waters (Zarkogiannis et al., 2022). For instance, G. truncatulinoides typically generates among the heaviest shells (see previous), as does G. inflata when compared to G. ruber and G. bulloides (Feldmeijer et., 2013). Consequently, certain current speeds may favor the settling of specific species specimens while others are dispersed elsewhere, potentially explaining the observed counts. In the central Mediterranean, G. bulloides is more prevalent in sediments (and thus resuspended sediment) than G ruber, contributing to a simultaneous peak in benthic foraminifera counts. Hydrodynamics may therefore account for discrepancies in cases where specific environmental drivers cannot be identified for certain species, leading to the invocation of other environmental controls in the text to explain the observed patterns. Indeed hydrodynamics in the area are particularly strong especially in spring (Gasparini et al., 2004), while sediment resuspension in the wider area is found to peak in spring (Grifoll et al., 2019) as well. Data from Copernicus also indicate increased flow speeds at 400m during spring of 2014.
MARGO site areas should be checked for sedimentation rates, as regions with high sedimentation will likely experience reduced hydrodynamics, facilitating the settling of lighter, surface-dwelling specimens. Additionally, for any inferences regarding ecosystem shifts in the Mediterranean, the sediment traps in other locations should be compared with nearby sedimentary material. Certainly, a pan-Mediterranean comparison should evaluate whether the data from the current study from the Sicily strait (area of high velocities) should be considered or disregarded.
Furthermore, in a future submission please change planktic to planktonic. The correct adjective form of plankton in Greek is planktonic. The adjectives of Greek nouns ending in -on get the suffix -ic in the end like plankton – planktonic, bion – bionic, lacon – laconic (preserved also in French words like Napoleon – Napoleonic). This is different to nouns ending in -os, which lose the ending -os to the previous consonant by replacing it with -ic, like bentos – benthic, cosmos – cosmic or chronos – chronic.
Zarkogiannis SD, Iwasaki S, Rae JWB, Schmidt MW, Mortyn PG, Kontakiotis G, Hertzberg JE and Rickaby REM (2022) Calcification, Dissolution and Test Properties of Modern Planktonic Foraminifera From the Central Atlantic Ocean. Front. Mar. Sci. 9:864801. doi: 10.3389/fmars.2022.864801
Feldmeijer, W., B. Metcalfe, P. Scussolini, and K. Arthur (2013), The effect of chemical pretreatment of sediment on foraminifera-based proxies, Geochem. Geophys. Geosyst., 14, 3996–4014, doi:10.1002/ggge.20233.
Gasparini, G. P., D. A. Smeed, S. Alderson, S. Sparnocchia, A. Vetrano, and S. Mazzola (2004), Tidal and subtidal currents in the Strait of Sicily, J. Geophys. Res., 109, C02011, doi:10.1029/2003JC002011.
Grifoll, M., Cerralbo, P., Guillén, J., Espino, M., Hansen, L. B. and Sánchez-Arcilla, A. (2019). "Characterization of bottom sediment resuspension events observed in a micro-tidal bay." Ocean Science 15(2): 307-319.
Citation: https://doi.org/10.5194/egusphere-2023-3101-RC1 -
RC2: 'Comment on egusphere-2023-3101', Anonymous Referee #2, 25 Apr 2024
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Dear
The editorial support team
Copernicus Publications
EGUsphere
I hereby you receive my report on the MS “Planktic foraminifera assemblage composition and flux dynamics inferred from an annual sediment trap record in the Central Mediterranean Sea” by Thibauld M. Béjard et al.
The authors analysed sediment trap data located in the eastern part of Sicily Channel spanning from November 2013 to October 2014. These data are compared with satellite-derived chlorophyll-a and Sea Surface Temperatures (SSTs). Moreover, the authors compared the sediment trap data with other sediment trap stations located in the Alboran Sea, Gulf of Lion and in eastern Mediterranean and also with coretop data from MARGO database closed to the study area. It is important to remark that in the Mediterranean, data on planktonic foraminifera from sediment traps and from plankton tow or multinet plankton samples are relay scarce. So that the short time series analysed by the authors is very important for the knowledge of planktonic foraminiferal ecology and also to use these information to interpret correctly the geological records.
The manuscript is well written and well organised. In my opinion the sediment trap data support the proposed ecological interpretation.
Comments:
In terms of missing references:
The sediment trap site is located in the same area analyzed by K. Schroeder, J. Chiggiato, S. A. Josey, M. Borghini, S. Aracri & S. Sparnocchia (2017). Rapid response to climate change in a marginal sea. Scientific Reports | 7: 4065 | DOI:10.1038/s41598-017-04455-5. It is very strange that this reference is not reported in the submitted manuscript.
In addition, concerning the oceanography of the Mediterranean is not reported as reference Pinardi et al., (2015). Mediterranean Sea large-scale low-frequency ocean variability and water mass formation rates from 1987 to 2007: A retrospective analysis. Progress in Oceanography 132 (2015) 318–332.
and in the following paper (Garcia-Solsona et al., 2020) one of the study station is very close to the sediment trap location. Maybe you could find additional information for the submitted manuscript.
Garcia-Solsona E., Pena L. D., Paredes E., Pérez-Asensio J.N., Quirós-Collazos L., Lirer F., Cacho I., (2020). Rare Earth Elements and Nd isotopes as tracers of modern ocean circulation in the central Mediterranean Sea. Progress in Oceanography, 185. https://doi.org/10.1016/j.pocean.2020.102340 (7) -
Chapter 3.3:
The authors report line 218 that: G. bulloides includes G. falconensis [(this is my interpretation when I see in the text Globigerina bulloides (+ G. falconensis)]. Maybe my interpretation is wrong. But, if it is right my interpretation, the question is that in the text (Table 1, or line 363) the authors consider separately these two species. Please try to control, if necessary.
In the plate is missing the picture of Turborotalita quinqueloba. Just a question: did you find only Globigerinoides with chambers lobulate (trilobus) or also with elongate and sac-like (sacculifer), and if you found both, what is the abundances of these morphotypes? Are the present in the same seasons?
Chapter 3.4:
Line 238-239: it means that the SST data in the same site of sediment trap are also from mooring line data. Is it wright? In several figures of the submitted manuscript, are from satellite derived or from data in situ the SST data? In is not clear for me.
Chapter 4.3:
Please in Fig. 3, report the planktonic foraminiferal species according to the % abundances: 1) G. inflata, 2) G. truncatulinoides, 3) G. bulloides, 4) G. ruber pink, 5) G. ruber white, 6) O. universa, 7) G. rubescens…….
I do not understand why the authors decided to plot in Fig. 3 the planktonic foraminiferal data also in %. I think that the authors have to plot only the flux (shells). In many cases the strong differences in shells, between the different months, produced an altered % signatures. The impact on % abundances it is possible to observe on G. truncatulinoides signal. According to the ecological niche of G. truncatulinoides is an indicative species of deep vertical mixing during the winter season, so that it is very strange to observe high abundance % of this species during the summer season of sediment trap data. Is there an explanation of this discrepancy? In fact, if you consider only the flux signals, G. inflata and G. truncatulinoides are in phase according to their ecological preferences.
Moreover, the Gulf of Lion sediment trap data, during winter season, document that the high abundance of G. truncatulinoides results almost in phase with high abundance of G. bulloides. And it has sense. In your record, this relation is not evident. Is there is an explanation for this discrepancy? The ascended of G. truncatulinolides to the euphotic zone, where it proliferates due to strong advection of nutrients from the nutrient-rich deeper layers and consequently high primary productivity could be supported by the increase in abundance of nutrient rich species G. bulloides. In the Sicily Channel this relation seems not documented, probably related to the occurrence of other oceanographic influence. Have you an idea?
Can the authors explain the criteria adopted for the season’s boundaries? I think that the spring season has to start with sample 5-april and not before and also 1-june is summer and not Spring.
Chapter 5.1:
I am very curious if in terms of benthic foraminifera, the authors found only these two species or is there a diversified benthic assemblage? If they found a diversified benthic assemblage, the identifies species are related to the same environment. Did you consider this issue?
In addition, the authors considered the benthic species a result of resuspended sediments process. The question is as follows: In the seasons where you find benthic species, did you also find altered assemblages in planktonic foraminifera (also change in size and/or not well preserved planktonic foraminifera)? I think that this is important to verify the reliability of the samples containing benthic foraminifera. The sediment trap is located ca. 400 meter deep and I can image that the suspended foraminifera could be related to strong LIW activity. Is it possible?
Chapter 5.4:
I have several doubts concerning the possibility to compare the sediment trap data with information from coretops from MARGO database. The age of these coretops is strongly different from the analysed sediment trap short time series. In fact, the authors reported that in seabed sediments Globigerina bulloides represents the main species.
Data on planktonic foraminifera (Margaritelli PhD thesis, 2016, Perugia, Italy) from a special gravity core system SW104, that allows the recovery of undisturbed and well-preserved water–sediment interface, show that the main species over the last 100 century (according to radionuclides chronology published in Margaritelli et al. 2020) is first Globorotalia inflata followed by Glibogerinoides ruber white variety, while G. bulloides represents the third abundance species. This discrepancy with coretops MARGO data is mainly related to the low resolution chronology of the coretops due to the missing of radionuclides ages (in my opinion). It means that you compare present day data with a mean signal over the latest part of the Holocene.
Incarbona et al 2019, analysed the planktonic foraminifera over the last four centuries and it is evident that the Sicily Channel is a complex system (from west to the east part). Anyway, the analyzed site (Site 342) in Incarbona et al. (2019) shows over the last century, high abundance values of G. ruber, G. bulloides and last G. inflata, conversely, Site 407 shows high abundance of G. inflata, G. ruber and last G. bulloides. The chronology of these sites is based on radionuclides ages and these data seem to support that the comparison with MARGO database could be questionable. Did you consider also the sites published in Incarbona et al 2019 (Paleoceanography and Paleoclimatology https://doi.org/10.1029/2018PA003529)?
I am reporting these data to stress the fact that the coretops database is useful to reconstruct glacial/interglacial sea surface temperature but, in my opinion, it is very difficult to use this database when comparing with sediment trap record. If the coretops data have a strong chronological control, obviously the comparison could be possible. Is the chronology of the MARGO coretops based con radionuclide ages?
The proposed interpretation concerning the discrepancy between sediment trap data and MARGO database could work but I am convincing that the comparison is questionable due to the different chronologies of the compared records.
General questions:
In Mallo et al. (2017) the authors reported that the most common size fraction of living planktonic foraminifera from bongo net is 150–350 µm and in the Sicily Channel they found the highest percentages of > 500 µm tests. In the submitted manuscript did you find the same size intervals from sediment trap record? And if no, could you suggest an explanation on this issue?
Concerning the size fraction used for the analysis, >150 micron, useful also related to MARGO database, it is clearly evident that according to this choice you did not consider in planktonic assemblage most of T. quinqueloba, G. glutinata and N. incompta. Did you try to consider the impact of this choice on the planktonic foraminiferal diversity, mainly related to productivity signal?
Please consider to use the term Sicily Channel and not Strait
Fig. 5: I think that for the reader it is much clearer if close to the terms Tropical/Subtropical, Temperate/Subtropical and Deep dwellers the authors report also the term group1, 2 and 3, as in the text of the manuscript.
My overall conclusion is that the manuscript is properly constructed, suitable for the journal and important for the international scientific community. In my opinion, this manuscript could be accepted for publication after moderate revision.
Citation: https://doi.org/10.5194/egusphere-2023-3101-RC2
Data sets
Béjard et al., 2023. Supplementary data. Planktic foraminifera population in a modern Central Mediterranean Sea sediment trap compared to basin-wide and seafloor assemblages Thibauld M. Béjard et al. https://doi.org/10.17632/tp4v6hm7dc.1
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