the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 11 Aug 2025
Wet and dry seasons modulate coastal coccolithophore dynamics off South-western Nigeria (Gulf of Guinea)
Abstract. Coccolithophores are calcifying unicellular phytoplankton at the base of the marine food web, playing a key role in pelagic calcium carbonate production. While their sensitivity to environmental change is well established, their ecological importance in tropical coastal systems remains underexplored, particularly along the African coastline. Here, we present the first multi-seasonal assessment of living coccolithophore communities off Lagos, southwest Nigeria, in the Gulf of Guinea. Periodic sampling was conducted at three coastal stations from December 2018 to April 2021 to evaluate species composition, standing stocks, diversity, and ecological drivers. Coccolithophore abundances showed clear seasonal patterns, with significantly higher (p < 0.05) standing stocks and diversity during the wet season. Total abundances ranged from 0.3 × 10³ cells L⁻¹ in the dry season to 5.5 × 10³ cells L⁻¹ in the wet season, with Gephyrocapsa oceanica dominating dry periods and Emiliania huxleyi prevailing during the wet season. Seasonal changes were linked to the migration of the Inter-tropical Convergence Zone (ITCZ), which modulates precipitation and current direction along the Gulf of Guinea. Interestingly, chlorophyll-a concentrations appeared decoupled from coccolithophore abundance, suggesting other phytoplankton groups may drive primary productivity in this region. Despite regional differences in oceanographic settings, the observed standing stocks fall within the global range of coastal coccolithophore assemblages, supporting the hypothesis that these communities are shaped by a set of common ecological constraints. As tropical coastal regions al-ready face multiple pressures from climate change, projected southward shifts of the ITCZ could alter precipitation regimes and current dynamics, with potential implications for coccolithophore community composition and coastal biogeochemical cycling.
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CC1: 'Comment on egusphere-2025-3201', Xiaobo Jin, 13 Aug 2025
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AC1: 'Reply on CC1', Michael Grelaud, 19 Sep 2025
We thank Dr. Jin for the insightful comment regarding the alternating occurrence of E. huxleyi and G. oceanica in our study. We agree that there are similarities with the succession patterns revealed by sediment traps in the South China Sea (Jin et al., 2019, 2022). In both systems (considering stations TJ-G and TJ-E in Jin et al., 2019), G. oceanica co-occurs with diatoms (e.g., December in Jin et al., 2019, and the dry season in our study), followed by E. huxleyi dominance (e.g., late February in Jin et al., 2019, and the wet season in our study). We will include the suggested references in the revised manuscript (Section 4.2, “Phytoplankton dynamics off the Nigerian coast”) when discussing the general ecological succession between diatoms and coccolithophores in coastal systems (l. 292–294 of the original manuscript).
We also agree that silicate availability plays a key role in shaping diatom presence. In our study, silicate is the only nutrient that shows a significant seasonal difference (Fig. 3, Table S4), while all other nutrients remain relatively stable and quite high across seasons (Fig. 3, Tables S3–S4). Since G. oceanica has strong affinities with high-nutrient conditions (Andruleit et al., 2003; Andruleit & Rogalla, 2002), its occurrence should remain relatively consistent at our study site, in line with the year-round stability of most nutrients, which is not the case.
Moreover, there are notable differences in timing and amplitude between the two systems. In the South China Sea, the alternation occurs mainly during winter (December–March; Jin et al., 2019), reflecting a sequential bloom, with low coccolithophore production during the rest of the year. In contrast, in the Gulf of Guinea, coccolithophores are present year-round, with abundances peaking during the wet season (Fig. 2). This alternation follows the large-scale seasonal cycle driven by the latitudinal displacement of the ITCZ, which strongly influences the regional oceanography. All this together, along with the direct influence of the ITCZ on regional oceanography, supports our interpretation that the observed seasonal assemblages are not only the result of local nutrient – phytoplankton interactions, but they likely reflect broader hydrographic controls, with wet and dry season communities being shaped by distinct water mass influences.
Citation: https://doi.org/10.5194/egusphere-2025-3201-AC1
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AC1: 'Reply on CC1', Michael Grelaud, 19 Sep 2025
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RC1: 'Comment on egusphere-2025-3201', Anonymous Referee #1, 23 Aug 2025
The manuscript, "Wet and dry seasons modulate coastal coccolithophore dynamics off South-western Nigeria (Gulf of Guinea)" by Falilu Adekunbi et al., presents a valuable dataset acquired from three coastal stations between 2018 and 2021. The study's objective is to enhance our understanding of living coccolithophore dynamics, particularly in coastal environments, by examining their relationship with environmental variables and oceanographic settings in the Gulf of Guinea. The authors effectively use a statistical analysis approach to highlight and clarify the key relationships between taxa and environmental variables.
The data acquisition is described in detail, and the findings are well-illustrated with an adequate number of high-quality figures. The references are up to date.
All my comments and suggestions are intended to improve the quality of the paper by clarifying specific sections of the text. I recommend the manuscript for publication after minor revision.
Minor Suggestions
Hyphenation: Several instances of hyphenated words appear to be incorrectly divided across lines (e.g., line 29). The authors should carefully check the entire text to correct all such instances.
Absence of Holococcolithophores: Lines 78-79 state, "Only heterococcolithophores were identified in all the samples (15 taxa, Table S1), as no holococcolithophore could be observed." While this is an interesting result, the authors do not provide an explanation for the absence of holococcoliths in the studied sites. Without this clarification, a reader (such as myself) might question the sampling strategy. For instance, the storage conditions mentioned in line 110 ("The seawater samples for coccolithophore community composition were stored shortly (<5 hours)") are critical and could be a contributing factor (e.g., exposure to sunlight or high temperatures). The authors should address this to rule out potential sampling issues and provide a more complete scientific explanation.
Citation: https://doi.org/10.5194/egusphere-2025-3201-RC1 -
AC2: 'Reply on RC1', Michael Grelaud, 19 Sep 2025
We thank the Anonymous Referee #1 for their constructive remarks and positive feedback.
The issue of hyphenated words originates from text formatting and will be corrected in the revised version.
Regarding the absence of holococcolithophores, we are confident that this is not related to sampling strategy. Standard protocols for coccolithophore analysis were followed, with samples processed within 5 hours of collection, and both light and scanning electron microscopy were used, minimizing the likelihood of missing holococcolithophores (liths or spheres) if they had been present.
We will clarify in the revised manuscript (section 4.2 “Phytoplankton dynamics off Nigerian coast”) that their absence is most likely ecological. Holococcolithophores, representing the haploid phase of coccolithophores, are typically associated with stable, nutrient-poor open-ocean conditions. In contrast, the eutrophic and dynamic coastal waters off Lagos are not limiting for the heterococcolithophores (diploid phase), making holococcolithophores unlikely to occur in our samples (Guerreiro et al., 2023; Penales et al., 2025).
Guerreiro, C. V., Ferreira, A., Cros, L., Stuut, J. B., Baker, A., Tracana, A., Pinto, C., Veloso, V., Rees, A. P., Cachão, M. A. P., Nunes, T., & Brotas, V. (2023). Response of coccolithophore communities to oceanographic and at-mospheric processes across the North and Equatorial Atlantic. Frontiers in Marine Science, 10, 1119488. doi:10.3389/fmars.2023.1119488
Penales, P. J. F., Skampa, E., Dimiza, M. D., Parinos, C., Velaoras, D., Pavlidou, A., Oikonomou, V. A., & Triantaphy-llou, M. V. (2025). Coccolithophore Assemblage Dynamics and Emiliania huxleyi Morphological Patterns During Three Sampling Campaigns Between 2017 and 2019 in the South Aegean Sea (Greece, NE Mediterrane-an). Geosciences, 15(7), 268. doi:10.3390/geosciences15070268
Citation: https://doi.org/10.5194/egusphere-2025-3201-AC2
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AC2: 'Reply on RC1', Michael Grelaud, 19 Sep 2025
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RC2: 'Comment on egusphere-2025-3201', Anonymous Referee #2, 11 Sep 2025
The manuscript “Wet and dry seasons modulate coastal coccolithophore dynamics off South-western Nigeria (Gulf of Guinea)“ by Adekunbi et al. explores the coccolithophore dynamics at the Nigerian coast (Gulf of Guinea) during the contrasting environmental conditions related to dry and wet seasons. Overall, the manuscript is very well written, data analysis and presentation are of high quality, and the dataset (both coccolithophore abundances and environmental data) is of interest for the wider coccolithophore research community. I recommend the manuscript for publication in Biogeosciences and suggest some revisions that I believe would improve the quality of the manuscript:
General comment
- There are instances of incorrect hyphenation/line breaks that should be corrected, e.g. Lines 29, 391, 392, and several cases in the References.
Introduction: A final paragraph stating the aims, hypotheses and impacts/relevance of the study is missing in the introduction section. I suggest the authors develop the final introduction paragraph from the sentence starting “In this study, for the first time…” (Line 63).
Materials and methods
- Line 73: incorrect unit format “m.s.-1”.
- In the Study area subsection, the authors discuss the months that belong to wet or dry season and argue that the duration of the seasons has changed in recent years. They should also state in the Methods which system was used to define wet or dry seasons in this study. This is shown in Table S6, but it should also be clarified in the Methods section. Was the Wet/Dry distinction predefined, or was it based on actual precipitation data from the studied period? If the latter is the case, it should be further clarified in the Methods section.
- Line 103: Change the salinity unit to PSU, also in Fig. 3B.
- Samples for SEM analysis were collected, but no SEM images or SEM data are shown. Were the 15 species identified only under Polarized LM analysis, or were they also observed by SEM? If there is SEM data available confirming the taxonomic identification of coccolithophore species found under LM analysis, consider adding these images to the supplement and listing any additional species observed under SEM. If SEM data is not included in the manuscript, perhaps you can remove the mention of sampling for SEM and SEM sample preparation from the Methods section altogether.
- Line 126: word “young” should be removed?
- Line 129: “2.4. Carbonate chemistry variables” should be removed
- Line 164: “2.8. Statistical analyses” should be removed
- Line 165-166: unclear sentence without a full stop.
Results
- Consider adding a supplementary figure showing LM images of 15 species observed during the analysis of the coccolithophore community if the images are available.
- Data from Table 2 would be easier to read if presented as a Figure/plot. Abbreviations for station labels should be provided in the caption rather than referring to the Table 1.
- Figure 3 caption: should begin with “Environmental parameters…”
- It is unclear whether the subsection “3.3. Seasonal differences” provides new data or summarizes environmental data from the previous section. If the latter is the case, I suggest you incorporate seasonal differences in coccolithophore community composition and environmental variables into previous subsections that are focusing on the respective topics.
- Figure 4: Caption is inadequate. Also, consider rearranging the three PCA plots the same way as in Fig. 2 (underneath each other) and making them larger and more readable. I also suggest you use different color coding for species names and environmental drivers in the PCA figures (if technically possible).
- Figure 6. Typo – Barents Sea is written wrong (“Barentz”)
Discussion
- Some parts of the first subsection of the discussion (“4.1. Factors modulating…”) read more like the results section, providing novel results and referring to tables that were not referred to or reported in the Results section. Consider revising these parts in order to have better distinction between Results and Discussion.
- Line 339: Consider that massive blooms of Emiliania huxleyi are regularly reported in the Barents Sea (e.g. Giraudeau et al 2016, ). Are any large scale coccolithophore blooms reported from the Gulf of Guinea? The standing stocks may be generally comparable, but the two systems are arguably very different when it comes to coccolithophore dynamics.
Citation: https://doi.org/10.5194/egusphere-2025-3201-RC2 -
AC3: 'Reply on RC2', Michael Grelaud, 22 Sep 2025
The manuscript “Wet and dry seasons modulate coastal coccolithophore dynamics off South-western Nigeria (Gulf of Guinea)” by Adekunbi et al. explores the coccolithophore dynamics at the Nigerian coast (Gulf of Guinea) during the contrasting environmental conditions related to dry and wet seasons. Overall, the manuscript is very well written, data analysis and presentation are of high quality, and the dataset (both coccolithophore abundances and environmental data) is of interest for the wider coccolithophore research community. I recommend the manuscript for publication in Biogeosciences and suggest some revisions that I believe would improve the quality of the manuscript:
We appreciate and thank the Anonymous Reviewer #2 for their very supportive comments and insightful feedback towards the improvement of our manuscript.
General comment
- There are instances of incorrect hyphenation/line breaks that should be corrected, e.g. Lines 29, 391, 392, and several cases in the References.
The issue of hyphenated words originates from text formatting and will be corrected in the revised version.
Introduction: A final paragraph stating the aims, hypotheses and impacts/relevance of the study is missing in the introduction section. I suggest the authors develop the final introduction paragraph from the sentence starting “In this study, for the first time…” (Line 63).
The end of the final paragraph of the introduction has been revised and extended to include the aims and relevance of the study as follow:
“In this study, we present the first periodic monitoring of coccolithophore communities and associated environmental parameters in coastal waters off Lagos, Nigeria, from December 2018 to April 2021, encompassing both the wet and dry seasons, characteristic of the region. The objectives were (1) to assess seasonal variability in coccolithophore abundance and community structure, and (2) to identify the environmental factors driving these dynamics, with the overarching goal of advancing our understanding of the ecology of this key group of marine primary producers in the Gulf of Guinea.”
Materials and methods
- Line 73: incorrect unit format “m.s.-1”.
We will change it for the correct format (m.s-1) in the revised manuscript.
- In the Study area subsection, the authors discuss the months that belong to wet or dry season and argue that the duration of the seasons has changed in recent years. They should also state in the Methods which system was used to define wet or dry seasons in this study. This is shown in Table S6, but it should also be clarified in the Methods section. Was the Wet/Dry distinction predefined, or was it based on actual precipitation data from the studied period? If the latter is the case, it should be further clarified in the Methods section.
We agree that a clarification is needed here. Indeed, the Wet/Dry season distinction was predefined in order to combine the “historical” seasonality (Nwankwo et al., 1996; Dry: November – April; Wet: May – October) and the fact that this seasonality has been shifted with the wet season that can extend up to November (Fasona et al., 2019). This will be clarified in the revised manuscript (last paragraph of the section 2.1 Study area) as follow:
“The rainfall in the Lagos coastal environment is characterized by a double maximum pattern where according to Nwankwo (1996) the dry season commences in November and extends until April while the wet season is from May to October with a break in August (Fig. 1). However, climate change has resulted already in local shifts in rainfall pattern, intensity and frequency, consequently the wet season now extends to November reaching a peak in July and September (Fasona et al., 2019). To take into account this shift, in this study the dry season is defined from December to April while the wet season is defined from May to November.”
- Line 103: Change the salinity unit to PSU, also in Fig. 3B.
The salinity unit will be changed to PSU in both main text and figure in the revised manuscript.
- Samples for SEM analysis were collected, but no SEM images or SEM data are shown. Were the 15 species identified only under Polarized LM analysis, or were they also observed by SEM? If there is SEM data available confirming the taxonomic identification of coccolithophore species found under LM analysis, consider adding these images to the supplement and listing any additional species observed under SEM. If SEM data is not included in the manuscript, perhaps you can remove the mention of sampling for SEM and SEM sample preparation from the Methods section altogether.
SEM was used indeed to confirm the species identified through light microscopy. SEM pictures of all the identified species will be added in a new section of the supplementary material word file as Figure S1. Moreover, reference to Fig. S1 will be added in the revised manuscript in section “3.1 Coccolithophore distribution”.
Figure S1 caption:
“Scanning electron Microscope (SEM) photomicrographs of coccolithophore off Lagos coastal waters A) Emiliania huxleyi, B) Gephyrocapsa oceanica, C) Gephyrocapsa ericsonii, D) Calciopappus rigidus, E) Umbilicosphaera hulburtiana, F) Syracosphaera histrica, G) Discosphaera tubifera, H) Calsiosolenia corselii, I) Michaelsarsia elegans, J) Syracosphaera tumularis, K) Syracosphaera sp., L) Umbellosphaera tenuis, M) Florisphaera profunda.”
- Line 126: word “young” should be removed?
The word "young" will be removed in the revised manuscript.
- Line 129: “2.4. Carbonate chemistry variables” should be removed
It will be removed in the revised manuscript.
- Line 164: “2.8. Statistical analyses” should be removed
It will be removed in the revised manuscript.
- Line 165-166: unclear sentence without a full stop.
It will be removed in the revised manuscript.
Results
- Consider adding a supplementary figure showing LM images of 15 species observed during the analysis of the coccolithophore community if the images are available.
Unfortunately, LM images of coccolithophores are not available, however, SEM images will be provided as Figure S1 of the supplementary material.
- Data from Table 2 would be easier to read if presented as a Figure/plot. Abbreviations for station labels should be provided in the caption rather than referring to the Table 1.
The data of Table 2 are now visually presented in Figure S2 of the supplementary material. Reference to Fig. S2 will be made in the revised manuscript in the last paragraph of the section 3.1 Coccolithophore distribution. Moreover the abbreviations will be clarified in the caption of Table 2 (BLA: Beyond Lagos Anchorage, LA: Lagos Anchorage, LBW: Lagos Break-Water).
Figure S2 caption:
“Average seasonal diversity index (top) and species richness (bottom) for three stations (BLA: Beyond Lagos Anchorage, LA: Lagos Anchorage, LBW: Lagos Break-Water) and all stations together (All)”.
- Figure 3 caption: should begin with “Environmental parameters…”
It will be corrected in the revised manuscript.
- It is unclear whether the subsection “3.3. Seasonal differences” provides new data or summarizes environmental data from the previous section. If the latter is the case, I suggest you incorporate seasonal differences in coccolithophore community composition and environmental variables into previous subsections that are focusing on the respective topics.
The section “3.3 Seasonal differences” presents the results of the statistical analysis that were carried out on our dataset. To make it clearer, we will change the title of the section for “3.3 Statistical assessment” in the revised manuscript. However, we prefer to keep this section separated from sections 3.1 and 3.2 which are more descriptive and separated between the coccolithophores and the environmental parameters, while the section 3.3 highlights the significant seasonal differences for all the parameters (biotic and abiotic) providing the base for our interpretations in the discussion.
Moreover, following another comment raised by the reviewer about the 1st section of the discussion (“4.1. Factors modulating the coccolithophore community off the Nigerian coast”; see below), we briefly introduce, in section 3.3, the results of the Pearson correlations performed between the environmental parameters and the total abundances of E. huxleyi, G. oceanica, coccolithophores, Shannon index and richness (Table S5) as well as the Principal Component Analysis (PCA):
“[…] Similar differences were observed for the stations LA and BLA while the costal station LBW didn’t show any significant difference between the two seasons. Finally, the results of the Pearson correlations (Table S5) suggest that precipitations is the only environmental parameter showing significant correlation with the total coccolithophore abundances, and to a lesser degree with the abundances of E. huxleyi and G. oceanica. Interestingly the highest correlation is observed between the abundances of G. oceanica and sea temperature during the wet season, which is not highlighted by the results of the PCA (Fig. 4).”
- Figure 4: Caption is inadequate. Also, consider rearranging the three PCA plots the same way as in Fig. 2 (underneath each other) and making them larger and more readable. I also suggest you use different color coding for species names and environmental drivers in the PCA figures (if technically possible).
We apologize for this mistake regarding the caption of Figure 4: The caption should read:
“Figure 4: Results of the principal component analysis (PCA) between the selected environmental variables, selected coccolithophore species and the diversity index, performed for the whole study (purple), the dry season (brown) and the wet season (blue). Samples distribution is shown for the whole study for both the dry (red) and wet (blue) seasons.”
The figure 4 has been reorganized vertically as suggested by the reviewer however, we could not change the colour for the species names.
- Figure 6. Typo – Barents Sea is written wrong (“Barentz”)
The Figure 6 has been corrected.
Discussion
- Some parts of the first subsection of the discussion (“4.1. Factors modulating…”) read more like the results section, providing novel results and referring to tables that were not referred to or reported in the Results section. Consider revising these parts in order to have better distinction between Results and Discussion.
All the tables mentioned in the section 4.1 were introduced in the section 3.3 Statistical assessment (former section “3.3 Seasonal differences”), except for the Table S5. This has been addressed in a previous comment. Now the results of the tables S3, S4 and S5 are presented in the revised section 3.3 as well as some results of the PCA (Fig. 4).
- Line 339: Consider that massive blooms of Emiliania huxleyi are regularly reported in the Barents Sea (e.g. Giraudeau et al 2016, ). Are any large scale coccolithophore blooms reported from the Gulf of Guinea? The standing stocks may be generally comparable, but the two systems are arguably very different when it comes to coccolithophore dynamics.
To date, no coccolithophore bloom has been reported in the Gulf of Guinea. We acknowledge that the systems compared in this section are very different from one to another. This is what we were suggesting by saying: “Despite very different oceanographic and climatic settings represented in the global dataset we extracted […], coccolithophore standing stocks off Lagos […] are of the same order of magnitude as those found in other coastal environments such as the South Pacific Ocean […], the Barents Sea […], or the Mediterranean Sea […].”
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AC3: 'Reply on RC2', Michael Grelaud, 22 Sep 2025
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The authors presented seasonal changes in living coccolithophore community off the coastal Nigeria. They found E. huxleyi dominated in wet seasons, and G. oceanica in dry seasons. This finding is interesting, with significant implications for coccolithophore ecology and biogeography.
Here I would present some comments for such alternating occurrences of the species. The authors found higher chlorophyll-a and silicate concentrations in the dry seasons, which may suggest a dominance of diatom in the phytoplankton community. Therefore, I speculate that the occurrence of E. huxleyi may result from their competition with diatoms. E. huxleyi cannot outcompete against diatoms in the dry seasons, and they would be found in the wet seasons. In contrast, G. oceanica may be a more efficient nutrient assimilator, and they would be co-exist with diatoms. A similar case can be found in the South China Sea as reported in Jin et al. (2019, JGR-BG) and Jin et al. (2022, JGR-BG). The biogeography and seasonal production succession of E. huxleyi and G. oceanica can be governed by their competition with diatoms.