I am a postdoc Fellow at the Mediterranean Institute for Advanced Studies working in coastal systems in the Dra. Fiona Tomas lab. My research interest is to understand how environmental factors or other ecological state changes affect the processes and mechanisms of coastal systems at different levels, from individuals to communities, and at different spatial and temporal scales. Most of my work focuses on marine macrophytes and fish communities and my research is motivated by dual goals of advancing ecological knowledge and influencing management decisions. To this end, I take an integrative perspective in physiology and ecology combining observational and experimental approaches, and modelling tools.

My current research is related to seagrass systems examining how ecological changes such as invasive species or human stressors affect them. Also, I am collaborating in a project looking at the dynamics and accumulation of microplastics in these systems.

In my previous postdoc at Florida State University I was working in the Dr. Rassweiller lab using quantitative ecology to examine the environmental and ecological drivers of change in coastal communities and also on improving methods for monitoring marine biodiversity.

In my doctoral work at the Mediterranean Institute for Advanced Studies, I studied the effects that changes in environmental factors (e.g., eutrophication, warming and acidification) have on seagrass-herbivore interactions by examining a wide range of plant responses and at different plant life stages and how some of these changes influenced herbivore behavior. During my PhD I visited different research groups and institutions collaborating with international researchers and I try to extend this collaborative spirit everywhere I work.

I enjoy sharing my research with the public and inspiring students. I have participated in outreach activities such as lab open doors and women in science events and judged regional science fairs. Also, I am a scientist you can Skype in the skype a scientist program. For more information see my list of publications and C.V.

[21] Hernan, G., E. Rodríguez, and F. Tomas (2024): Identifying patterns of microplastic accumulation in coastal vegetated habitats: A systematic review and meta-analysis, Science of The Total Environment, 954, doi.org/10.1016/j.scitotenv.2024.175985 Link [PDF]

Abstract: Microplastic (plastics < 5 mm; MP) contamination in the marine environment has gained global attention due to its continuous accumulation and serious threats to ecosystems. This review evaluates patterns of MP accumu- lation in seagrasses, mangroves, and saltmarshes to provide an integrated view of MP pollution. Since 2011, studies have examined the sources, distribution, characterization, and fate of MPs in these habitats. We found an unequal geographic distribution with most studies conducted in the Northern Hemisphere and in mangroves, which have the highest MP concentrations compared to saltmarshes and seagrass beds, particularly near urban centers and fishing zones. Almost 40 % of the outcomes of our meta-analysis show a higher MP accumulation in vegetated than unvegetated sites. Also, degraded and highly-degraded sites exhibited higher amounts of MPs than less-degraded areas. In addition, secondary MPs are the dominant form, with less dense polymers (poly- ethylene, polystyrene, and polypropylene) being more abundant and blue, black, and transparent the most common colors. Methodological differences in reporting units, sampling depths, and extraction methods reduce study comparability and increase variability. This review provides a comprehensive understanding of MP research in coastal ecosystems, revealing critical knowledge gaps affecting MP distribution, such as vegetation density, diversity, and hydrodynamics, and emphasizes the need for standardized methodologies for accurate comparisons.

[20] Toma, F., G. Hernan, J. Mañez-Crespo, A. Arona, D. Haverbeck Meléndez, X. Reynés, J. Delgado, G. Procaccini, and E. Ballesteros (2024): Mass flowering and unprecedented extended pseudovivipary in seagrass (Posidonia oceanica) after a Marine Heat Wave. Marine Pollution Bulletin. doi.org/10.1016/j.marpolbul.2024.116394 Link

Abstract: Seagrasses are marine flowering plants that create critical coastal ecosystems and are threatened by warming. Clonal expansion is generally the dominant strategy for meadow recovery, while sexual reproduction strongly differs among species (e.g., monoecious and diecious species, some creating seed banks, viviparous seedlings). In 2022, the Western Mediterranean underwent unprecedented warming, and, associated with it, we observed flowering (100 %) across 11 Posidonia oceanica meadows in Mallorca, Balearic Islands. Furthermore, 64 % of the sites also exhibited pseudovivipary, an extremely rare phenomenon in angiosperms whereby plantlets replace sexual reproductive structures, producing clones of the maternal plant. Our results support the notion that P. oceanica flowering and pseudovivipary (genetically confirmed) are triggered by warming, never before being pseudovivipary reported across multiple sites in a marine plant. Considering the negative impacts that warming can have on seagrasses, existence of widespread pseudovivipary is a critical aspect to consider for understanding mechanisms of resilience in seagrasses.

[19] McHenry, J., A. Rassweiler, G. Hernan, A.K. Dubel, C. Curtin, J. Barzak, N. Varias, S.E. Lester (2023): Geographic variation in organic carbon storage by seagrass beds. Limnology and Oceanography. doi:10.1002/lno.12343 Link

Abstract: Since seagrasses are efficient sinks for marine organic carbon, there is growing interest in incorporating seagrass protection and restoration into climate mitigation schemes, that is, offering credit for accumulated carbon to offset carbon dioxide emissions. However, patterns and drivers of organic carbon storage by seagrasses are not well resolved, especially at scales relevant to management decisions. Here, we quantified geographic variation in stand- ing stocks of sedimentary organic carbon (Mg Corg ha-1) associated with seagrasses along the northern Florida Gulf Coast using field surveys and sediment cores. We measured plant biomass, organic carbon, and sediment composi- tion in each core. Using a multivariate modeling approach, we evaluated the relative importance of ecological, physical, oceanographic, and seascape drivers, developing the first spatially explicit predictions of seagrass- associated carbon stocks for this region. Applying model predictions to confirmed seagrass beds and potential recovery areas, we also estimated the carbon storage value of potential seagrass conservation and restoration as the resulting stock enhancement value per hectare of seagrass (Δ Mg Corg ha-1). We found that organic carbon stored by seagrass sediments varied considerably across this region, with stocks significantly increasing with seagrass cover, proximity to oyster reefs, and distance from river outlets, highlighting potential synergies for coordinated management. We also found that current seagrass beds could offer nearly double the carbon storage value of poten- tial recovery areas, emphasizing the importance of conservation as well as restoration. Our results have important implications for management, restoration, and understanding biogeographic patterns of seagrass ecosystem services.

[18] Mañez-Crespo, Tomas F., Fernandez-Torquemada Y., Royo L., Espino F., Antich L., Bosch N. E., Castejón I., Hernán G., Marco-Méndez C., Mateo-Ramírez A., Pereda-Briones L., del Pilar-Ruso Y., Terrados J., Tuya F. (2022): Variation in fish abundance, diversity and assemblage structure in seagrass meadows across the Atlanto-Mediterranean province. Diversity 14 (10), 808. doi:10.3390/d14100808 Link [PDF]

Abstract: Seagrasses worldwide provide key habitats for fish assemblages. Biogeographical disparities in ocean climate conditions and seasonal regimes are well-known drivers of the spatial and temporal variation in seagrass structure, with potential effects on associated fish assemblages. Whether taxonomically disparate fish assemblages support a similar range of ecological functions remains poorly tested in seagrass ecosystems. In this study, we examined variation in the abundance, diversity (from a taxonomic and functional perspective), and assemblage structure of fish community inhabiting nine meadows of the seagrass Cymodocea nodosa across three regions in the Mediterranean (Mallorca and Alicante) and the adjacent Atlantic (Gran Canaria), and identified which attributes typifying the structure of meadows, and large-scale variability in ocean climate, contributed most to explaining such ecological variation. Despite a similar total number of species between Mallorca and Gran Canaria, the latter region had more taxonomically and functionally diverse fish assemblages relative to the western Mediterranean regions, which translated into differences in multivariate assemblage structure. While variation in the abundance of the most conspicuous fish species was largely explained by variation in seagrass structural descriptors, most variation in diversity was accounted for by a descriptor of ocean climate (mean seasonal SST), operating at regional scales. Variation in fish assemblage structure was, to a lesser extent, also explained by local variability in seagrass structure. Beyond climatic drivers, our results suggest that lower temporal variability in the canopy structure of C. nodosa meadows in Gran Canaria provides a more consistent source of food and protection for associated fish assemblages, which likely enhances the more abundant and diverse fish assemblages there.