My research is rooted in the marine sciences, with a particular focus on marine palynology and paleoceanography. I am especially interested in dinoflagellate cysts—my preferred group of palynomorphs—which I use as key proxies to reconstruct past marine productivity and environmental conditions from the sedimentary record. These reconstructions are enriched by integrating other microfossil groups, such as pollen, freshwater algal spores, and fungal remains, to provide a more comprehensive picture of past ecosystems. Lately, I am also exploring the potential of ciliate palynomorphs (loricae and cysts) as past indicators, being involved in studies of their distribution and ecology.
A central aspect of my work involves studying the modern distribution of dinoflagellate cysts along environmental gradients. This allows us to better understand their ecological preferences and to develop robust reference datasets for interpreting sedimentary signals. Because these resistant cysts are formed as part of the dinoflagellates’ life cycle, their presence in marine sediments often reflects, to a certain extent, the spatial distribution of their planktonic stages. Sediment cyst assemblages may accurately reflect the prevailing environmental conditions when they were formed.
In addition to paleoenvironmental reconstructions, I investigate how physical oceanographic processes—such as currents, internal waves, and water column stratification—influence the transport, distribution, and accumulation of dinocysts in both coastal and open ocean settings. This dual approach, linking modern dynamics with sedimentary archives, helps bridge present and past oceanographic conditions.
My primary research area is the Atlantic Iberian Margin (AIM), with a special focus on coastal ecosystems such as the Galician Rías. However, I have also contributed to research projects in other regions, including the northwest African upwelling system off Cape Blanc and the Canadian Arctic.
Dinoflagellate cysts (left) observed in sediments of the Atlantic Iberian Margin (AIM), like the Galician Rías (right).
I currently work on three main lines of research in which the dinoflagellate cyst analysis is the main study tool:
- Palaeoceanographic reconstructions from dinoflagellate cyst (fossil) records.
Dinoflagellate cysts are the resting structures formed (presumably by sexual reproduction) by the microalgae called dinoflagellates. Dinoflagellates are a major component of marine phytoplankton that greatly contributes to ocean primary productivity. Some are toxin-producers and their blooms can cause harmful events in coastal ecosystems. Organic-walled and calcareous dinoflagellate cysts are widespread in modern marine sediments and are known to have a long geological record that gives invaluable insights into the ecological and evolutionary history of the species. Cyst morphology can be recognized under light microscopy at a low taxonomic level. This, together with their high diversity and the presence of species with different environmental affinities make them an excellent palaeoenvironmental proxy, commonly used in biostratigraphy to reconstruct past marine conditions (palaeotemperatures, palaeosalinities, stratification, eutrophication, etc.).
Dinoflagellate cyst morphology can be preserved in sediments for millions of years. Sedimentary cyst records provide information on past environmental conditions, such as sea-surface temperature, salinity, productivity, eutrophication and other forms of pollution, sea level, etc., because cyst sediment assemblages reflect, to a great extent, the distribution of their respective motile stages and the prevailing environmental conditions in the upper column at the times they were produced.
Palaeoceanographic reconstructions from dinoflagellate cyst sedimentary records help to investigate how past climate and other environmental changes affected marine ecosystems (e.g., Eynaud et al., 2000; Matthiensen et al., 2001; Pospelova et al., 2005; García-Moreiras et al., 2019). To understand the impact that anthropogenic activities are having on marine ecosystems recently, it is important to know how ecosystems functioned in the past in response to natural climate variability. This baseline information is crucial in the context of current climate change, to develop management and conservation strategies for marine and coastal ecosystems.
For instance, reconstructing the history of toxic, cyst-producing dinoflagellate species from the signal they left in the sediments through their cysts can help to increase the knowledge about their past population dynamics and the (environmental) factors that can trigger their blooms (Amorim and Dale, 2006). Harmful algal blooms threaten local economic activities that depend on the marine ecosystem (such as shellfish production or tourism). The reconstruction of past harmful algae bloom dynamics would contribute to a better understanding of their future dynamics (Dale, 2001; Anderson et al., 2012, 2014) and performing prediction models and management strategies of coastal ecosystems.
2. Improving dinoflagellate cysts as a paleoceanographic tool.
The accuracy of interpreting fossil assemblages retrieved from sedimentary records relies on our comprehension of the dinoflagellates’ ecology and the processes that transform ‘dinoflagellate signals’ from surface waters into the corresponding ‘cyst signals’ embedded in the sediment. Analyzing cyst distributions in ‘surface’ sediment, assumed to reflect contemporary production conditions, in conjunction with the physical properties of the overlying surface waters has provided valuable insights into the general biogeography of prevalent taxa and trends in their environmental preferences.
This application is based on a wealth of knowledge of dinoflagellate cyst distributions in modern sediments. However, dinoflagellate communities and their response to environmental changes can vary greatly between regions, and some regions are underrepresented in such modern datasets, such as the AIM (e.g., De Vernal et al., 2020; van Nieuwenhove et al., 2020). Moreover, interpretations of the environmental signals from sediment cyst records in coastal and oceanic environments can be biased by sedimentary processes (pre- and post-depositional), such as transport by currents and resuspension and selective degradation of organic matter. Therefore reference information about modern cyst distributions-environment relationships must be obtained in the region where the palaeoceanographic reconstruction is intended, to obtain valuable reference information that may assist in the interpretation of the past sedimentary record and more accurate paleoreconstructions.
In the Atlantic Iberian margin (AIM) there was very little data on the distribution of cysts in modern sediments (Blanco, 1989, 1995; Amorim and Dale, 1998; Sprangers et al., 2004), to which my recent studies in the Ría de Vigo and W of Portugal have been added (García-Moreiras et al., 2015, 2021, 2023). However, there are still many environmental gradients to investigate, in particular, in the highly productive and hydrodynamically complex Galician Rías.
Moreover, in the AIM no sediment trap studies that provide information on the cyst production of individual species in comparison to environmental conditions were carried out. In recent decades, there has been a growing interest in utilizing sediment trap records to enhance our understanding of dinoflagellate ecology. These records, capable of capturing the seasonality in cyst production by collecting settling particulate material at regular intervals, contribute to refining our knowledge through in situ observations. In order to improve our understanding of dinoflagellate cyst ecology, we need to put efforts in investigating the effect of environmental factors on the seasonal production of cysts, something that I would like to carry out in the future -an for the first time- in the rías.
Furthermore, many cysts species show morphological plasticity. Cyst morphology (of the same cyst species) can change depending on environmental conditions. In this sense, some research has demonstrated that the length processes of a cyst species can be used to trace past salinities from stratigraphic records (Mertens et al., 2009). In the future I would like to expand this line of research to further explore the potential of different cyst morphometrics of various cyst species as (paleo)environmental indicators.
3. Cyst dispersal and bloom dynamics in coastal and oceanic systems.
Resting cysts are a survival and dispersal strategy of many dinoflagellates, that make them very successful in coastal environments. Dinoflagellate cysts are non-motile and, after being produced in the pelagic environment they act as passive particles and eventually sink to the ocean floor, where they can remain viable for a long time (could be even a century! Lundholm et al., 2019). Cysts can form in the seabed more or less extensive reservoirs that can re-establish the planktonic population by germination.
Harmful algal blooms (HAB) are a recurrent feature on the Atlantic Iberian margin (AIM) and some of these HAB-forming species and other potentially toxic species in this region are cyst producers, such as Alexandrium minutum, Gymnodinium catenatum, Lingulodinium polyedra and Protoceratium reticulatum. Few studies have demonstrated that the blooming of some of these species relies on the existence of a cyst reservoir, well in the sediments or suspended in the near-bottom nepheloid layer (e.g., Nehrin, 1996, Anderson et al., 2005). The assessment of the biological and physical mechanisms involved in the seeding and initiation of blooms (such as inoculum from cysts) is critical for coastal management.
The cyst source-to-sink trajectories may be affected by lateral transport and other oceanic processes, as it was demonstrated in some regions (e.g. Zonneveld et al., 2018; Nooteboom et al., 2019). However, in the AIM data on the dinoflagellate cyst distribution and their dynamics is very scarce. Hence, our understanding of dinoflagellates’ present-day and past ecology is rather limited.
Different hypotheses have been proposed to explain the blooms of the HAB-forming species G. catenatum in the Galician Rías, such as in situ germination of cysts and advection of shelf populations (Figueiras and Pazos, 1991; Sordo et al., 2001). Life-cycle studies during blooms in the Ría de Vigo (Figueroa et al., 2008) did not support the in-situ germination hypothesis for this species. In contrast, for Alexandrium minutum local germination of cysts accumulated in the sediments has been identified as the main mechanism of bloom formation (Bravo et al., 2010, Anderson et al., 2005). On the NW Portuguese margin, new data evidenced that suspended cysts near the bottom can form a reservoir with the potential of seeding new planktonic populations (García-Moreiras et al., 2023). The scarce studies available suggest that bloom seeding strategy is variable between species, and may change between regions/environments, thus further research on this topic is required to better understand cyst and motile population dynamics of dinoflagellates.