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Spatial-temporal characterisation of the surface and near-surface dynamics of the island of Santo Antão - Cape Verde
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É essencial compreender as condições de base das ilhas remotas de oceano aberto, uma vez que este conhecimento é frequentemente escasso. Esta informação é essencial para orientar as iniciativas de conservação e restauro para salvaguardar estes ecossistemas distintos e frágeis. O presente estudo foi realizado no âmbito do projeto BiodivRestore ERA-NET "COnservation of mArine ecosystems around Santo Antão, Cabo Verde: implications for policy and society" (COAST) e teve como objetivo caraterizar a variabilidade espacial e temporal dos aspetos biofísicos superficiais e sub-superficiais das águas em redor da ilha de Santo Antão. Utilizando um período de 20 anos de imagens MODIS-Aqua Level-2 com resolução espacial de 1 km para dados de Cor do Oceano (nomeadamente, Clorofila a – Chl a, Coeficiente de Extinção da Luz aos 490 nanómetros – Kd490, Radiação Disponível para a Fotossíntese – PAR, Carbono Inorgânico Particulado – PIC, e Carbono Orgânico Particulado – POC) e parâmetros do Infravermelho Térmico (nomeadamente, Temperatura da Superfície do Mar – SST), neste estudo foram realizadas análises estatísticas para avaliar a variabilidade anual, sazonal e mensal ao longo dos 20 anos. Foram ainda analisados os gradientes espaciais sazonais e mensais. Com base no ciclo sazonal da SST, foi possível definir duas estações principais: o Período de Arrefecimento, de dezembro a junho, e o Período de Aquecimento, de julho a novembro. Os resultados deste estudo mostram que, ao longo de um período de 20 anos, os parâmetros físicos das águas permaneceram estáveis, enquanto os parâmetros biópticos exibiram a maior variabilidade. As análises sazonais revelaram ainda padrões distintos entre os períodos de Aquecimento e de Arrefecimento estabelecidos. Em geral, a Chl a, o POC, o PAR e o Kd490 apresentaram concentrações mais reduzidas durante o Período de Aquecimento, enquanto a SST e o PIC comportaram-se de forma inversa. O Período de Aquecimento coincidiu com o período de diminuição dos valores da Chl a e de PAR. Dado que as águas de Santo Antão podem ser classificadas como oligotróficas, com ciclos sazonais menos pronunciados para alguns parâmetros, e com maior disponibilidade de luz, estes resultados sugerem que os blooms de fitoplâncton, e os processos e parâmetros associados, ocorrem durante eventos esporádicos de aporte de nutrientes que coincidem com episódios de afloramento, sazonalidade dos ventos ou padrões de circulação média derivados de correntes de larga escala. Foram também observados gradientes espaciais semelhantes entre a Chl a, o Kd490 e o POC, sugerindo uma forte ligação entre estes, como seria de esperar em águas do Tipo-1. Em todas as análises, as concentrações de PIC mostraram uma divergência notável em relação aos outros parâmetros biópticos, apresentando uma variabilidade distinta ao longo dos anos e dos meses, sem um padrão discernível ao longo do tempo. As análises de tendências não revelaram uma relação linear clara entre as anomalias da SST, do PAR (parâmetros abióticos) ou do PIC (parâmetro biótico) e a variável "tempo". No entanto, para as anomalias da Chl a, do Kd490 e do POC, observaram-se ligeiras tendências de aumento destes parâmetros ao longo da série temporal. Isto significa que as águas em torno de Santo Antão não sofreram aquecimento com o tempo e que a produtividade primária não diminuiu entre 2003 e 2022, desafiando as noções pré-existentes e abrindo novos caminhos para futuras investigações neste domínio. Este estudo explorou as características espaciais e temporais das águas oceânicas superficiais e sub-superficiais em torno de Santo Antão, fornecendo novos conhecimentos para a compreensão da dinâmica oceânica da região. De acordo com o conhecimento atual, este foi o primeiro estudo que utilizou tecnologias avançadas de observação remota do oceano para caraterizar a dinâmica espácio-temporal da superfície e da sub-superfície em torno da ilha de Santo Antão. Em geral, esta investigação sublinhou a necessidade de monitorização contínua para desenvolver estratégias de conservação e gestão eficazes em ecossistemas insulares remotos como os de Cabo Verde.
ABSTRACT: Understanding the baseline conditions of open ocean remote islands is essential as this knowledge is often scarce. This information is a cornerstone for guiding conservation and restoration initiatives to safeguard these distinctive and fragile ecosystems. The present study was conducted within the framework of the BiodivRestore ERA-NET "COnservation of mArine ecosystems around Santo AnTão, Cabo Verde: implications for policy and society" (COAST) project and aimed to characterise the spatial and temporal variability of surface and near-surface biophysical aspects of the waters surrounding Santo Antão Island. Using 20-year of MODIS-Aqua Level-2 imagery with 1 km spatial resolution for Ocean Colour data (namely, Chlorophyll a – Chl a, Diffuse Attenuation Coefficient Downwelling Irradiance at 490 nanometres – Kd490, Photosynthetically Available Radiation – PAR, Particulate Inorganic Carbon – PIC, and Particulate Organic Carbon – POC), and Thermal Infrared parameters (namely, Sea Surface Temperature – SST), this study conducted statistical analyses to assess annual, seasonal, and monthly variability within the time-series. The seasonal and monthly spatial gradients were also analysed. Based on the seasonal cycle of SST, it was possible to define two main seasons: the Cooling Period from December to June, and the Warming Period from July to November. This study showed that over 20 years, the physical parameters of the waters remained stable, while the bio-optical parameters exhibited the highest variability. Seasonal analyses revealed distinct patterns between the established Warming and Cooling Periods. Overall, Chl a, POC, PAR and Kd490 exhibited lower concentrations during Warming Period, while SST and PIC behaved the other way around. The Warming Period coincided with the period of decreasing Chl a and PAR values. As the waters of Santo Antão can be classified as oligotrophic, with less pronounced seasonal cycles for some parameters, and high light field, these results suggest that phytoplankton blooms, and associated processes and parameters, occur during occasional nutrient entrainment events that coincide with coastal upwelling events, wind-mixing or mean long-term circulation patterns. Similar spatial gradients were observed among the Chl a, the Kd490, and the POC suggesting a strong link between these bio-optical parameters, as it would be expected for Case-1 waters. In all analyses, PIC concentrations displayed notable divergence from other bio-optical parameters, showcasing distinct variability across years and months and no discernible pattern over time. The trend analyses revealed no clear linear relationship between SST, PAR (abiotic parameters), or PIC (biotic parameter) anomalies, and the variable "time". However, Chl a, the Kd490, and POC anomalies showed a slight upward trend over the 20 years. This means that the waters around Santo Antão have not warmed over time, and primary productivity has not decreased between 2003 and 2022, challenging pre-existing notions and opening new avenues for future research in this field. As this study explored the spatial and temporal characteristics of surface and near-surface ocean waters around Santo Antão, new insights were provided for understanding the region's ocean dynamics, highlighting the study's significance. To the best of knowledge, this was the first study that used advanced remote ocean-observing technologies to characterise the spatial-temporal surface and near-surface dynamics around Santo Antão Island. Overall, this research underscored the necessity of continuous monitoring for effective conservation and management strategies in remote island ecosystems like Cape Verde.
ABSTRACT: Understanding the baseline conditions of open ocean remote islands is essential as this knowledge is often scarce. This information is a cornerstone for guiding conservation and restoration initiatives to safeguard these distinctive and fragile ecosystems. The present study was conducted within the framework of the BiodivRestore ERA-NET "COnservation of mArine ecosystems around Santo AnTão, Cabo Verde: implications for policy and society" (COAST) project and aimed to characterise the spatial and temporal variability of surface and near-surface biophysical aspects of the waters surrounding Santo Antão Island. Using 20-year of MODIS-Aqua Level-2 imagery with 1 km spatial resolution for Ocean Colour data (namely, Chlorophyll a – Chl a, Diffuse Attenuation Coefficient Downwelling Irradiance at 490 nanometres – Kd490, Photosynthetically Available Radiation – PAR, Particulate Inorganic Carbon – PIC, and Particulate Organic Carbon – POC), and Thermal Infrared parameters (namely, Sea Surface Temperature – SST), this study conducted statistical analyses to assess annual, seasonal, and monthly variability within the time-series. The seasonal and monthly spatial gradients were also analysed. Based on the seasonal cycle of SST, it was possible to define two main seasons: the Cooling Period from December to June, and the Warming Period from July to November. This study showed that over 20 years, the physical parameters of the waters remained stable, while the bio-optical parameters exhibited the highest variability. Seasonal analyses revealed distinct patterns between the established Warming and Cooling Periods. Overall, Chl a, POC, PAR and Kd490 exhibited lower concentrations during Warming Period, while SST and PIC behaved the other way around. The Warming Period coincided with the period of decreasing Chl a and PAR values. As the waters of Santo Antão can be classified as oligotrophic, with less pronounced seasonal cycles for some parameters, and high light field, these results suggest that phytoplankton blooms, and associated processes and parameters, occur during occasional nutrient entrainment events that coincide with coastal upwelling events, wind-mixing or mean long-term circulation patterns. Similar spatial gradients were observed among the Chl a, the Kd490, and the POC suggesting a strong link between these bio-optical parameters, as it would be expected for Case-1 waters. In all analyses, PIC concentrations displayed notable divergence from other bio-optical parameters, showcasing distinct variability across years and months and no discernible pattern over time. The trend analyses revealed no clear linear relationship between SST, PAR (abiotic parameters), or PIC (biotic parameter) anomalies, and the variable "time". However, Chl a, the Kd490, and POC anomalies showed a slight upward trend over the 20 years. This means that the waters around Santo Antão have not warmed over time, and primary productivity has not decreased between 2003 and 2022, challenging pre-existing notions and opening new avenues for future research in this field. As this study explored the spatial and temporal characteristics of surface and near-surface ocean waters around Santo Antão, new insights were provided for understanding the region's ocean dynamics, highlighting the study's significance. To the best of knowledge, this was the first study that used advanced remote ocean-observing technologies to characterise the spatial-temporal surface and near-surface dynamics around Santo Antão Island. Overall, this research underscored the necessity of continuous monitoring for effective conservation and management strategies in remote island ecosystems like Cape Verde.
Descrição
Dissertação de Mestrado, Estudos Integrados dos Oceanos, 23 de setembro de 2024, Universidade dos Açores.
Palavras-chave
Santo Antão (Cape Verde) Ocean space and time variability Surface and near-surface dynamics Spatial patterns Climate studies
Contexto Educativo
Citação
PACHECO, Cassandra dos Santos. (2024). Spatial-temporal characterisation of the surface and near-surface dynamics of the island of Santo Antão - Cape Verde. Horta: Universidade dos Açores, 2024, 155 p. Dissertação de Mestrado em Estudos Integrados dos Oceanos. Disponível em http://hdl.handle.net/10400.3/8977