Oceanic response to Hurricane Irma (2017) in the Exclusive Economic Zone of Cuba and the eastern Gulf of Mexico

An understanding of the oceanic response to tropical cyclones is of importance for studies on climate change, ecological variability and environmental protection. Hurricane Irma (2017, Atlantic Ocean) broke many records, including the fact that it was the first category 5 hurricane making landfall in Cuba since 1924. In this study, we assess the oceanic response of the waters of the Cuban Exclusive Economic Zone (EEZ) and the eastern Gulf of Mexico (GoM) to the passage of this hurricane. Overall, Irma led to a weak sea surface cooling in the EEZ, which was associated with the thermal structure of its waters and the fact that it was affected by the left-side quadrants of this hurricane. This cooling was driven by mixing and upwelling processes. In contrast, the chlorophyll-a (chl-a) concentration increase was comparable with climatological records, suggesting that horizontal advection of coastal waters and entrainment of chl-a rich waters from remote regions of the GoM influenced the post-storm chl-a concentration. Moreover, Irma increased the chl-a concentration in the northeastern GoM and stimulated the offshore transport of these chl-a-rich waters to the interior GoM. A high chl-a plume (HCP) extended southward across the eastern GoM during the first post-storm week of Irma, and these waters reached the northwestern Cuban coast following the Loop Current. An intensification of the geostrophic currents of an anticyclonic eddy at the upper front of the Loop Current, the formation of an anticyclonic-cyclonic eddy pair in the northeastern GoM and wind-driven advection governed the extension of this HCP

Oceanic response to the consecutive Hurricanes Dorian and Humberto (2019) in the Sargasso Sea

Understanding the oceanic response to tropical cyclones (TCs) is of importance for studies on climate change. Although the oceanic effects induced by individual TCs have been extensively investigated, studies on the oceanic response to the passage of consecutive TCs are rare. In this work, we assess the upper-oceanic response to the passage of Hurricanes Dorian and Humberto over the western Sargasso Sea in 2019 using satellite remote sensing and modelled data. We found that the combined effects of these slow-moving TCs led to an increased oceanic response during the third and fourth post-storm weeks of Dorian (accounting for both Dorian and Humberto effects) because of the induced mixing and upwelling at this time. Overall, anomalies of sea surface temperature, ocean heat content, and mean temperature from the sea surface to a depth of 100 m were 50 %, 63 %, and 57 % smaller (more negative) in the third-fourth post-storm weeks than in the first-second post-storm weeks of Dorian (accounting only for Dorian effects), respectively. For the biological response, we found that surface chlorophyll a (chl a) concentration anomalies, the mean chl a concentration in the euphotic zone, and the chl a concentration in the deep chlorophyll maximum were 16 %, 4 %, and 16 % higher in the third-fourth post-storm weeks than in the first-second post-storm weeks, respectively. The sea surface cooling and increased biological response induced by these TCs were significantly higher (Mann-Whitney test, p < 0.05) compared to climatological records. Our climatological analysis reveals that the strongest TC-induced oceanographic variability in the western Sargasso Sea can be associated with the occurrence of consecutive TCs and long-lasting TC forcing

Spatio-temporal variability of oceanographic conditions in the Exclusive Economic Zone of Cuba

In this work we assessed the spatio-temporal variability of the oceanographic conditions in the period 1998–2017 in four sectors of the Exclusive Economic Zone (EEZ) of Cuba, referred to as the northwest, northeast, southwest and southeast sectors (i.e., NWS, NES, SWS and SES, respectively). We relied upon remote-sensing observations of wind speed, sea surface temperature (SST), sea surface height anomaly (SSHA) and chlorophyll-a (chl-a) concentration along with model predictions of mixed layer depth (MLD), nitrate concentration and net primary production (NPP). We found evidence of the spatially heterogeneous oceanographic conditions of the EEZ of Cuba, since the analysed variables fluctuated spatially with meridional and zonal gradients. Overall, the southern sectors were warmer and less productive than the northern ones, thereby showing the influence of the Caribbean Sea. The NWS showed the highest variability of the physical and biological variables analysed, given the convergence of the Caribbean and the Gulf of Mexico waters and the Loop Current variability. The NES was strongly influenced by coastal conditions given its limited area and long coast line. Besides, a significant decadal increase in SSHA was linked to that in SST. Still, this warming trend was not reflected in the chl-a concentration and NPP trends, which is consistent with the fact that there has been no change over the studied 20 years in mean wind regime and MLD. Furthermore, the strong El Niño Southern Oscillation events of 1997–1998 and 2015–2016 appear to have considerably impacted the surface chl-a concentration, which was partially governed by the variability of the MLD

The Impact of Hurricanes on the Oceanographic Conditions in the Exclusive Economic Zone of Cuba

In this work, we analysed the satellite-based responses of sea surface temperature (SST) and chlorophyll-a (chl-a) concentration in the waters of the Exclusive Economic Zone (EEZ) of Cuba to hurricanes that crossed the EEZ between 1998 and 2016. We considered two spatial scales to capture the spatially heterogeneous nature of the effects of hurricanes. A first more fine-grained one where we considered 120 km radius disks centered at every consecutive hurricane position within the EEZ (scale 1) and a second more coarse grained one enclosing the entire EEZ (scale 2). We conclude that the hurricanes induced a weak cooling since 75 and 85% of the SST anomalies at scale 1 and 2, respectively, were smaller than -1{\deg}C. The cooling was mainly caused by the wind, inducing mixing and/or upwelling of subsurface cool waters. The maximum chl-a responses were recorded in the first and second post-storm weeks, with 60% ranging between -0.01 and 0.04 mg m$^{-3}$ at scale 1, and between -0.07 and 0.02 mg m$^{-3}$ at scale 2. During those post-storm weeks SST and chl-a anomalies were 18 and 44% higher at scale 1 than at scale 2, respectively. We argue that the transport of chl-a from the deep chlorophyll maximum and/or the rich coastal waters are the dominant mechanisms determining the post-storm chl-a response in the EEZ. We also found that the magnitude of the Island Mass Effect in the EEZ after the passage of the hurricanes was 89% higher than before its passage.Comment: 33 pages, 14 figures. Submitted to Remote Sensing of Environmen

Variación espacio-temporal del coeficiente de atenuación de la luz en la bahía de Cienfuegos, Cuba.

In this work, 16 stations in Cienfuegos Bay were optically classified in the period from 2009 to 2014 and the spatio-temporal variation of the average attenuation coefficient of light was determined, as well as its relationship with chlorophyll a and suspended solids. The average attenuation coefficient was estimated from an empirical relationship with the Secchi depth and was compared with the corresponding ones determined by Jerlov (1976) to determine the optical type of water of each station. A predominance of C9 (more turbid) water was obtained, which was mainly reported in the mouths of the rivers, mainly in the northern lobe of the bay subjected to anthropization. The greatest variation of the attenuation was detected between the periods of drought and rain, showing the highest values in rain due to the increase of the nutrient concentration and the turbulence in the water column. The spatial variation indicated that stations 1 and 16 showed minimum and maximum attenuation values respectively due to their hydrodynamic characteristics. The relationship of the attenuation coefficient with suspended solids on the surface and in turn the attenuation coefficient with chlorophyll a showed a marked multifactorial relationship

Marine phytoplankton response to environmental stressors associated with climate change

The open ocean accounts for nearly 70% of Earth’s surface and represents the largest habitat in the biosphere. Phytoplankton, which are drifting microorganisms with the capacity to perform oxygenic photosynthesis, support life in this vast environment. Besides, they are a key component of marine ecosystems as they drive the oceanic biological pump, influence biogeochemical cycles and modulate fishing yields. However, climate change modifies the environmental drivers governing phytoplankton photosynthesis and consequently alters their productivity, diversity and community structure. Understanding the phytoplankton response to environmental stressors is mandatory to ascertain the implications of current and future climate changes on marine ecosystems in general. Important tools in this respect are remote sensing satellite observations and mathematical models. The former provide high-resolution spatial-temporal observations of key ocean variables, while the latter allow to extrapolate knowledge from the laboratory and sparse field observations to global and regional scales. Hence, the main focus of this thesis is the assessment of the marine phytoplankton response to environmental stressors associated with climate change on the basis of multi-platform datasets, i.e. satellite observations and outputs of mathematical models. More specifically, the response of dominant phytoplanktonic cyanobacteria genera on Earth (Prochlorococcus and Synechococcus) to ultraviolet (UV) radiation is investigated as well as the perturbations induced by hurricanes (strongest tropical cyclones (TCs)) on phytoplankton assemblages in several areas of the western North Atlantic Basin in the period 1998–2019. On the basis of biological weighting function (BWF)/photosynthesis-irradiance (P-E) models, we found that UV accounts for roughly two-thirds of the potential photosynthetic inhibition of Prochlorococcus and Synechococcus in the oceanic photoactive layer in the latitudinal band 40º N/S. Prochlorococcus showed a higher inhibition and integrated photosynthetic potential throughout the water column than Synechococcus, since the former is more vulnerable to UV damage at the surface and more successful at greater depths compared to the latter. On the other hand, we demonstrated that hurricanes trigger vertical and horizontal transport of phytoplankton and nutrients leading to an increased satellite chlorophyll (Chl) a concentration (a proxy for phytoplankton biomass) in the waters surrounding Cuba, the eastern Gulf of Mexico and the western Sargasso Sea. Besides, we illustrated that hurricanes drive connectivity of phytoplankton assemblages between coastal and oceanic environments. Climatological analyses showed that the strongest TC-induced Chl a increases in the western Sargasso Sea have been mainly associated with consecutive TCs as they superimpose effects on the upper-ocean response. Finally, data of phytoplankton functional types (PFTs) derived from a biogeochemical ocean general circulation model revealed that Prochlorococcus and Synechococcus respond modestly to post-storm nutrient enrichment in the tropical Sargasso Sea as compared to coccolithophores, diatoms, diazotrophs, mixotrophic dinoflagellates, and picoeukaryotes, whose concentrations increase significantly after a hurricane passage. Besides, a significant post-storm increase of the Shannon diversity index values was also observed indicating that a moderate post-storm nutrient increase in this oligotrophic area positively impacts phytoplankton diversity limiting exacerbated productivity of opportunistic species. Overall, this thesis provides a baseline against which future phytoplankton responses to environmental stressors can be evaluated. Its findings fit the needs of future studies on climate change, ecological variability, environmental protection and fisheries oceanography

The impact of hurricanes on the oceanographic conditions in the Exclusive Economic Zone of Cuba

In this work, we analysed the satellite-based responses of sea surface temperature (SST) and chlorophyll-a (chl-a) concentration in the waters of the Exclusive Economic Zone (EEZ) of Cuba to hurricanes that crossed the EEZ between 1998 and 2016 as well as the environmental drivers governing the post-storm responses. We considered two spatial scales to capture the spatially heterogeneous nature of the effects of hurricanes. A first more fine-grained one where we considered 120 km radius disks centered at every consecutive hurricane position within the EEZ (scale 1) and a second more coarse grained one enclosing the entire EEZ (scale 2). We conclude that the hurricanes induced a weak cooling since 75 and 85% of the SST anomalies at scale 1 and 2, respectively, were smaller than -1 degrees C. The sea surface cooling was mainly caused by wind-driven processes. The maximum chl-a responses were recorded in the first and second post-storm weeks, with 60% ranging between -0.01 and 0.04 mg m(-3) at scale 1, and between -0.07 and 0.02 mg m(-3) at scale 2. During those post-storm weeks SST and chl-a anomalies were 18 and 44% higher at scale 1 than at scale 2, respectively. We argue that the transport of chl-a from the deep chlorophyll maximum and/or the rich coastal waters are the dominant mechanisms determining the post-storm chl-a response in the EEZ. We also found that the magnitude of the Island Mass Effect (i.e., increase of chl-a concentration in waters surrounding islands) after the passage of the hurricanes was 89% higher in the EEZ than before its passage

Response of phytoplankton functional types to Hurricane Fabian (2003) in the Sargasso Sea

Understanding how tropical cyclones affect phytoplankton communities is important for studies on ecological variability. Most studies assessing the post-storm phytoplankton response rely on satellite observations of chlorophyll a concentration, which inform on the ocean surface conditions and the whole phytoplankton community. In this work, we assess the potential of the Massachusetts Institute of Technology marine ecosystem model to account for the response of individual phytoplankton functional types (PFTs, coccolithophores, diatoms, diazotrophs, mixotrophic dinoflagellates, picoeukaryotes, Prochlorococcus and Synechococcus) in the euphotic zone to the passage of Hurricane Fabian (2003) across the tropical and subtropical Sargasso Sea. Fabian induced a significant mean concentration increase (t-test, p < 0.05) of all PFTs in the tropical waters (except for Prochlorococcus), which was driven by the mean nutrient concentration increase and by a limited zooplankton grazing pressure. More specifically, the post-storm nutrient enrichment increased the contribution of fast-growing PFTs (e.g. diatoms and coccolithophores) to the total phytoplankton concentration and decreased the contribution of slow-growing dominant groups (e.g. picoeukaryotes, Prochlorococcus and Synechococcus), which lead to a significant increase (t-test, p < 0.05) of the Shannon diversity index values. Overall, the model captured the causal relationship between nutrient and PFT concentration increases in the tropical waters, although it only reproduced the most pronounced PFT responses such as those in the deep euphotic zone. In contrast, the model did not capture the oceanic perturbations induced by Fabian as observed in satellite imagery in the subtropical waters, probably due to its limited performance in this complex oceanographic area

Assessing the effects of ultraviolet radiation on the photosynthetic potential in Archean marine environments

In this work, the photosynthesis model presented by Avila et al. in 2013 is extended and more scenarios inhabited by ancient cyanobacteria are investigated to quantify the effects of ultraviolet (UV) radiation on their photosynthetic potential in marine environments of the Archean eon. We consider ferrous ions as blockers of UV during the Early Archean, while the absorption spectrum of chlorophyll a is used to quantify the fraction of photosynthetically active radiation absorbed by photosynthetic organisms. UV could have induced photoinhibition at the water surface, thereby strongly affecting the species with low light use efficiency. A higher photosynthetic potential in early marine environments was shown than in the Late Archean as a consequence of the attenuation of UVC and UVB by iron ions, which probably played an important role in the protection of ancient free-floating bacteria from high-intensity UV radiation. Photosynthetic organisms in Archean coastal and ocean environments were probably abundant in the first 5 and 25 m of the water column, respectively. However, species with a relatively high efficiency in the use of light could have inhabited ocean waters up to a depth of 200 m and show a Deep Chlorophyll Maximum near 60 m depth. We show that the electromagnetic radiation from the Sun, both UV and visible light, could have determined the vertical distribution of Archean marine photosynthetic organisms

Modeling the effect of ultraviolet radiation on the photosynthetic potential of Prochlorococcus and Synechococcus cyanobacteria

We used mathematical models of photosynthesis to quantify the effects of ultraviolet (UV) radiation on the photosynthetic potential of Prochlorococcus and Synechococcus marine cyanobacteria living at 0 degrees and 40 degrees N/S latitude. We show that UV is an environmental stressor for these organisms near the ocean surface, accounting for roughly two-thirds of the potential photo synthetic inhibition. Prochlorococcus showed a higher inhibition and integrated photo synthetic potential throughout the water column than Synechococcus, since the former is more vulnerable to UV damage at the surface and more successful at greater depths compared to the latter. The maximum photosynthetic activity was reached beneath the photoactive zone, largely due to the harmful effects of UVA. UV inhibition varies with latitude, due to variability in repair capacity for Synechococcus, and the existence of more diverse mechanisms of acclimation to irradiance and temperature for Prochlorococcus. The lowest photoinhibition is estimated to occur at 0 degrees latitude, since the interactive effects of high temperature and irradiance have a positive effect on photo acclimation to UV damage