Basin-Scale Control on the Phytoplankton Biomass in Lake Victoria, Africa

The relative bio-optical variability within Lake Victoria was analyzed through the spatio-temporal decomposition of a 1997–2004 dataset of remotely-sensed reflectance ratios in the visible spectral range. Results show a regular seasonal pattern with a phase shift (around 2 months) between the south and north parts of the lake. Interannual trends suggested a teleconnection between the lake dynamics and El-Niño phenomena. Both seasonal and interannual patterns were associated to conditions of light limitation for phytoplankton growth and basin-scale hydrodynamics on phytoplankton access to light. Phytoplankton blooms developed during the periods of lake surface warming and water column stability. The temporal shift apparent in the bio-optical seasonal cycles was related to the differential cooling of the lake surface by southeastern monsoon winds. North-south differences in the exposure to trade winds are supported by the orography of the Eastern Great Rift Valley. The result is that surface layer warming begins in the northern part of the lake while the formation of cool and dense water continues in the southern part. The resulting buoyancy field is sufficient to induce a lake-wide convective circulation and the tilting of the isotherms along the north-south axis. Once surface warming spreads over the whole lake, the phytoplankton bloom dynamics are subjected to the internal seiche derived from the relaxation of thermocline tilting. In 1997–98, El-Niño phenomenon weakened the monsoon wind flow which led to an increase in water column stability and a higher phytoplankton optical signal throughout the lake. This suggests that phytoplankton response to expected climate scenarios will be opposite to that proposed for nutrient-limited great lakes. The present analysis of remotely-sensed bio-optical properties in combination with environmental data provides a novel basin-scale framework for research and management strategies in Lake Victoria

Climatic patterns and physical controls of chlorophyll-a in the Northeast Atlantic

Tese de Doutoramento, Física, 17 de Dezembro de 2013, Universidade dos Açores.Chlorophyll-a concentration is one of the most commonly used indicators of phytoplankton biomass, the basis of the marine food chain. Patterns of chlorophyll-a can be driven by many environmental factors, such has meteorological and oceanographic processes that regulate the available light and nutrients for phytoplankton growth. The identification of physical mechanisms responsible for observed chlorophyll-a patterns is key to predict and understand the effect of climate on phytoplankton. Satellite data, with their unprecedented spatial and temporal coverage, provides an unique source of observations to elucidate on these physical-biological interactions. This study describes the distribution and variability of chlorophyll-a in the Northeast Atlantic using satellite images of ocean colour and identifies underlying physical mechanisms using complementary satellite, in-situ and model data. Long-term changes in oceanographic properties were firstly analyzed for a better understanding of chorophyll-a variability. Simultaneous reductions in the southward flow and increases in temperature in the upper ocean (0-500 m) were observed in the eastern basin. These results support the hypothesis that the colder and fresher waters of the early 1990's were replaced by warmer and more saline waters after 2000, as a result, at least partially, of stronger subtropical inflows. Positive (and weak negative) wind stress curl was related to the reduction of southward flow providing a link between large-scale changes in atmospheric patterns and the observed warming. After the severe winter of 2009/2010, surface temperature (0-300 m) dropped to the values of 1998-2000, possibly due to an exceptional deep winter mixing. Regarding the distribution of satellite-derived chlorophyll concentration (SCHL), it was found that the climatological annual pattern was remarkably similar to the nutrient pattern resulting from the combination of the subsurface nutrient field and the mixed layer depth (MLD) field. This suggests that the supply of nutrients during winter has a control over annual SCHL in the region and that this nutrient supply is regulated not only by the depth of winter mixing, but also by the subsurface nutrient field, which in turn is related to ocean circulation and lower in subtropical waters. The seasonal variability of SCHL was different in each group of Azores islands. During winter, a gradual increase of SCHL related to shallower MLD ( 200 m) was observed in the Western Group. In spring, stronger late blooms were observed in the Western Group, possibly associated with the deep mixing. In the Eastern Group weaker spring blooms could be attributed to lower winter nutrient supply as a result of shallower MLD, but also to a reduced subsurface nutrient content due to its proximity to the Azores Front. The timing of the seasonal SCHL increase in autumn followed heat losses from the ocean, which indicates surface cooling and the start of convective mixing. Inversely, the timing of SCHL increases in winter and spring occurred during oceanic heat gain, which indicates surface warming and reductions in turbulence. Regarding interannual variability of spring bloom magnitude, a decreasing trend was observed between 1998 and 2009 in the eastern basin. Although west of Azores, stronger winter mixing was related to increased spring bloom magnitude, to the east there was no clear relation. One possible explanation is that subtropical inflows after 2000, changed the subsurface nutrient content in the eastern basin during the period of SCHL observation and caused different biological responses to winter mixing. Finally, SCHL around the islands was slightly higher than surrounding areas, which provides evidence that the Azores islands produce an island-mass effect throughout the year. This study provides observational evidence of the effect of vertical mixing and horizontal transport on the patterns of chlorophyll-a in the Northeast Atlantic. These results illustrate direct routes between climate and the basis of the marine food chain, contributing to the understanding of the transition region between the subtropical and subpolar biomes, and the physical-biological coupling in the ocean.Plano Integrado para a Ciência e Tecnologia, Eixo 3.1. – Bolsas de Apoio à Investigação Científica e Tecnológica do Programa de Apoio à Formação Avançada (FORMAC)

Florida Bay Science Program: a synthesis of research on Florida Bay

This report documents the progress made toward the objectives established in the Strategic Plan revised in 1997 for the agencies cooperating in the program. These objectives are expressed as five questions that organized the research on the Florida Bay ecosystem: Ecosystem History What was the Florida Bay ecosystem like 50, 100, and 150 years ago? Question 1—Physical Processes How and at what rates do storms, changing freshwater flows, sea level rise, and local evaporation and precipitation influence circulation and salinity patterns within Florida Bay and exchange between the bay and adjacent waters? Question 2—Nutrient Dynamics What is the relative importance of the influx of external nutrients and of internal nutrient cycling in determining the nutrient budget for Florida Bay? What mechanisms control the sources and sinks of the bay’s nutrients? Question 3—Plankton Blooms What regulates the onset, persistence, and fate of planktonic algal blooms in Florida Bay? Question 4—Seagrass Ecology What are the causes and mechanisms for the observed changes in the seagrass community of Florida Bay? What is the effect of changing salinity, light, and nutrient regimes on these communities? Question 5—Higher Trophic Levels What is the relationship between environmental and habitat change and the recruitment, growth, and survivorship of animals in Florida Bay? Each question examines different characteristics of the Florida Bay ecosystem and the relation of these to the geomorphological setting of the bay and to processes linking the bay with adjacent systems and driving change.This report also examines the additional question of what changes have occurred in Florida Bay over the past 150 years

Life on the ice-edge: Paleoenvironmental significance of the radiolarian species Amphimelissa setosa in the northern hemisphere

The high-latitude Northern Hemisphere is a key region in the global climate balance. Variations in sea-ice extent affect biological productivity, CO2 exchange and carbon drawdown. Marine proxies indicative of proximity of the ice-marginal zone are therefore essential to understand these processes. Amphimelissa setosa is nowadays a dominant radiolarian species in the Arctic basin and very abundant in the high-latitude North Atlantic. This species, now absent from the North Pacific, has been widely used as a qualitative proxy of modern and past environmental conditions in the high-latitude Northern Hemisphere. Using our new and published data on the distribution of A. setosa in plankton, sediment trap, surface sediment and downcore samples, we provide a quantitative ecological context for the occurrence of this species. We find that the optimal depth and season of A. setosa in the modern North Atlantic and the Chukchi Sea are 160 m and the late boreal summer/early fall (August–October), respectively. A regression model combining environmental variables (temperature, salinity, silicate and chlorophyll-a concentrations, apparent oxygen utilization, sea-ice) at that season and depth, are able to explain 43% of the distribution of this species in surface sediments. Based on these new findings, we conclude that the presence of A. setosa in surface sediments is closely related to high primary production in the proximity of the sea-ice and areas of ice rafting. The onset of this species started at ca. 1.5 Ma in the North Pacific, linked to a gradual cooling, increased silica availability and southward advance of the ice-margins since the Early Pleistocene. Amphimelissa setosa’s decline in this region was likely caused by the development of a quasi-permanent halocline, perennial sea-ice and depletion of silica during marine isotope stage 4. In the high-latitude North Atlantic, the relative abundance of A. setosa appears to be related to cooling and supply of dissolved silica from the continent during ice-rafting events. The comprehensive approach taken in this study suggest that A. setosa is a useful proxy to explore past variations in the ice-cover in the high-latitude Northern Hemisphere.Shirshov Institute of Oceanology | Ref. 0149-2019-000

Coccolithophores in coastal waters : Lisbon Bay, Portugal =Cocolitóforos em águas costeiras : Baía de Lisboa, Portugal

Tese de doutoramento em Biologia (Ecologia), apresentada à Universidade de Lisboa através da Faculdade de Ciências, 2009Disponível no document

Influência dos processos físicos na produção primária ao longo da costa noroeste da Península Ibérica

Doutoramento em FísicaA costa noroeste da Península Ibérica (PI) e caracterizada por intensa atividade hidrológica e biogeoquímica, resultante em importantes recursos de peixe e marisco. Neste contexto, o principal objetivo deste trabalho consiste no estudo da influência dos processos físicos na produção primária da região através de detecção remota e modelação numérica. Com este propósito, a linha de costa foi dividida em três domínios: segmentos costeiros oeste, intermédio e norte. Para cada um destes segmentos foi analisada a variabilidade do transporte de Ekman, do índice de afloramento (IA), da temperatura da superfície do mar (TSM) e da concentração de Cloro la-a (Clo-a) em termos de médias sazonais, anuais e mensais a partir de dados de detecção remota. A influência das condições meteorológicas, definidas através de padrões de circulação atmosférica (PCA), no IA e na concentração de Clo-a foi também avaliada e quantificada através de uma análise de probabilidade condicionada. Foi também implementado e validado um sistema de modelação de circulação e biogeoquímica para a área de estudo. A sua validação foi efectuada através da comparação das previsões numéricas com dados in situ e de sat elite de várias variáveis físicas e biogeoquímicas. Esta implementação foi utilizada para investigar os padrões de toplâncton durante dois eventos de afloramento costeiro distintos, observados principalmente nos segmentos costeiros oeste e intermédio. Os resultados da análise aos dados de detecção remota sugerem que o transporte de Ekman, a TSM e a concentração da Clo-a têm diferentes ciclos anuais em cada segmento costeiro, apresentando elevada variabilidade. Em geral, as condições mais favoráveis a ocorrência de afloramento ocorrem nos segmentos costeiros oeste e intermédio durante os meses de primavera-verão e, consequentemente, e observada uma lente de agua mais fria junto a costa do que ao largo, assim como níveis de Clo-a elevados. Pelo contrário, no segmento costeiro norte a TSM e elevada, devido ao aquecimento de verão na zona sudeste do Golfo da Biscaia, e a concentração de Clo-a e baixa. Durante o inverno são observados elevados níveis de Clo-a nos três segmentos costeiros, que estão relacionados com o efeito cumulativo de eventos de afloramento e de descargas fluviais, que favorecem a entrada de nutrientes na costa. Os resultados da análise de probabilidades efetuada evidenciam que ao longo do segmento oeste as relações mais fortes entre os PCA, o IA e a concentração de Clo-a foram obtidas na primavera-verão, quando padrões atmosféricos Norte, Nordeste e Noroeste ocorrem. Nos segmentos costeiros intermédio e norte verificam-se elevadas probabilidades de IA e de Clo-a quando os padrões atmosférico Este e Sul ocorrem, para as duas estações do ano referidas. O modelo numérico implementado demonstrou capacidade em reproduzir com precisão a dinâmica da região em estudo, quer a superfície, quer ao longo da coluna de agua. A análise dos resultados de modelação revela a existência de duas células de afloramento nos dois eventos estudados, sendo os padrões de toplâncton diferentes para cada evento. De acordo com os resultados, quando ocorrem ventos fortes favoráveis ao afloramento o tempo de residência na zona costeira e muito baixo, não permitindo um crescimento significativo do toplâncton. As aguas ricas em nutrientes e toplâncton são então advetadas para o largo, onde o tempo de residência e maior e existem condições favoráveis para a fotossíntese. Para eventos de afloramento fortes e observado um fluxo direcionado para o equador junto a costa ao longo da coluna de agua, enquanto para eventos mais fracos este fluxo e em direção ao pólo, promovendo a retenção do toplâncton na zona costeira. Em suma, a análise efetuada aos dados de deteção remota e de modelação numérica contribuíram para melhorar o conhecimento dos principais processos que influenciam a produção primária ao longo da costa noroeste da PI, a escala sazonal e de eventos. Como trabalho futuro destaca-se a exploração com maior detalhe da dinâmica das células de afloramento detetadas e da resposta do ecossistema a sua presença, assim como o estudo da influência de descargas fluviais na produção primária da região, tirando partido da configuração numérica desenvolvida.The northwestern coast of Iberian Peninsula (IP) is a region of great hydrologic and biogeochemical activity, resulting in important sh and shell sh resources. In this context, the main aim of this work is to study the in uence of physical processes on primary production of the region through remote sensing imagery and numerical modelling. For this propose, the coastline was divided in three domains: western, intermediate and northern coastal segments. For each coastal segment was analysed the variability of Ekman transport, upwelling index (UI), sea surface temperature (SST) and chlorophyll (Chl-a) concentration in terms of seasonal, annual and monthly averages of remote sensed data. The in uence of meteorological conditions, de ned by circulation weather types (CWTs), on UI and Chl-a concentration was also evaluated and quanti ed, through a conditional probability analysis. Afterwards, a coupled circulation and biogeochemical model was implemented and validated along the study area. Validation was performed through the comparison of numerical predictions with in situ and satellite data for several physical and biogeochemical variables. This implementation was then used to investigate phytoplankton patterns during two distinct upwelling events, mainly observed at western and intermediate coastal segments. Results from remote sensed data analysis suggest that Ekman transport, SST and Chl-a concentration had di erent annual cycles within each coastal segment and high seasonal variability. Generally, the most upwelling favourable conditions occur along the western and intermediate coastal segments during spring-summer months and consequently a lens of water colder than o shore is observed in the coastal region along with high Chl-a levels. Otherwise, at northern coastal segment high SST, related to the summer warming at the southeastern Bay of Biscay, and low Chla concentration are observed. During winter high Chl-a levels were also observed at the three coastal segments, that are related to the cumulative e ect of upwelling events and river runo , favouring the entrance of nutrients onto the coast. Results from the probability analysis show that along the western coastal segment the strongest relations between CWTs, UI an Chl-a concentration were obtained in spring-summer when weather types Northerly, Northeasterly and Northwesterly occur. At northern and intermediate segments results reveal high probabilities of UI and Chl-a when weather types Easterly and Southerly were observed, for both stations referred. The numerical model showed the capability to reproduce accurately the dynamics of the study region, either at surface or along the water column. The modelling results analysis reveals the existence of two upwelling cells for both events studied, being phytoplankton patterns di erent for each event. According to results, when upwelling favourable winds are strong, the residence time near coast is very short, not allowing signi cant phytoplankton growth. Nutrient and phytoplankton rich waters are then advected o shore, where the residence time is higher and favourable conditions for photosynthesis occur. For strong upwelling events an equatorward ow develops near coast along the water column, while for weaker events this ow is poleward, promoting the retention of phytoplankton in the coastal region. In summary, the analysis performed to remote sensed data and numerical modelling predictions, have contributed to improve the knowledge about the main processes that in uence primary production along the northwestern coast of IP, at seasonal and event scales. For future, is highlighted the exploitation in detail of the upwelling cells dynamics and the ecosystem response to their presence, as well as the study of river runo in uence in the primary production of the region, taking advantage of the numerical application developed

Drainage of deep Mediterranean Water, its transition through the Strait of Gibraltar: spatial and temporal variability

La presente tesis está organizada en cuatro capítulos en los que se estudia el flujo de agua saliente del Mar Mediterráneo a través del Estrecho de Gibraltar, para ello se han abordado varios procesos relacionados con el intercambio de aguas Atlánticas y Mediterráneas a través del Estrecho. El primer objetivo es identificar los mecanismos que influyen en la ventilación de las aguas profundas Mediterráneas residentes en el Mar de Alborán. Para ello en primer lugar se investiga el papel que juegan las estructuras de escala regional en la ventilación de aguas profundas mediante datos medidos in-situ, datos de satélite y simulaciones numéricas realizadas a tal fin. De éste estudio se deduce que la dinámica del Estrecho de Gibraltar es fundamental en la ventilación de las aguas profundas del Mar de Alborán, la cual a su vez es facilitada por la presencia casi permanente del giro anticiclónico del Mar de Alborán Occidental. Seguidamente, mediante modelos numéricos regionales y a nivel de cuenca se estudia el modo en que el forzamiento mareal interfiere no solo en la circulación en el propio Estrecho sino también en la ventilación de aguas profundas desde el mar de Alborán o en los procesos de formación de estas mismas aguas. Se concluye que la dinámica mareal debe ser resulta en los modelos a escala regional y de cuenca que pretenden reproducir algunas características relevantes del intercambio a largo término y, muy probablemente, otros importantes procesos que tienen lugar más allá del Estrecho, como la formación de agua profunda en el Golfo de León. A lo largo del tercer capítulo se ha estudiado en qué medida la marea en el Estrecho de Gibraltar afecta a los procesos biogeoquímicos de la vecina cuenca del Mar de Alborán, inusualmente productiva en comparación con el resto del Mar Mediterráneo. El resultado es una productividad primaria en el Mar de Alborán que se reproduce satisfactoriamente cuando el forzamiento mareal se incluye en un modelo regional físico‐ecológico. Más específicamente se encuentra que la diferencia entre tener o no en cuenta las mareas puede suponer hasta un 60% más de producción primaria en la cuenca Oeste de Alborán. Finalmente se realiza un intensivo análisis de datos de temperatura potencial y salinidad medidos en transeptos meridionales a lo largo del Estrecho. De éste modo aplicando un análisis de clusters se determinar cuál es el patrón espacio-temporal de los hasta seis tipos de agua que intervienen en el intercambio de flujos en el Estrecho. En particular se han detectado cuatro masas de agua Mediterráneas en el flujo saliente en la sección del contorno Este del Estrecho, a pesar de que una de ellas, puede no estar presente en determinados años. La señal de estas aguas se suaviza lentamente a medida que fluyen hacia el Oeste, y lo hacen abruptamente una vez sobrepasan el umbral de Camarinal. Según lo anterior, al Oeste del umbral sería más apropiado hablar de una única agua Mediterránea. Con respecto a la variabilidad temporal se observa que las señales estacionales son más evidentes en las aguas Atlánticas, mientras que la variabilidad interanual es más notable en las aguas Mediterráneas. Estas cuestiones ofrecen una nueva visión sobre la dinámica y las propiedades del flujo Mediterráneo de agua que abandona éste mar fluyendo a través del Estrecho