Resumen Se analizó la abundancia, diversidad y distribución de eufáusidos y su relación con procesos hidrodinámicos entre febrero de 2011 y octubre de 2012 en el cañón de Campeche, sur del golfo de México. Se muestrearon un total de 55 estaciones para las cuales se utilizaron redes de arrastre horizontales para capturar organismos de zooplancton y variables ambientales (temperatura, salinidad y oxígeno). Los niveles de fluorescencia del fitoplancton se registraron a diferentes profundidades. Se identificaron 24 especies de eufáusidos de los géneros Euphausia, Nematobrachion, Nematoscelis, Stylocheiron y Thysanopoda. Stylocheiron carinatum fue abundante en febrero y octubre; la etapa de furcilia fue siempre la más abundante. El análisis batimétrico permitió la regionalización en 4 zonas; la zona III (eje y cabecera del cañón) tuvo mayor diversidad de eufáusidos a 100 y 200 m de profundidad con 2.14 bits Ind-1 en febrero y 1.99 bits Ind-1 en octubre. La distribución espaciotemporal de los eufáusidos fue heterogénea en cuerpos de agua con temperaturas entre 15° y 22 °C y bajas concentraciones de oxígeno, que se distribuyeron fuera de las concentraciones máximas de clorofila-a (Chl-a) (2,127 mg m-3), y con afinidad por los frentes geostróficos entre los remolinos ciclónicos y anticiclónicos.

Abstract We analyzed the abundance, diversity, and distribution of euphausiids and their relationship with hydrodynamic processes from February 2011 to October 2012 in the Campeche Canyon in the southern Gulf of Mexico. Sampling was conducted at 55 oceanographic stations and zooplankton were captured using horizontal trawls. Environmental variables (temperature, salinity, and oxygen) and phytoplankton fluorescence were recorded at different depths. Twenty-four species of euphausiids representing the genera Euphausia, Nematobrachion, Nematoscelis, Stylocheiron, and Thysanopoda were identified. Stylocheiron carinatum was abundant in February and October, with the furcilia stage always the dominant one. Differences were observed among the 4 zones identified based on the bathymetric criteria; zone III (axis and head of the canyon) presented the highest diversity of euphausiids between 100 and 200 m with 2.14 bits Ind-1 in February and 1.99 bits Ind-1 in October. The spatial-temporal distribution of euphausiids was heterogeneous in water bodies with temperatures between 15 and 22 °C and low dissolved oxygen concentrations, which were distributed outside the maximum concentrations of chlorophyll-a (2,127 mg m-3) and had an affinity for the frontal regions between cyclonic and anticyclonic eddies.

Resumen Introducción: Bahía Salinas se encuentra embebida en la piscina de agua cálida del Pacífico Tropical Oriental (PTO) o del Este (PTE), caracterizada por aguas superficiales cálidas, poco salinas, una termoclina fuerte y poco profunda y una importante diversidad biológica. La productividad primaria de la región está influenciada por el afloramiento oceánico, el cual se presenta durante el invierno boreal como resultado del fortalecimiento de los vientos alisios. Objetivo: Estudiar la dinámica espacial y temporal de parámetros físico-químicos en siete estaciones hidrográficas en Bahía Salinas, Costa Rica, a través del análisis de datos de CTD; y relacionar los eventos cálidos y fríos con las condiciones atmosféricas presentes al medir los datos. Métodos: Se muestrearon y seleccionaron siete estaciones hidrográficas en Bahía Salinas entre agosto de 2008 y diciembre de 2014. Las variables procesadas para su análisis son temperatura, densidad, salinidad, oxígeno, clorofila-a y turbidez. Una vez procesados los datos se graficaron 42 diagramas tipo Hovmöller. Resultados: Todas las variables, excepto la turbidez, presentaron una periodicidad estacional asociada al afloramiento. En general, se observaron aguas más frías y densas, mayores concentraciones de salinidad y clorofila-a y valores menores de oxígeno disuelto durante la estación seca, cuando el afloramiento estaba activo. Por el contrario, durante la estación lluviosa las masas de agua eran más cálidas y menos densas, las concentraciones de salinidad y clorofila-a disminuyeron y los valores de oxígeno disuelto tendieron a aumentar. Conclusiones: La dinámica espacial y temporal de la hidrología en Bahía Salinas fue influenciada por los eventos de afloramiento costero. La región también presentó una variabilidad interanual, como la asociada con el ENOS. La variabilidad estacional y la interanual pueden contrarrestar sus efectos sobre los parámetros oceanográficos cuando coinciden temporalmente.

Abstract Introduction: Salinas Bay is located in the warm pool of the Eastern Tropical Pacific (ETP), characterized by warm, shallow surface waters, a strong and shallow thermocline, and an important biological diversity. The primary productivity of the region is influenced by the coastal upwelling, which occurs during the boreal winter as a result of the strengthening of trade winds. Objective: To study the spatial and temporal dynamics of physical and chemical parameters at seven hydrographic stations in Salinas Bay, Costa Rica, through the analysis of CTD data, and relate the warm and cold events to the regional atmospheric conditions present when measuring the data. Methods: Seven hydrographic stations, sampled at Salinas Bay between August 2008 and December 2014, were selected. The variables processed for analysis are temperature, density, salinity, oxygen, chl-a and turbidity. Once the data was processed, 42 Hovmöller kind diagrams were plotted. Results: All variables, except turbidity, presented a seasonal periodicity associated with the upwelling. In general, colder and denser waters, higher salinity and chl-a concentrations and lower dissolved oxygen values were observed during the dry season, when the upwelling was active. Whereas, during the rainy season water masses were warmer and less dense, salinity and chl-a concentrations decreased and dissolved oxygen values tended to increase. Conclusions: The spatial and temporal dynamics of the hydrology in Salinas Bay was influenced by the coastal upwelling events. The region also presented an interannual variability associated with ENSO. Seasonal and interannual variability can counteract their effects on the oceanographic parameters when they coincide temporally.

Resumen Introducción: Las agregaciones esporádicas de crustáceos decápodos ocurren por varias razones, y en algunos casos no se conocen los factores que las provocan. Objetivos: En esta publicación presentamos nuestras observaciones sobre un evento aparentemente no muy común, de una agregación masiva de cangrejos pelagicos Euphylax dovii. Métodos: Durante una expedición científica a Isla del Coco (5°32’34” N, 87°05’06” W), encontramos grandes cantidades de cangrejos pelágicos en las aguas superficiales en las noches del 6 al 7 de mayo, 2015. Colectamos algunos individuos para identificarlos antes de liberarlos nuevamente. Resultados: Identificamos cangrejos de ambos sexos, pero ninguna hembra en estado ovígero. Los cangrejos se agregaron alrededor de una pequeña panga que utilizamos para realizar actividades de muestreo científico entre las 4pm y las 8 pm cada tarde, y alrededor de nuestro buque de investigación durante la noche, presumiblemente atraídos por las luces. En ambas ocasiones, la agregación disipó antes del amanecer. No se avistaron cangrejos en las noches anteriores o posteriores. Conclusiones: Las temperaturas superficiales marinas en Isla del Coco durante este periodo fueron altas, sin termoclina hasta al menos 40 m, coincidiendo con el inicio del evento El Niño de 2015. Parece que la agregación se formó en respuesta a las condiciones del Niño, pero las razones biológicas dicha agregación no se conocen.

Abstract Introduction: Sporadic aggregations of decapod crustaceans can occur for various reasons and in some cases their drivers are poorly understood. Objective: Here, we present our observations of an apparently rare massive aggregation of pelagic swimming crabs, Euphylax dovi. Methods: During a research cruise at Cocos Island (5°32’34” N, 87°05’06” W), we encountered large numbers of E. dovii in surface waters in the evenings of May 6-7, 2015, some of which we collected for identification prior to returning to the water. Results: Crabs of both sexes were identified, yet no females were bearing eggs. Crabs aggregated around a small dinghy used for scientific surveying from 4pm through 8pm each evening, and around our research vessel throughout the night, presumably attracted by the lights. On both occasions, the aggregation dissipated around dawn. Crabs were not seen prior to or after the two nights. Conclusions: Sea surface temperatures at Cocos during this period were markedly warm, with no perceptible thermocline down to at least 40 m, coinciding with the onset of the 2015 El Niño event. It appears that the aggregation formed in response to El Niño conditions, yet the biological reason behind it remains unknown.

RESUMEN En los lagos tropicales, los cambios estacionales en la dinámica del plancton son mínimos e irregulares, ya que las condiciones climáticas son bastante estables durante todo el año, en la mayoría de los casos solamente estudios a largo plazo permiten observar patrones en la dinámica del plancton. El lago de Río Cuarto es un lago meromícto tipo maar localizado en las llanuras del norte de Costa Rica, en el pasado se han realizado estudios limnológicos no mayores a un año de muestreo, en este trabajo presentamos datos de la composición del fitoplancton y del zooplancton a lo largo de cuatro años, del 2013 al 2017, con el fin de analizar cambios en la comunidad del plancton en un periodo de tiempo mayor, asociados a variables ambientales. Para el fitoplancton se tomaron muestras subsuperficiales y se fijaron con lugol, y para el zooplancton se hicieron arrastres verticales de 20 m con una red con 64 µm de poro, que se preservaron con formalina y luego se pasaron a alcohol de 95 %. El fitoplancton incluyó 54 taxa, la mayoría fue Chlorophyta, seguidos por Cyanobacteria y Bacillariophyta. Las taxa más comunes fueron: Synechococcus spp., Aphanocapsa sp. Coenochloris fotii, Chlorella sp. y Discostella stelligera. La abundancia total del fitoplancton disminuyó de 2013 a 2017, principalmente debido a una disminución en las Cyanobacteria. El zooplancton estuvo compuesto por 14 taxa: 5 cladóceros, 4 copépodos, 1 díptero y 4 rotíferos. Los más abundantes fueron: Keratella americana, Brachionus falcatus, Eubosmina tubicen y Arctodiaptomus dorsalis. Se observó un cambio en la diversidad y abundancia del zooplancton, al principio (2013-2014) K. americana y E. tubicen se alternaron en dominancia, y luego (2015-2016) A. dorsalis, Diaphanosoma sp. y Ceriodaphnia sp. fueron dominantes, con un regreso de K. americana en el 2017. Los cambios observados en el periodo de estudio parecen estar relacionados con cambios en la condición del lago, pues la termoclina se hundió y el lago se volvió más trasparente durante el estudio. La abundancia, de rotíferos y del cladócero Eubosmina tubicen, disminuyó asociada a una disminución de Cyanobacterias. Esos cambios no siguieron un ciclo anual, más bien parecen cambios que ocurren a largo plazo, tal y como se ha propuesto en la literatura sobre lagos tropicales.

Abstract In tropical lakes, seasonal changes in plankton dynamics are minimal and irregular, since climatic conditions are fairly stable throughout the year, in most cases only long-term studies allow observing patterns in the dynamics of the plankton. Lake Río Cuarto is a maar-type meromic lake located in the plains of Northern Costa Rica, in the past there have been no limnological studies more than a year of sampling, in this work we present data on the composition of phytoplankton and zooplankton over four years, from 2013 to 2017, with the aim of analyzing changes in the plankton community in a longer period, associated with environmental variables. Phytoplankton was studied from subsurface samples, preserved in Lugol’s solution and zooplankton from 20 m vertical hawls of a 64 µm mesh net, and preserved with formaline and later transferred to 95 % ethanol. Phytoplankton composition included 54 taxa, most of them were Chlorophyta, followed by Cyanobacteria and Bacillariophyta. The most abundant were: Synechococcus spp., Aphanocapsa sp. Coenochloris fotii, Chlorella sp. and Discostella stelligera. Total phytoplankton decreased from 2013 to 2017, mainly due to a decrease in Cyanobacteria. Zooplankton comprised 14 taxa, 5 cladocerans, 4 copepods, 1 diptera and 4 rotifers. The most abundant were: Keratella americana, Brachionus falcatus, Eubosmina tubicen and Arctodiaptomus dorsalis. There was a substitution of dominant species through time, at first (2013-2014) K. americana and E. tubicen alternated in dominance, and later (2015-2016), A. dorsalis, Diaphanosoma sp. and Ceridaphnia sp. became dominant, with a return of K. americana by 2017. The changes that occurred during the time of the study seem to be related to changes in the conditions of the lake, since its thermocline deepened with time and became more transparent. The abundance, of rotifers and Eubosmina tubicen, decreased associated to a decrease of Cyanobacteria. Those changes were not cyclical on an annual basis, but seemed to occur more on a long-term basis, as has been recognized earlier in the literature about tropical lakes.

Abstract Lakes are excellent sensors of climate changes because their thermal behavior is controlled by the annual temperature patterns. Deep tropical lakes stratify during the warm part of the year, mixing during winter. This mixing pattern can be altered by warmer climatic conditions such as global warming and/or “El Niño”. In this article we aim to correlate climate variability with changes in the mixing pattern of lake Alberca de Tacámbaro, Michoacán, México during an interval that involved two “El Niño” events (2009-2010 and 2015-2016) and with changes in the diatom assemblages preserved in the sediments between 1988 to 2015. The lake showed a winter mixing pattern, with a shallower thermocline in spring that deepened towards the autumn. Temperature data show a warming trend since AD 2000 and the impact “El Niño” years is associated to nutrient limitation (phosphorus and nitrogen), that we associate to a reduced vertical mixing. These changes are linked with an increase in hypolimnion temperature in 2015 compared to 2019 and modifications in the diversity and species composition of the diatom community of this lake between 1988 to 2015. This study allowed to identify recent changes in the lake and its diatom assemblages that are linked with global warming trends and “El Niño” events.

Resumen Los lagos están entre los ecosistemas más sensibles al cambio climático, ya que el comportamiento termodinámico de su columna de agua es controlado por la marcha anual de la temperatura. Los lagos tropicales profundos presentan una columna de agua estratificada durante los meses más cálidos, sin embargo, en invierno esos estratos desaparecen al mezclarse la columna de agua. El patrón de mezcla de un lago se puede alterar cuando se presenta un incremento en la temperatura, ya sea debido al calentamiento global y/o durante el fenómeno meteorológico conocido como “El Niño”. El objetivo de este trabajo fue correlacionar cambios en el clima con los del lago La Alberca de Tacámbaro, Michoacán, México, durante un período que involucra dos eventos de “El Niño” (2009-2010 y 2015-2016) y con variaciones en las asociaciones de diatomeas preservadas en los sedimentos entre 1988 a 2015. El lago presentó un patrón de mezcla en invierno, con una termoclina somera en primavera que se profundiza en el otoño. Los datos muestran una tendencia que se refleja en la limitación de dos nutrientes, fósforo y nitrógeno (P y N), asociada con una menor mezcla vertical causada por años más calientes desde el 2000 y el efecto de los años en los que se presenta el fenómeno “El Niño”. Estos cambios correlacionan con un calentamiento del hipolimnion en 2015 comparado con 2009 y con alteraciones en la diversidad y composición de las diatomeas entre 1988 a 2015. Este estudio permitió documentar los cambios recientes del lago y asociarlos con el impacto del calentamiento global y los eventos de “El Niño”.

Abstract Sixteen monthly cruises were carried out at a station 6.5 km to the NW of the mouth of Magdalena Bay (Mexico) to study the temporal variability of phytoplankton biomass and primary production rates, and the response of phytoplankton to the prevailing hydrographic characteristics. During each cruise, temperature and salinity were measured in the water column, and discrete samples were collected to measure inorganic nutrient concentrations (nitrate, phosphate, and silicate) and chlorophyll a content (as a proxy for phytoplankton biomass); in addition, in situ incubations were performed to estimate primary production (PP) rates using the 14C technique. During spring and early summer (March-July), the water column at the study site was cold (14.4 ºC at 80 m depth and 17.7 ºC at the surface) and well mixed, with high nutrient concentrations (nitrate = 6.39 μM, phosphate = 0.91 μM, and silicate = 11.87 μM) at the surface, typical of intense upwelling systems. On the contrary, from August to February the water column was stratified (23.5 ± 3.2 ºC above the thermocline and 17.0 ± 1.7 ºC at 80 m depth) and had low nutrient concentrations (nitrate = 3.90 μM, phosphate = 0.47 μM, and silicate = 10.30 μM) at the surface. Phytoplankton abundance, chlorophyll a concentration, and PP rates in the euphotic zone were higher during the upwelling season, with maximum cell density of 1.7 × 106 cells·L-1 (nanoplankton fraction), integrated chlorophyll a content of 231 mg·m-2, and integrated PP rates of 553 mg Cm-2·h-1). Abundance of cells <20 μm, as determined by flow cytometry, revealed the importance of the nanoplankton and picoplankton fractions in this region. This study showed the seasonal variability of phytoplankton communities (biomass and productivity) in waters off Magdalena Bay and the coupling of phytoplankton to environmental variability. Seasonal variability was similar to that observed for other typical coastal upwelling regions, that is, higher phytoplankton abundances and PP rates in spring and summer and lower values in autumn and winter.

Resumen En el periodo de la primavera de 2013 al verano de 2014 se llevaron a cabo 16 muestreos mensuales a 6.5 km al noroeste de la boca de bahía Magdalena (México) con el propósito de estudiar la variabilidad temporal de la biomasa y las tasas de producción primaria del fitoplancton y la respuesta del mismo a las características hidrográficas predominantes. En cada muestreo se midieron la temperatura y salinidad en la columna de agua y se determinaron las concentraciones de nutrientes inorgánicos (nitrato, fosfato y silicato) y el contenido de clorofila a (como proxy de la biomasa del fitoplancton); además, se hicieron incubaciones in situ para estimar la producción primaria (PP) con el método de 14C. En primavera y principios de verano (marzo-julio), el sitio presentó, en promedio, una columna de agua fría (14.4 ºC a 80 m de profundidad y 17.7 ºC en superficie) y mezclada con concentraciones de nutrientes altas en superficie (nitrato = 6.39 µM, fosfato = 0.91 µM y silicato = 11.87µM; valores característicos de zonas de afloramiento intenso). Por el contrario, de agosto a febrero se observó una columna de agua estratificada (23.5 ± 3.2 ºC arriba de la termoclina y 17.0 ± 1.7 ºC a 80 m de profundidad) con menor contenido de nutrientes en superficie (nitrato = 3.90 µM, fosfato = 0.47 µM y silicato = 10.30 µM). La abundancia del fitoplancton, la concentración de clorofila a y las tasas de PP en la zona eufótica fueron mayores en la temporada de surgencias, con densidad celular máxima de 1.7 × 106 cél·L-1 (fracción del nanoplancton), contenido de clorofila a integrada de 231 mg·m-2 y tasa de producción integrada de 553 mg C·m-2·h-1. Las abundancias de células <20 µm determinadas por citometría de flujo mostraron la importancia de las fracciones del nano y picoplancton en la región. Este estudio mostró la variabilidad estacional de las comunidades del fitoplancton (biomasa y productividad) en las aguas frente a bahía Magdalena y su acoplamiento al cambio ambiental. La variabilidad estacional fue similar a la observada en otras regiones características de afloramientos costeros, esto es, mayores abundancias y tasas de PP en primavera y verano y valores menores en otoño e invierno.

Resumo Objetivo Este estudo procura analisar o padrão de migração vertical das espécies de Cladocera e relacionar com os parâmetros físicos e químicos da água. Métodos As amostragens foram realizadas em intervalos de três horas por um período de 24 h no dia 15 de janeiro de 2014 no compartimento do rio Sapucaí, reservatório de Furnas, Minas Gerais, Brasil. Estas amostras foram coletadas em cinco profundidades, da superfície ao fundo (0, 2, 4, 6, 8 e 9 m), totalizando 54 amostragens para cada variável limnológica (temperatura, pH, oxigênio dissolvido, condutividade elétrica, teor de clorofila-a, índice de estado trófico, transparência da água e Cladocera). As amostragens foram realizadas na região limnética em um ponto com profundidade total de 9 metros. Resultados As estratificações térmicas e químicas da coluna de água e as ocorrências da termoclina foram registradas no meio do dia. Entre as 14 espécies de Cladocera, apenas Ceriodaphnia cornuta e C. silvestrii apresentaram um padrão de migração significativo, principalmente relacionado às concentrações de clorofila a e oxigênio dissolvido. Diaphanosoma birgei e D. spinulosum foram encontrados em altas densidades na superfície e meio da coluna d'água. A migração de Diaphanosoma spp foi diretamente relacionada à concentração de clorofila a e pH, que atuaram como um fator limitante. A alimentação em altas temperaturas na camada superficial e a assimilação nas camadas mais profundas podem conferir vantagens metabólicas às espécies migratórias verticais. Conclusões Neste estudo, um padrão bem definido de migração vertical foi encontrado para duas espécies de Cladocera e suas possíveis causas foram hipotetizadas. Para espécies com menor capacidade locomotora, as estratificações térmicas e químicas da água podem ter atuado como uma barreira à migração e distribuição através da coluna d'água. Este padrão pode provavelmente ser encontrado para espécies zooplanctônicas em outros compartimentos do reservatório de Furnas, com propriedades físicas e químicas semelhantes.

Abstract Aim This study seeks to analyze the vertical migration pattern of Cladocera species as related with the physical and chemical parameters of the water. Methods Samplings were carried out at 3-h intervals for 24 h in January 15 on the Sapucaí River compartment of Furnas reservoir, Minas Gerais State, Brazil. These samples were taken at five depths, from the surface to the near-bottom layer (0, 2, 4, 6, 8, and 9 m) totalizing 54 samplings for each limnological variable (temperature, pH, dissolved oxygen, electrical conductivity, chlorophyll-a content, Index of Trophic State, water transparency and Cladocera populations abundance). The samplings were performed at the limnetic region at a point with total depth of 9 meters. Results Water column thermal and chemical stratifications and thermocline occurrences were registered in the middle of the day. Among the 14 Cladocera species, only Ceriodaphnia cornuta and C. silvestrii displayed a significant migration pattern, which was mainly related to chlorophyll-a and dissolved oxygen concentrations. Diaphanosoma birgei and D. spinulosum are found at high densities near the surface and low ones in depth. The migration of Diaphanosoma was directly related to the concentration of chlorophyll a and pH, thus acting as a limiting factor. Feeding at high temperatures in the surface layer and assimilating at lower ones in depth, usually confer metabolic advantages to the vertically migratory species. Conclusions In this study, a well-defined pattern of vertical migration was found for two Cladocera species and its possible causes were hypothesized. To the species with less locomotory capacity, thermal and chemical stratifications can act as barriers to migration and distribution through the water column. This pattern may probably be found for zooplankton species in other compartments of Furnas reservoir, with similar physical and chemical properties.

RESUMO Os transportes verticais de nutrientes, compostos químicos e microrganismos causados pela presença de ondas internas causam um grande efeito no estado trófico de reservatórios. Poucas pesquisas buscam verificar a ocorrência de ondas internas em reservatórios, sistemas muitas vezes mais dendrítico que lagos. O objetivo desta pesquisa foi identificar ondas internas no reservatório do Vosorroca (Paraná), utilizando, para isso, séries de temperatura da coluna de água coletados entre os meses de Agosto e Novembro de 2012. Foi utilizado o método de duas camadas para a identificação da periodicidade das ondas internas. Através da relação de dispersão foram obtidos os períodos teóricos das ondas internas em águas rasas. As densidades espectrais de potência da série de temperatura na profundidade média da termoclina auxiliaram na identificação dos picos de frequências. Foi possível observar um grande pico de frequência durante o mês de Novembro, mês que apresentou maior estratificação térmica. O pico identificado como uma onda interna de periodicidade de 8 horas condiz com o período teórico de uma onda V1H1, obtido a partir do modelo. Ventos de baixa intensidade (≈ 1 m/s) predominantemente direcionados à nordeste excitaram uma onda V1H1. De acordo com os espectros de potência, após a passagem desta onda foram verificadas ondas internas de modos baroclínicos superiores e uma possível degeneração da onda interna de periodicidade de 8h.

ABSTRACT The vertical movements caused by internal waves in lakes and reservoirs have chemical and biological consequences for these ecosystems. The vast majority of studies that investigate internal waves are conducted on large lakes. There are just few researches that investigate this phenomenon on dendritic reservoirs. The purpose of this research was to identify internal waves (baroclinic mode) in the Vossoroca reservoir by using temperature time series recorded between May to November 2012. A two-layer method was used which considered rigid upper and lower boundaries. Moreover, the potential flow theory was used for both layers since the flow within each layer was considered irrotational. From the dispersion relation, we obtained the theoretical shallow internal wave period. The power spectral density (PSD) of temperature series of thermocline depth, provided by fast Fourier transform, helped in the identification on the frequency peak. Subsequently, the theoretical period was compared with the frequency spectra. Using a careful analysis (excluding the interference of solar radiation and intensity of wind), we observed a clear peak in November due to an internal wave with period around 8 hours, which matched the theoretical calculation from the dispersion relation equation for V1H1 mode. Weak winds from southwest excited a V1H1 baroclinic mode. According to spectral analysis, after the passage of this long-basin internal seiches, we identified the formation of higher vertical internal seiche modes. In addition, we observe indications of V1H1 mode degeneration.

Abstract Although the semidiurnal internal tide dominates internal motions at all sites studied to date off Mexico, a diurnal signal was detected on the shelf of the California Current System off Baja California Sur, even though the barotropic tide is exponentially stronger in the semidiurnal frequency along this coastline. In this study, diurnal thermocline shoaling events were measured in kelp beds in late summer and fall of 2004, very near their southern limit in the Northern Hemisphere. Sub-thermocline water bathed the base of the kelp beds at a diurnal frequency. Nitrate concentrations up to 1.4 μmol·L-1 were found in this sub-thermocline water of almost 20 ºC entering the kelp beds, which is a much greater concentration of nitrate at this temperature than is typical for the California Current. Nitrate-rich pulses lasted from 35 min to 19 h and reduced bottom temperatures up to 5 ºC in 1 h. The periodicity of temperature variability at the moorings showed significant coherence with the diurnal variability in zonal winds (>95% confidence interval) and was not coherent with the semidiurnal tide. We propose that wind-forced, diurnal internal motions may provide an important nutrient source for primary producers during reduced upwelling periods and merit further study as a fertilization source in the southern part of the California Current System.

Resumen A pesar de que la marea semidiurna predomina en las ondas internas de casi todos los sitios estudiados hasta la fecha en México, se detectó una marea interna diurna sobre la plataforma continental del Sistema de la Corriente de California frente a las costas de Baja California Sur, aunque la marea barotrópica es exponencialmente más fuerte en la frecuencia semidiurna. En este estudio, se midió la subida diurna de la termoclina en los mantos de kelp, localizados cerca del límite sureño de su distribución en el hemisferio norte, durante el verano y el otoño de 2004. Agua profunda cubrió la base de los mantos de kelp una vez al día donde se encontraron concentraciones de nitrato de hasta 1.4 μmol·L-1 en agua de casi 20 ºC, lo cual representa una concentración elevada de nitrato a esa temperatura para la corriente de California. Los pulsos de agua rica en nitrato duraron de 35 min a 19 h en los mantos y redujeron la temperatura del fondo hasta 5 ºC en 1 h. La periodicidad de la variabilidad de la temperatura mostró una coherencia significativa con la variabilidad diurna de los vientos zonales (>95% intervalo de confianza) y no fue coherente con la marea semidiurna. Proponemos que las ondas internas diurnas, forzadas por el viento, podrían proveer una fuente importante de nutrientes a los productores primarios durante periodos en los cuales la actividad de surgencias es reducida y requieren de más estudio para evaluar su capacidad de actuar como una fuente de fertilización en la parte sur del Sistema de la Corriente de California.

Abstract Experiments were carried out at a coastal upwelling site (ENSENADA station) off northern Baja California (México) during autumn 2015 (OCT-15) and spring 2016 (APR-16) to estimate phytoplankton daily growth (µ o) and mortality (m) rates and to assess microzooplankton grazing impact (m:µ o) on the phytoplankton community and specific autotrophic groups. In accordance with regional seasonality and under an environmental warming scenario due to the El Niño 2015-2016 event, significant differences in both hydrographic conditions and the growth-mortality dynamics of the phytoplankton community were observed between the 2 study periods. The µ o and m estimates were, respectively, 0.120 ± 0.012 d-1 and 1.145 ± 0.049 d-1 for OCT-15 and 1.186 ± 0.002 d-1 and 0.409 ± 0.086 d -1 for APR-16. The results of this study suggest that the effects of the anomalous warming on the phytoplankton community were more evident in OCT-15. During that period, growth of the larger autotrophic components (diatoms) was severely controlled by the environmental limitation of nutrients caused by the sinking of the thermocline that resulted from the entrance of warm water to the region. Furthermore, microzooplankton exerted active grazing pressure on phytoplankton biomass (72% of chlorophyll a [Chla]) and primary production (PP = 0.20 µg Chla L-1 d-1), with grazing impact >100% of PP. In APR-16, when the ecosystem apparently started returning to the spring conditions, a high value for PP (3.73 µg Chla L-1 d-1) was estimated, with only one third of it being consumed by microzooplankton (34% of PP). The results of this research evidence the high dynamism of multivorous food webs coupled to the seasonal and interannual variability of coastal upwelling systems.

Resumen En un sitio de surgencia costera (estación ENSENADA) frente a la región norte de Baja California (México) se realizaron experimentos durante el otoño de 2015 (OCT-15) y la primavera de 2016 (ABR-16) para estimar las tasas diarias de crecimiento (µ o) y mortalidad (m) del fitoplancton, que permitieron evaluar el impacto (m:µ o) que genera el pastoreo diario del microzooplancton sobre la comunidad de fitoplancton y grupos autótrofos específicos. Acorde con la estacionalidad de la región y bajo el escenario de un calentamiento ambiental debido al evento El Niño 2015-2016, se observaron notables diferencias en las condiciones hidrográficas y en la dinámica de crecimiento y mortalidad de la comunidad de fitoplancton entre los 2 periodos de estudio. Las estimaciones de µ o y m fueron, respectivamente, 0.120 ± 0.012 d -1 y 1.145 ± 0.049 d-1 para OCT-15 y 1.186 ± 0.002 d-1 y 0.409 ± 0.086 d-1 para ABR-16. Los resultados de este estudio sugieren que los efectos del calentamiento anómalo sobre la comunidad del fitoplancton fueron más evidentes durante OCT-15. En ese periodo, los componentes autotróficos más grandes (diatomeas) estuvieron fuertemente restringidos en su crecimiento, posiblemente por una limitación ambiental de nutrientes ocasionada por el hundimiento de la termoclina debido al mayor aporte de agua cálida. Además, se estimó un activo consumo del microzooplancton sobre la biomasa (72% de clorofila a [Chla]) y producción primaria (PP = 0.20 µg Chla L-1 d-1) de fitoplancton (>100% de la PP). Para ABR-16, cuando aparentemente se empezaron a manifestar las condiciones propias de primavera en el ecosistema, se estimó una PP alta (3.73 g Chla L-1 d-1) y sólo una tercera parte de ésta fue consumida por el microzooplancton (34% de la PP). Los resultados de esta investigación revelan el alto dinamismo de las tramas tróficas multívoras acoplado a la variabilidad estacional e interanual de los sistemas de surgencia costera.

Resumo Para avaliar o padrão sucessional de organismos incrustantes, três hipóteses foram testadas: 1) uma termoclina é causada por eventos de ressurgência e, portanto, a profundidade influencia na trajetória sucessional da comunidade incrustante; 2) a redução na intensidade de luz natural no substrato influencia a composição da comunidade e a trajetória sucessional; 3) a predação por peixes influencia a composição da comunidade e sua trajetória sucessional. Durante um ano, faces inferiores e superiores de painéis de PVC em gaiolas abertas, parcial e totalmente fechadas em profundidades de 1,5 e 3,5 metros foram amostradas mensalmente por fotografia digital para determinar a composição da comunidade por ponto-contato e estimar a porcentagem de cobertura dos organismos. Diferentes massas de água resultantes da ressurgência e diferentes intensidades de luz foram fatores determinantes na trajetória sucessional da comunidade. Após a instalação de gaiolas parcial e totalmente fechadas, foram identificadas diferenças nas respectivas trajetórias. O resultado deste estudo sugere que cada fator físico ou processo biológico pode alterar a trajetória de desenvolvimento de uma comunidade, e o modelo sucessional (ex. convergente, divergente, paralelo ou cíclico) depende da magnitude dos fatores determinantes que atuam em cada etapa de sua trajetória.

Abstract To assess the successional pattern of fouling organisms three hypotheses were tested: 1) a thermocline is caused by seasonal upwelling events, and therefore, depth influences the successional trajectory of the fouling community; 2) a reduction in the intensity of natural light of the substrate influences the fouling composition and the successional trajectory; 3) fish predation influences the community composition and its successional trajectory. During one year, up-facing and down-facing PVC panels on open, partially caged or fully caged, and placed at depths of 1.5 and 3.5 meters were monthly sampled by digital photograph to determine the community composition and by contact point to estimate the percent coverage of organisms. The upwelling impact provided different water masses, and light intensity was also a determining factor of the overall successional trajectory of the fouling community. After the installation of full and partial cages, differences were identified in the respective successional trajectories. The results of this study suggest that each physical factor or biological process can change the successional trajectory of the community, and the successional model (e.g., convergent, divergent, parallel, or cyclic) depends on the magnitudes of the determinants that act on the community at each stage of its trajectory.

Abstract: There exist previous studies on phytoplankton, its taxonomic groups and species, its biomass and primary productivity, mainly in winter and spring, but the structure of the phytoplankton from the Gulf of Tehuantepec in summer is poorly known. The composition and distribution of the phytoplankton photosynthetic pigments in summer conditions are provided in this paper. Hydrographic data from nine stations, during an oceanographic cruise in June, 20013, and analysis of five major phytoplanktonic pigments, Fucoxanthin, Prasinoxanthin, Violaxanthin, Zeaxanthin (marker of Synechococcus, picoplanktonic prokaryote) and Divinyl-chlorophyll a (diagnostic of Prochlorococcus, another picoplanktonic prokaryote) are given. Column water was well-stratified: surface layers had a thickness between 20 and 35 m, with well-defined thermoclines at those depths. Values of all pigments were low, but they showed similar vertical distribution patterns, with subsurface maxima peaks (between 30 and 40 m), especially Divinyl-chlorophyll a in most stations, except 3 and 4, where Fucoxanthin was the most important pigment. These peaks did not always coincide with the depths of the main thermocline at each station, buy usually they were found below the thermocline. This scenery shows the picoplankton as a very important size-fraction group in summer (at least in June), strongly contrasting with the winter-spring conditions, of intense turbulent mix and upwellings, where microplankton and diatoms appear to predominate.

Resumen: Existen estudios previos sobre fitoplancton y sus grupos taxonómicos y especies, así como de su biomasa y productividad primaria, principalmente en invierno y primavera, pero se conoce poco la estructura del fitoplancton del Golfo de Tehuantepec en verano. Aquí se dan a conocer la composición y distribución de pigmentos fotosintéticos fitoplanctónicos en condiciones de verano. Se obtuvieron datos hidrográficos en nueve estaciones durante un crucero oceanográfico en junio, 2003, y se analizaron cinco pigmentos, Fucoxantina, Prasinoxantina, Violaxantina, Zeaxantina (indicador de Synechococcus, procarionte del picoplancton) y Divinil-clorofila a (diagnóstico de Prochlorococcus, otro procarionte del picoplancton). La columna de agua se encontró estratificada: la capa superficial, definida por la profundidad del tope de la termoclina, varió entre 20 y 35 m de espesor. Los valores de todos los pigmentos fueron bajos, pero mostraron patrones similares de distribución vertical, con un pico de concentración subsuperficial (entre 30 y 40 m), destacando Divinil-clorofila a en la mayoría de las estaciones, excepto en la 3 y 4, donde la Fucoxantina fue el principal pigmento. Estos picos no siempre coincidieron con la profundidad de la termoclina en cada estación, generalmente éstos se encontraron por debajo de ella. Este escenario muestra al picoplancton como una fracción importante en verano (al menos en junio), contrastando notablemente con la condición de invierno-primavera, de intensa mezcla turbulenta y surgencias, donde el microplancton y las diatomeas predominan.

Se investigó el zooplancton del océano Pacífico colombiano durante junio-julio (2001a) y agosto-septiembre (2001b). Puesto que Oncaea venusta y O. media predominaron en la comunidad de copépodos, se evaluó su distribución y abundancia. Los organismos se extrajeron de muestras de mesozooplancton, tomadas en la superficie, con una red cónica (50cm de apertura, malla 363µm). Ambas especies se distribuyeron ampliamente con las mayores abundancias, en especial, en aguas neríticas (3°-5°N). El análisis factorial reveló correlaciones negativas y positivas entre las abundancias y las variables abióticas, según el período de estudio. El análisis de agrupamiento mostró tres grupos, en 2001a y cinco, en 2001b, dependientes, principalmente, de las abundancias diurnas y nocturnas y la profundidad y amplitud de la termoclina, sugiriendo la influencia de estos factores sobre Oncaea venusta y O. media. En análisis futuros es necesario considerar las diferentes formas de O. venusta y O. media, para establecer la importancia del fitoplancton y otros organismos en la dieta de las dos especies. Las mayores abundancias nocturnas o diurnas aluden a la posible migración vertical de estos microcrustáceos a la superficie. La disminución en la abundancia general de O. venusta de 2001a a 2001b (66.4% a 47.5%) contrastó con el incremento de O. media (33.6% a 52.5%). Estos cambios, se podrían explicar por la diferente respuesta biológica de las dos especies a la variabilidad, a corto plazo, en las condiciones oceanográficas y el acaecimiento de un episodio débil de La Niña.

Zooplankton of the Colombian Pacific Ocean was investigated during June-July (2001a) and August-September (2001b). Since Oncaea venusta and O. media predominated in the copepod community, their distribution and abundance were evaluated. Organisms were extracted from surface mesozooplankton samples taken with a conic net (50cm opening, 363µm mesh). Both species were widely distributed with highest abundances mainly in neritic waters (3°-5°N). Factor analysis revealed negative and positive correlations among abundances and abiotic variables depending on each period. Cluster analysis showed three groups in 2001a and five ones in 2001b, essentially consistent with night and day abundances, and thermocline depth and width suggesting the influence of these factors on O. venusta and O. media. In future analysis it may be necessary to consider the different forms of O. media and O. venusta to establish the importance of phytoplankton and other organisms in their diet. Higher night or day abundances allude to the possible diel vertical migration of these microcrustaceans to the surface. The decrease in the general abundance of O. venusta from 2001a to 2001b (66.4% to 47.5%) contrasted with the increase of O. media (33.6% to 52.5%). These changes could be explained by the different biological response of both species to short-term variability in oceanographic conditions and the occurrence of a weak La Niña event.

Em ecossistemas de água doce, os padrões de estratificação térmica têm uma influência considerável sobre a dinâmica populacional do fitoplâncton. Neste estudo nós investigamos como os padrões de estratificação térmica e os processos hidrodinâmicos afetaram a distribuição vertical do fitoplâncton durante dois ciclos nictemerais consecutivos em um reservatório urbano na cidade de São Paulo. As amostragens e medidas das variáveis físicas e químicas foram realizadas a cada três horas durante um período de 48 horas, ao longo de um perfil vertical. Ocorreram dois eventos de estratificação e subsequente abaixamento da termoclina acarretando alterações na distribuição vertical do fitoplâncton. Aphanocapsa delicatissima Nägeli foi a principal espécie dominante durante todo o período avaliado. No segundo ciclo nictemeral o gradiente de densidade criado pelas diferenças de temperatura impediu o afundamento de Mougeotia sp. C. Agardh para as camadas mais profundas. Por outro lado Pseudoanabaena galeata Böcher se manteve nas camadas de 4.0 e 5.5 m. A estrutura térmica da água foi diretamente afetada pelos fatores meteorológicos temperatura do ar e velocidade do vento. As mudanças na densidade e na distribuição vertical do fitoplâncton foram controladas pelos eventos térmicos e hidrodinâmicos.

In lentic freshwater ecosystems, patterns of thermal stratification play a considerable part in determining the population dynamics of phytoplankton. In this study we investigated how these thermal patterns and the associated hydrodynamic processes affect the vertical distribution of phytoplankton during two consecutive diel cycles in a warm polymictic urban reservoir in the metropolitan region of São Paulo, Brazil. Water samples were taken and physical, chemical and biological data collected at half-meter intervals of depth along a water column at a fixed site, every 3 hours throughout the 48-hour period. Two events of stratification, followed by deepening of the thermocline occurred during the study period and led to changes in the vertical distribution of phytoplankton populations. Aphanocapsa delicatissima Nägeli was the single dominant species throughout the 48-hour period. In the second diel cycle, the density gradient induced by temperature differences avoided the sedimentation of Mougeotia sp. C. Agardh to the deepest layers. On the other hand, Pseudanabaena galeata Böcher remained in the 4.0-5.5 m deep layer. The thermal structure of the water was directly affected by two meteorological factors: air temperature and wind speed. Changes in the cell density and vertical distribution of the phytoplankton were controlled by the thermal and hydrodynamic events.

CTD measurements were processed to analyze the thermohaline variability, presence of water masses and geostrophic circulation during a period of neutral ENSO. The abrupt stratification and the steep thermocline, halocline and pycnocline in May and the thermocline and pycnocline in July, were generated by the entry of water of the gulf, originating from coastal upwelling, due to southerly winds. In October, the salinity decrease with a minimum and a maximum subsurface, was caused by water intrusion from the gulf, as the bay is considered an evaporation basin without river discharge with evaporation of 176.16 mm and null precipitation in this month. The mixed layer and stratification decreased in February, was the result of homogenization and cooling water by winds from the north and the lowest solar radiation received. The water exchange with the gulf, on May, July and February, involved Gulf of California Water on the surface and Subtropical Subsurface Water in the deep; exceptionally in October, the Tropical Surface Water, replaced the Gulf of California Water. The bathymetric North Mouth's sill prevented the flow incursion of Pacific Intermediate Water into the bay. In May, July and October, the geostrophic circulation in the bay, was dominated by a cyclonic gyre, which facilitated the exchange of water with the gulf, while in February the circulation was anticyclonic.

Se procesaron mediciones de CTD para analizar la variabilidad termohalina, presencia de masas de agua y circulación geostrófica durante un período de ENSO neutral. La estratificación y la abrupta termoclina, haloclina y picnoclina en mayo y la escarpada termoclina y picnoclina en julio, fueron generadas por la entrada de agua del golfo, originada en la surgencia costera, producto de los vientos del sur. En octubre, la disminución de la salinidad con un mínimo y un máximo subsuperficial, fue causada por la intrusión de agua del golfo, puesto que la bahía es considerada una cuenca de evaporación, sin descargas fluviales, con evaporación de 176,16 mm y precipitación nula en este mes. La capa de mezcla y la disminución de la estratificación en febrero, fue resultado de la homogenización y el enfriamiento del agua por los vientos del norte y la menor radiación solar recibida. El intercambio de agua con el golfo, en mayo, julio y febrero, involucró Agua del Golfo de California en la parte superficial y Agua Subsuperficial Subtropical en la parte profunda; de manera excepcional en octubre, el Agua Superficial Tropical, sustituyó al Agua del Golfo de California. El umbral batimétrico de la Boca Norte, impidió la incursión del Agua Intermedia del Pacífico a la bahía. La presencia de un remolino ciclónico en la bahía durante mayo, julio y octubre dominó la circulación geostrófica, lo que facilitó el intercambio del agua con el golfo, mientras que en febrero la circulación fue anticiclónica.