María Adela Monreal-Gómez

Environmental forcing in a tropical estuarine ecosystem: the Palizada River in the southern Gulf of Mexico

Environmental characteristics of the fluvial–deltaic–lagoon–estuarine system of the Palizada River were determined by the assessment of physical, environmental and hydrological variability. Environmental information comprised daily time series of precipitation (1971–85), Usumacinta River discharge (1948–94), sea level (1956–91) and wind patterns (1984–87). Hydrology was studied by weekly measurement of salinity, transparency,temperature, dissolved oxygen and pH at 18 stations during 19 months (September 1985 to March 1987). Water temperature reflected the climatic seasons: dry, rainy and winter frontal storms (‘Nortes’). During the rainy season fresh water dominated the whole area, but during the dry season a salt wedge entered as far as Del Este Lagoon.Northern winds allowed saline water to reach intermediate regions and occasionally the upstream freshwater head of the estuary. There are annual, 6-month and 3.2-month oscillations in the environmental records: the annual signal is modulated by precipitation, fluvial discharge and sea level; the 6-month signal has an implicit periodicity driven by the changes in the wind direction that affect all other parameters; and the 3.2-month period seems to be modulated by the rainy, dry and ‘Nortes’ seasons. Cluster analysis identified eight ecological regions based on the hydrological variables. Resumen. Las caracteristicas ambientales del sistema fluvio–deltaico–lagunar estuarino del Rio Palizada fueron determinadas a traves del estudio de su variabilidad fisica, ambiental e hidrologica. La informacion ambiental comprende series de tiempo diarias de precipitacion pluvial (1971–85), descarga fluvial del Rio Usumacinta (1948-94), nivel del mar (1956–91) y direccion y velocidad del viento (1984–87). La hidrologia comprende mediciones semanales de salinidad, transparencia, temperatura, oxigeno disuelto y pH en 18 estaciones durante 19 meses (Septiembre de 1985 a Marzo de 1987). La temperatura del agua refleja las estaciones climaticas: secas, lluvias y frentes invernales (‘Nortes’). Durante la epoca de lluvias el agua dulce predomina en toda el area, sin embargo durante la estacion de secas la cuna salina llega hasta la Laguna del Este. Los vientos del norte promueven el avance del agua salina a las regiones intermedias y ocasionalmente hasta la cabecera del estuario. Las oscilaciones ambientales muestran periodos anuales, semestrales y de 3.2 meses: el ciclo anual esta modulado por la precipitacion pluvial, la descarga fluvial y el nivel del mar; el ciclo semestral tiene una periodicidad implicita gobernada por cambios en la direccion del viento que afecta los demas parametros y el ciclo de 3.2 meses parece estar modulado por las estaciones climaticas de lluvias, secas y nortes. Los analisis Cluster basados en las variables hidrologicas permiten la identificacion de ocho regiones ecologicas.

Role of Near-Bottom Currents in the Distribution of Sediments within the Southern Bay of Campeche, Gulf of Mexico

Abstract The sediment distribution in the southern Bay of Campeche is mainly governed by wind-induced currents and mesoscale cyclonic eddies. An increase in sediment grain size toward the sea is attributed to the presence of a convergence zone that sorts the sediments from heavier to lighter. A small anticyclonic eddy near the coast leads to a concentration of sediments westward of the Grijalva-Usumacinta River. We combined current profiles and charts of the distribution of magnetic susceptibility within sediments with a hydrodynamic numerical model to elucidate the distribution of the major sedimentary units in the Southern Bay of Campeche, Gulf of Mexico. Acoustic Doppler current profiles and geostrophic velocity data were used to set the water velocity at the open boundaries of the applied numerical model. The model was forced by the dominant winds. Sediment on the southeastern side of the bay is dominated by carbonates because of a lack of river discharge and other systems that contribute terrigenous sediments. In the southernmost side of the bay, where the major rivers are located, the bottom sediments are mainly of terrigenous origin.

Differential distribution of diatoms and dinoflagellates in a cyclonic eddy confined in the Bay of La Paz, Gulf of California

The differential distribution of diatoms and dinoflagellates in the Bay of La Paz, Gulf of California, Mexico, was analyzed in summer of 2009, when a cyclonic eddy confined in the bay dominated the circulation. An uplift of the nutricline in the eddy drove high concentrations of nutrients to the euphotic layer. A differential phytoplankton distribution was observed to be associated with the eddy: there was an abundance of dinoflagellates close to the center of the cyclonic eddy, whereas diatoms were more abundant at the periphery. A significant inverse correlation (R = −0.62, p < 0.002) was found between the temperature at 25 m depth and the dinoflagellates abundance. Based on the temporal evolution of chlorophyll measured by MODIS satellite images, and a conceptual model proposed for the lifecycle of eddies, the cyclonic eddy may have been an old decaying structure. The effect of the cyclonic eddy on the phytoplankton distribution in this small semienclosed region was apparently similar to that found in larger eddies in the open ocean, but this is the first time such a differential distribution has been found associated to a confined eddy.

Geographical patterns in species richness of the benthic polychaetes in the continental shelf of the Gulf of California, Mexican Pacific

The present study is the first attempt to describe meso-scale patterns in the species richness of polychaetes along the Gulf of California, which stretches from about 23°N to 31°N. We examine herein the spatial changes in species distribution and explore the overlapping of species’ ranges towards the centre of the Gulf, to test whether the mid-domain effect (MDE) could explain an expected mid-domain peak in species richness. The faunal composition and the latitudinal range of 244 species of polychaetes recorded along the continental shelf of the Gulf of California were analysed in latitude bands of 1°. The species composition changes around the Gulf’s archipelago (~29°N), and the highest values of species richness are found at the 25° (197 species) and 26° (193 species) of latitude. Although the species richness pattern could be described by a parabolic shape, the regional trend was not strongly consistent with the peak of diversity at 27°N (176–191 species) predicted by the mid-domain effect: the random sorting of species’ ranges within spatial domain does not explain satisfactorily the geographical patterns of diversity. Nevertheless, a partial contribution of MDE to these natural patterns of diversity could be detected, and the increase in species richness towards middle latitudes was basically determined by species with distribution ranges larger than 6°. The low level of significance between the empirical species richness pattern and the mid-domain model prediction for polychaetes in the Gulf does not restrict their use as a model for exploring the randomness of the diversity patterns.

Impact of the “Godzilla El Niño” Event of 2015–2016 on Sea-Surface Temperature and Chlorophyll-a in the Southern Gulf of California, Mexico, as Evidenced by Satellite and In Situ Data

Abstract: During 2015–2016, a strong El Niño event, nicknamed the “Godzilla El Niño,” occurred in the Pacific Ocean. Using satellite imagery, in this article we assess impacts of this event on sea-surface temperature and chlorophyll-a concentrations in the southern Gulf of California. Daily images of sea-surface temperatures and chlorophyll-a were obtained by satellite from the Moderate Resolution Imaging Spectroradiometer for the period from January 2013 to December 2017. A circular area ≈ 46.8 km in diameter in the central part of the gulf was selected to evaluate monthly variation of both parameters. Hydrographic data generated during a research cruise in November 2016 were used to evaluate water mass distributions. Results revealed strong seasonal variability, with high chlorophyll-a concentrations recorded during winter and low values during summer. Contrary to predictions, the “Godzilla El Niño” event apparently did not have as large an impact on the phytoplankton biomass, expressed as chlorophyll-a, in this region in comparison to other areas and to previous strong El Niño events. This is likely related to gulf dynamics and to the mechanism of productivity enhancement, although further observations are required to confirm this theory. Results presented contribute to a better understanding of the highly productive and unique Gulf of California ecosystem.