The Bay of La Paz (BLP) is located in the southwestern portion of the Gulf of California (GC), the largest and deepest coastal environment along the gulf. The BLP is recognized for its high biological diversity and is a place of refuge, growth, and feeding for several emblematic species, some of which are critically endangered. However, what makes the BLP productive? This paper presents a synthesis and review of the physical processes and mechanisms related to planktonic production, which are directly linked to productivity at higher trophic levels, including top predators. This review reveals that the BLP has a high richness and abundance of plankton species. Several hydrodynamic processes at different scales exert a marked influence on these organisms, including internal waves, hydraulic jumps, thermohaline fronts, the presence of a dipole eddy, and a quasi-permanent cyclonic eddy that is confined to the bay, which induces Ekman pumping that determines the availability of nutrients in the euphotic zone. This eddy induces a differential phytoplankton distribution, with a predominance of dinoflagellates in its center and the dominance of diatoms in its periphery. This arrangement is closely linked to that of zooplankton. Herbivorous zooplankton are highly abundant in the periphery of the eddy, while omnivorous zooplankton are dominant in the center. In contrast, carnivorous zooplankton are dominant in the GC. Although significant progress has been made in understanding the dynamics of the BLP over the last two decades, gaps remain unaddressed. This review highlights the areas of opportunity to continue advancing this endeavor, achieve a better scientific understanding of the region, and propose better management and conservation plans for this highly diverse area.

Los copépodos, como uno de los principales grupos que componen el zooplancton marino, juegan un papel importante por su posición en la red trófica. Se evaluó la biomasa de copépodos en un vórtice ciclónico durante 2 estaciones climáticas contrastantes en la bahía de La Paz, al sur del golfo de California, la cual se caracteriza por una alta productividad biológica. Durante 2 campañas oceanográficas en invierno de 2006 y verano de 2009, se determinó la estructura física de la columna de agua mediante un CTD y se recolectaron muestras de zooplancton mediante arrastres oblicuos. Se utilizaron datos de satélite para visualizar los patrones de distribución de clorofila a (Cla). Los resultados mostraron un vórtice ciclónico en ambas estaciones, con un diámetro de ~25 km y velocidades geostróficas >50 cm·s–1 en su periferia, con elevada Cla (~3 mg·m–3 en invierno). La máxima biomasa de calanoides ocurrió en invierno, con hasta 6.6 mg·100 m–3 en el oeste de la bahía, cerca de la costa; además, su distribución coincidió con el patrón de Cla y circulación, ya que formó un anillo siguiendo la periferia del vórtice, con un segundo pico cerca de la conexión con el golfo. Los valores medios de biomasa de copépodos mostraron un patrón de alternancia calanoides-ciclopoides entre invierno y verano dentro del vórtice ciclónico, con más biomasa de calanoides que ciclopoides en invierno y lo contrario en verano. Los resultados resaltan el impacto del vórtice en el ecosistema planctónico a través de su influencia sobre las condiciones hidrográficas en la columna de agua. Otros factores, como las interacciones ecológicas, la dinámica poblacional y los hábitos alimenticios, también pueden jugar un papel importante. El comportamiento alimentario es afectado por altas concentraciones de Cla, que representan una fuente de alimento para copépodos observados alrededor del vórtice.

This study examined the ingestion of microplastics (MPs) by copepods in the Bay of La Paz, the largest and deepest coastal environment in the Gulf of California (GC), Mexico. The research was based on zooplankton samples collected during oceanographic cruises from 2003 to 2016 at a location near the urban area of La Paz city. In the laboratory, adult copepods were selected from the samples, and a dilution process was used to extract the MPs from their bodies. Were extracted 428 MPs, consisting of fibers, films, fragments, and pellets (microspheres). No evidence was found to suggest that the ingestion of MPs has increased over time. The year 2009 recorded the highest number of MPs, with 120 items, which may be attributed to the circulation pattern during that period, which was influenced by an old, decaying cyclonic eddy. In contrast, 2014 had the fewest MPs, with only 44 items. Throughout the study, transparent fibers were the most conspicuous material. Research on the ingestion of MPs by copepods in Mexico is still in its early stages. According to the available literature, this study represents the first documented report on this subject in coastal waterbodies within the GC.

This study evaluated how the summer circulation pattern in the Southern Gulf of California influences copepod communities. The evaluation was based on hydrographic data and zooplankton samples collected during a multidisciplinary research cruise conducted in June and July of 2019. The results revealed the presence of a cyclonic circulation with a diameter of approximately 100 km, located near the entrance of the Gulf, affecting the upper 200 m layer. A total of 30 copepod species were identified, including 20 from the order Calanoida and 10 from Cyclopoida. The most abundant Calanoida species were Canthocalanus pauper, Clausocalanus furcatus, and Subeucalanus subcrassus, with respective densities of 2316.80, 1593.60, and 1584.64 ind m⁻³. The most abundant Cyclopoida species were Oithona setigera, Dioithona rigida, and Oncaea venusta, which had densities of 963.44, 290.56, and 235.52 ind m⁻³, respectively. The horizontal distribution of these species showed variations influenced by the cyclonic circulation. Specifically, low abundance values were observed at the center of cyclonic circulation, while higher values were found at its periphery. This pattern was consistent among the dominant species, indicating that they do not benefit from the cold subsurface waters induced by circulation. In fact, the distribution of some species was higher in a band of warm water located in the eastern portion of the study area. Overall, our findings shed light on how the summer cyclonic circulation in the Southern Gulf of California affects the copepod community, an aspect that has not been previously explored. This research enhances our understanding of the processes influencing this group of organisms in a highly dynamic environment.

Zooplankton biomass is an indirect indicator of secondary production and a measure that allows estimates to be made about the availability of matter in the food web. This study compares the wet biomass of zooplankton within of the Bay of La Paz, the largest and deepest coastal water body in the Gulf of California, Mexico. It analyzes the influence of the physical forcing in zooplankton biomass and distribution. Two research cruises were conducted in February 2006 and August 2009, recording high-resolution hydrographic data and collecting zooplankton. The hydrographic properties of the seawater column changed in both seasons. The circulation pattern was dominated by a well-defined cyclonic eddy with different azimuthal velocities, being more intense in summer (75 cm s-1) than in winter (20 cm s-1). Zooplankton biomass showed slightly lower values in winter (43.4 g 100 m-3) than in summer (45.5 g 100 m-3). Its horizontal distribution showed that the highest values were observed in the shallow stations, close to the coast, and in the connection with the Gulf of California. However, high secondary values were observed at the eddy’s peripheries, describing a circular distribution pattern following their circumference, which could be attributed to 1) eddies retain zooplankton organisms and advect them to their periphery, and 2) mixing processes at the eddies peripheries ensure food (phytoplankton) for zooplankton organisms.