Assessments of picoplankton carbon biomass in the pelagic ecosystem over the deep region of the southern Gulf of Mexico were conducted during three consecutive summer cruises. Notably, the relationship between carbon distribution of Prochlorococcus (PRO) and Loop Current (LC) dynamics was evaluated. Seawater samples were collected from the euphotic zone (~150 m) for estimating the abundance of the picoplankton populations using flow cytometry analyses. Carbon biomass estimates were based on cell abundance and variable conversion factors computed across stations and depths. On average, about half of the total depth-integrated carbon biomass of picoplankton was attributed to heterotrophic bacteria (HB, 54%) and three autotrophic populations (Prochlorococcus, Synechococcus, and pico-eukaryotes, 46%). In agreement with previous winter assessments, PRO was the dominant component of abundance (~90%) and pico-phytoplankton community biomass (>70%). Based on molecular analyses, distinct ecotypes of high-light PRO and low-light (LL) PRO were found within the euphotic zone, vertically distributed along light and nutrient gradients. Also, PRO distributions were related to hydrographic conditions strongly modulated by mesoscale dynamics. LL-PRO subgroups, located close to the nutricline under LL conditions, were associated with the westward propagation of anticyclonic eddies that episodically detach from the LC. This study highlights the role of the LC and its eddies in the transport and distribution of carbon biomass into the Gulf of Mexico, as represented by the deep subgroups of the dominant, tiniest autotroph within this oligotrophic ecosystem.

Microzooplankton grazing impact on the phytoplankton community at a coastal upwelling station off northern Baja California, MexicoImpacto del pastoreo del microzooplancton sobre la comunidad fitoplanctónica en una estación de surgencia costera localizada en la región norte de Baja California, México 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 calenta...

A. In order to compare phytoplankton biomass (satellite-derived chlorophyll, Chl sat) and production (PP), and sea surface temperature (SST) between the regions north and south of the Ensenada Front, time series were generated from satellite imagery for 250-km transects perpendicular to the coast: one off La Jolla (TLJ), southern California (USA), and the other off San Quintín Bay (TSQB), Baja California (Mexico). Moderate Resolution Imaging Spectroradiometer (MODIS) SST and Chl sat monthly composites and PP monthly composites were used for the 2002-2016 period. An "average year" was generated for each transect and each variable as an approximation to the climatology. Data show spatial variation in Chl sat and PP with higher values in the coastal zone (>10.0 mg•m-3 and >4.0 g C•m-2 •d-1 , respectively) than offshore (~0.1 mg•m-3 and 0.4 g C•m-2 •d-1 , respectively), while SST showed, in general, minimum values in the coastal zone (~15.0 ºC) and maxima offshore (~21.0 ºC). In the coastal zone, Chl sat values were higher on TSQB than on TLJ. However, sometimes phytoplankton biomass was higher in the offshore waters of TLJ than in the offshore waters of TSQB because of the effect of high Chl sat plumes coming from Point Conception into the Southern California Bight. The SST, Chl sat , and PP variations had clear seasonal and interannual components. Spectral analysis shows that the seasonal component of variation was dominant for the 3 variables. The 2004 central Pacific type of El Niño, the 2014 "Blob", and the 2015-2016 eastern Pacific type of El Niño had very strong effects on phytoplankton biomass and production along both transects. Nevertheless, the effects generally tended to be stronger at TLJ than at TSQB. There are spatial variations in the coastal dynamics of the California Current System, and thus the effects of seasonal and interannual events will not be the same at different geographic locations.

This study assessed the cell carbon content and biomass for genera of dinoflagellates and diatoms in the oceanic ecosystem of the Southern Gulf of Mexico. Carbon content estimates were based on biovolume calculations derived from linear dimension measurements of individual cells and the approximate geometric body shape of each genus. Then, biomass assessments were performed for both groups in two gulf regions (Perdido and Coatzacoalcos) using these carbon content factors and cell abundances. After four seasonal cruises, 11,817 cells of dinoflagellates and 3,412 cells of diatoms were analyzed. Diverse body shapes and cell sizes were observed among 46 dinoflagellate genera and 37 diatom genera. Nano-cells of dinoflagellates (68% <20 μm) and micro-cells of diatoms (77% 20–200 μm, mostly 50–75 μm) were predominant. According to this cell-size structure, on average, diatoms contained 40% more carbon per cell than dinoflagellates. Contrasting carbon content estimates were observed within the genera of both microalgae. Large carbon averages (>10,000 pg C cell-1) were attributed to Gonyaulacal and some occasional genera of dinoflagellates (e.g., Pyrocystis and Noctiluca) and centric diatoms. In contrast, values up to 3 orders of magnitude lower were found for Peridinial and Gymnodinial dinoflagellates and pennate diatoms. Based on these carbon content estimates, which can be considered representative for most of this oceanic ecosystem, seasonal and regional differences were found in the biomass assessments conducted for these functional groups. Overall, dinoflagellates (mostly low-carbon Gymnodinales) had larger depth-integrated biomass than diatoms (mainly rich-carbon centric forms) within the euphotic zone. An exception to it was the late-summer cruise at the Coatzacoalcos region when a surface bloom of centric diatoms was observed in stations influenced by river runoff. This work contributes useful reference information for future ecological studies and models for understanding the biogeochemical functioning of this open-ocean ecosystem.