The absorption or scattering of photosynthetically active radiation (PAR) in the ocean by its components is crucial in determining the extent of the euphotic zone. This stratum is the ocean's uppermost layer, where light availability governs primary productivity in marine ecosystems. This study aims to bio-optically categorize Gulf of Mexico oceanic waters based on regional differences in water transparency and the euphotic zone depth using remotely sensed diffuse attenuation coefficient data (Kd490) validated with in situ observations (KdPAR) from cruises conducted between May 2016 and June 2019. An overestimation of about 15 % of remotely sensed depths was found, which was significantly reduced using in-water records. A 22-year climatological analysis on the calibrated euphotic zone depths derived from satellite-based Kd identified three optical regions in the Gulf of Mexico (I, IA, and IB). Deeper euphotic zones were observed in Region I (in central waters), while Region IB (in the Bay of Campeche) had shallower euphotic zones. The differences in PAR light penetration were linked to variations in chlorophylla, which were modulated by mesoscale circulation and river discharges. Our findings support the extent of the euphotic zone down to 0.1 % of incident surface light, especially in Region I, where the large anticyclonic eddies detached from the Loop Current have the most influence in the oceanic waters of the Gulf of Mexico. This study contributes to understanding important optical variables that can be used to improve models for estimating primary productivity and carbon fluxes in oligotrophic surface layers.

Marine protists are key components of biogeochemical cycles and microbial food webs, which respond quickly to environmental factors. In the Gulf of Mexico (GoM), the Loop Current intensifies in summer and supplies the gulf with warm and oligotrophic waters. However, the cyclonic eddies within the GoM create favorable conditions for biological productivity by bringing nutrient-rich water to the subsurface layer. In this study, we investigated the response of the protist community to the regional physicochemical conditions, its spatial and temporal variability, the influence of mesoscale structures, and its ecological roles in the mixed layer (ML) and deep chlorophyll maximum (DCM). This is the first study to conduct a V9-18S rRNA gene survey for this community in the Mexican Exclusive Economic Zone of the GoM. The regional distribution, temporal changes, and mesoscale structures significantly affected the structure of the protist community in the ML. In contrast, only mesoscale structures significantly affected the protist community in the DCM. Different protist assemblages were also present between the ML and DCM, with the Alveolata representing similar to 60% of the community in both layers, followed by haptophytes and MAST (Marine Stramenopiles) in the ML; pelagophytes and radiolarians were the more prevalent taxa in the DCM. Finally, co-occurrence analyses revealed that competition, parasitism, and predation were the potential interactions shaping these communities at both depths. The protist community of the Mexican GoM shows regional distribution and temporal changes influenced by depth-specific physicochemical conditions and mesoscale structures that differently shape the community and its interactions.

The microbial plankton community is an integral part of the pelagic ecosystem. It hosts essential functional groups that play a vital role in organic carbon production, release, uptake, and degradation within open-ocean ecosystems. Given its significance, carbon biomass estimates are urgently needed, especially in oligotrophic regions, to provide and enhance our knowledge of biogenic carbon pools. They also aid in validating biogeochemical models that characterize the functioning of these extensive marine ecosystems within the global carbon cycle. This study addresses the temporal variability of microbial community biomass in two oceanic zones: the west-central (Perdido) and southern (Coatzacoalcos) areas of the Gulf of Mexico. During three seasonally contrasting periods (nortes, rainy, and dry seasons), seawater samples were collected from the euphotic zone in both regions to estimate the carbon biomass of different pico- (<2–3 µm), nano-, and microplankton groups (>3–200 µm). Carbon biomass assessments for the microbial groups were based on their abundance and carbon conversion factors. Overall, we found a significant contribution of pico-prokaryotic components (heterotrophic bacteria, Prochloroccocus, and Synechoccocus) to the total microbial carbon stock of the euphotic zone (84–89 % global estimates). The finding suggests these microorganisms are key functional groups that drive carbon production and fate in the Gulf of Mexico ecosystem. Pico-cyanobacteria, especially Prochloroccocus, were the dominant primary producers (68–82 % total autotrophic carbon), mainly in the upper layer of the oligotrophic euphotic zone. This vertical pattern implies that the deep chlorophyll-a maximum (DCM) depth level was unrelated to a net increase in phytoplankton biomass in the three study periods. The distribution of microbial carbon biomass exhibited striking differences associated with winter mixing (the nortes season), high river discharge accompanied by cross-shelf transport (the rainy season), and the dynamics of mesoscale structures. Ecological aspects, such as the habitat preference of the organisms and the seasonal complementary development of mixotrophic and heterotrophic grazers and their prey, were also essential drivers in regulating the microbial carbon pool of both oceanic regions. The microbial carbon assessments conducted in this study contribute to identifying and quantifying key planktonic functional groups involved in the biogeochemical carbon cycle in the Gulf of Mexico open-ocean ecosystem.

Calanoid copepods (CC) are key contributors to the biological carbon pump and pelagic trophic dynamics. The deep-water regions of Perdido and the Bay of Campeche in the western and southern Gulf of Mexico (GM), respectively, differ in hydrography and productivity, leading to potential differences in copepod biomass and community structure. Zooplankton (0-200 m) were collected from the shelf edge to the deep-water region during the winter and summer autumn 2016. Calanoids contributed 38-60% of total zooplankton biomass and 55-70% of overall copepod abundance. The Bay of Campeche had the highest total zooplankton biovolume (287 +/- 120 ml 1000 m(-3)) and total mean copepod abundance (CC and non-calanoids similar to 146,000 ind. 1000 m(-3)) during summer-autumn, likely resulting from cross-shelf nutrient transport fueling local productivity. Adult females dominated calanoid numerical abundance (43-50%), thus suggesting a high reproductive potential. Cluster analysis showed differences between seasons (similar to 40% dissimilarity) but not regions. Environmental conditions explained 22% of the variability in community composition; the winter assemblage was significantly related to oxygen concentrations, whereas the summer-autumn community was related to warmer conditions and higher integrated chlorophyll-a concentrations. The CC community responded to seasonal changes more than regionally related hydrographic differences, with likely implications for organic matter cycling and export.

Summary In oligotrophic environments, interactions among eukaryotic microorganisms are highly complex. In the Gulf of Mexico (GoM), the Loop Current intensifies in summer and supplies the Gulf with warm and oligotrophic waters. However, mesoscale eddies within the GoM create favorable conditions for biological productivity by bringing nutrient-rich water to the subsurface layer. This study aimed to determine the structure, variability, and ecological roles of the protist in the mixed layer (ML) and deep chlorophyll maximum (DCM), representing the first V9-18S rRNA survey studying the protist community from the Southern GoM. Results revealed different assemblages between the ML and DCM. In the ML, species abundance was highly and positively correlated with temperature but negatively correlated with the nitrate concentration, whereas the opposite pattern was observed in the DCM. Alveolata represented ∼60% in both the ML and DCM, while Haptophytes and MAST dominated the ML, and Pelagophytes and Radiolarians dominated the DCM. Interestingly, Ostreococcus abundance increased under upwelling conditions suggesting that it may act as an indicator of the vertical nitrate flux and that picoeukaryotes respond to this instead of diatoms. Finally, our analyses revealed high levels of competition, parasitism, and predation with a high proportion of self-exclusion relationship (30%) in both depths.

Abstract. Surface chlorophyll concentrations inferred from satellite images suggest a strong influence of the mesoscale activity on biogeochemical variability within the oligotrophic regions of the Gulf of Mexico (GoM). More specifically, long-living anticyclonic Loop Current Eddies (LCEs) are shed episodically from the Yucatan Chanel and propagate westward. This study addresses the biogeochemical response of the LCEs to seasonal forcing and show their role in driving phytoplankton biomass distribution in the GoM. Using an eddy resolving (1/12°) interannual regional simulation based on the coupled physical-biogeochemical model NEMO-PISCES that yields a realistic representation of the surface chlorophyll distribution, it is shown that the LCEs foster a large biomass increase in winter in the upper ocean. The primary production in the LCEs is larger than the average rate in the surrounding open waters of the GoM. This behavior cannot be directly identified from surface chlorophyll distribution alone since LCEs are associated with a negative surface chlorophyll anomaly all year long. This anomalous biomass increase in the LCEs is explained by the mixed-layer response to winter convective mixing that reaches deeper and nutrient-richer waters.

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.

We report the seasonal variability of temperature, nutrients, and total and size-fractionated chlorophyll-a (Chl-a) in nearshore waters off northern Baja California (nBC), under conditions of the marine heatwave and El Nino that occurred in the northeastern Pacific during 2014-2015. Compared with the mean annual cycle (2008-2015), our study period was characterized by warmer waters, nitrate-impoverished, and with very low Chl-a concentration, which was closely associated with strong stratification and reduced upwelling conditions off nBC. Temperature anomalies were >2.0 degrees C by the end of 2014 when the marine heatwave prevailed, decreased during the spring-early summer upwelling season of 2015, and returned to >2.5 degrees C by the end of 2015 when El Nino appeared along the coast of nBC. As in 2008-2012 and closely coupled with upwelling, a seasonal cycle of total Chl-a was recorded under these abnormally warm conditions. However, relative to the mean annual cycle, total Chl-a and nitrate concentrations were significantly reduced (19%-55% and 40%-81%, respectively), with negative anomalies throughout the study period. Moreover, the seasonal evolution of the size-fractionated Chl-a concentration showed that smaller cells (<5 mu m) systematically contributed with the largest fraction (>60%) of the total Chl-a. Our findings indicate that the sequential occurrence of the marine heatwave in 2014 and El Nino in 2015 had a significant and sustained impact limiting the nitrate supply and reducing the total Chl-a in nearshore waters off nBC. In conclusion, our data reveal that a shift toward an oligotrophic state occurred in coastal waters off nBC during the warm period of 2014-2015.

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. Plain Language Summary Picoplankton communities are composed of microalgae and heterotrophic bacteria. They constitute an integral part of the marine food webs and play a significant role in oceanic biogeochemical cycles. In this study, we identify the importance of picoplankton carbon biomass within the pelagic ecosystem in open ocean Gulf of Mexico waters. Remarkable is the biomass contribution (>70%) of the tiniest photosynthetic cyanobacterium, Prochlorococcus, relative to the pico-phytoplankton carbon stocks. A partitioned vertical distribution of Prochlorococcus is found in the upper 150 m, showing the coexistence of two subgroups within the euphotic zone: high-light and lowlight ecotypes. Special interest is focused on deeper ecotypes (low-light Prochlorococcus), which are found close to the nutricline, living at low-light conditions (1% of surface irradiance) and, likely, exploiting a nutrient source (nitrate) that is commonly associated with larger phytoplankton cells. Additionally, we identify how these deep PRO populations seem to be introduced into gulf waters by warm anticyclonic eddies that episodically detach from the Loop Current, named Loop Current Eddies. The physical dynamics revealed here highlights the relevance of the Loop Current system as a mechanism of transport of microbial populations, regulating their carbon standing stocks, with significant implications for the carbon cycle of this ecosystem.

We evaluated the factors that control the variability in dissolved cadmium (Cd-d), phosphate (PO43-) and the Cd-d/PO43- ratio in the upper 1000 m of the deep region of the Gulf of Mexico (GoM), a marginal sea where trace element data are limited. These variables were compared to data from studies of adjacent areas (in the Western Central Atlantic), related to physical processes (mesoscale structures and upwelling events) occurring inside the GoM, and to dinoflagellate and diatom abundances in surface waters of this region. Vertical distributions of Cd-d concentrations and the Cd-d/PO43- ratio displayed the typical nutrient profile that has been extensively reported for different oceans. Cd-d and PO43- concentrations in the GoM were higher than those in the Western Central Atlantic. The slope of the Cd-d vs. PO43- relationship for samples within the Loop Current (LC) in the GoM was similar (242 pmol mu mol(-1), R-2 = 0.96) to that calculated for the Western Central Atlantic waters (same depth range) that reach the GoM through the LC (235 pmol mu mol(-1), R-2 = 0.98). The linear regression for all the GoM samples, though having a similar slope (249 pmol mu mol(-1)), presented a larger deviation (R-2 = 0.91). We propose that this deviation is due to spatial inhomogeneity that can be explained by 1) preferential remineralization of PO43- vs. Cd-d at subsurface waters in some regions, 2) Cd-d inputs from the north, transported by mesoscale processes, and 3) interactions with the continental slope at the minimum O-2 layer. Local changes in the relationship between Cd-d and PO43- were traced with Cd*, which represents the deviation of measured Cd to that expected by the deep-water Cd-d vs. PO43- relationship. Cd* presented mainly negative values at surface and subsurface waters (similar to 150 m) indicating preferential uptake of Cd-d vs. PO43- by the phytoplankton at the surface and preferential remineralization of PO43- vs. Cd-d from the organic matter at the subsurface waters. At deeper waters (similar to 400-1000 m) Cd-d was preferentially remineralized. In order to understand the inhomogeneity of the horizontal distribution of Cd-d, PO43- and the Cd-d/PO43- ratio in surface waters, three regions were defined considering their biological and physical characteristics. These were: I) the Oligotrophic region (in the northwest GoM) where the variability in the Cd-d/PO43- ratio was largely explained by salinity, II) the Upwelling region (off the coast of the Yucatan Peninsula) where the variability in the Cd-d/PO43- ratio was principally due to the diatom abundance and, III) Semi-permanent Cyclonic Gyre region (Bay of Campeche) where the variability in Cd-d and the Cd-d/PO43- ratio were primarily influenced by the upwelling generated by the cyclonic gyre, and the variability in PO43- was likely due to the consumption of this nutrient by phytoplankton. In summary, physical processes, river inputs, and phytoplankton abundance appear to play an important role in explaining the spatial variation in the Cd-d/PO43- ratio.

Experiments were carried out at a coastal upwelling site (ENSENADA station) off northern Baja California (Mexico) during autumn 2015 (OCT-15) and spring 2016 (APR-16) to estimate phytoplankton daily growth (μo) and mortality (m) rates and to assess microzooplankton grazing impact (m:μo) on the phytoplankton community and specific autotrophic groups. In accordance with regional seasonality and under an environmental warming scenario due to the El Nino 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 μo 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. En un sitio de surgencia costera (estacion ENSENADA) frente a la region norte de Baja California (Mexico) se realizaron experimentos durante el otono de 2015 (OCT-15) y la primavera de 2016 (ABR-16) para estimar las tasas diarias de crecimiento (μo) y mortalidad (m) del fitoplancton, que permitieron evaluar el impacto (m:μo) que genera el pastoreo diario del microzooplancton sobre la comunidad de fitoplancton y grupos autotrofos especificos. Acorde con la estacionalidad de la region y bajo el escenario de un calentamiento ambiental debido al evento El Nino 2015–2016, se observaron notables diferencias en las condiciones hidrograficas y en la dinamica de crecimiento y mortalidad de la comunidad de fitoplancton entre los 2 periodos de estudio. Las estimaciones de μo 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 anomalo sobre la comunidad del fitoplancton fueron mas evidentes durante OCT-15. En ese periodo, los componentes autotroficos mas grandes (diatomeas) estuvieron fuertemente restringidos en su crecimiento, posiblemente por una limitacion ambiental de nutrientes ocasionada por el hundimiento de la termoclina debido al mayor aporte de agua calida. Ademas, se estimo un activo consumo del microzooplancton sobre la biomasa (72% de clorofila a [Chla]) y produccion 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 estimo una PP alta (3.73 g Chla·L–1·d–1) y solo una tercera parte de esta fue consumida por el microzooplancton (34% de la PP). Los resultados de esta investigacion revelan el alto dinamismo de las tramas troficas multivoras acoplado a la variabilidad estacional e interanual de los sistemas de surgencia costera.

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 (µo) and mortality (m) rates and to assess microzooplankton grazing impact (m:µo) 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 µo 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.

In order to characterize the carbon biomass spatial distribution of autotrophic and heterotrophic picoplankton populations linked to mesoscale dynamics, an investigation over an extensive open-ocean region of the southern Gulf of Mexico (GM) was conducted. Seawater samples from the mixed layer were collected during wintertime (February–March 2013). Picoplankton populations were counted and sorted using flow cytometry analyses. Carbon biomass was assessed based on in situ cell abundances and conversion factors from the literature. Approximately 46% of the total picoplankton biomass was composed of three autotrophic populations (Prochlorococcus, Synechococcus, and pico-eukaryotes), while 54% consisted of heterotrophic bacteria populations. Prochlorococcus spp. was the most abundant pico-primary producer (>80%), and accounted for more than 60% of the total pico-autotrophic biomass. The distribution patterns of picoplankton biomass were strongly associated with the mesoscale dynamics that modulated the hydrographic conditions of the surface mixed layer. The main features of the carbon distribution pattern were: (1) the deepening of picoplankton biomass to layers closer to the nitracline base in anticyclonic eddies; (2) the shoaling of picoplankton biomass in cyclonic eddies, constraining the autoprokaryote biomasses to the upper layers, as well as accumulating the pico-eukaryote biomass in the cold core of the eddies; and (3) the increase of heterotrophic bacteria biomass in frontal regions between counter-paired anticyclonic and cyclonic eddies. Factors related to nutrient preferences and light conditions may as well have contributed to the distribution pattern of the microbial populations. The findings reveal the great influence of the mesoscale dynamics on the distribution of picoplankton populations within the mixed layer. Moreover, the significance of microbial components (especially Prochlorococcus) in the southern GM during winter conditions was revealed, indicating that they may play an important role in the pelagic food web, and that they may have a substantial impact on the carbon cycle in oligotrophic regions.

A slide preparation method for seawater samples preserved in acid Lugol's is presented here as an alternative to the traditional Utermöhl settling chamber method for microplankton analysis. This preparation maintains the integrity of fragile cells, such as ciliates, resolves issues associated with the transience of samples prepared in settling chambers, and allows the use of automated image acquisition methods. Samples are filtered onto polycarbonate membranes and analyzed with transmitted light microscopy. The visibility of pore outlines is minimized by using mounting oil (Cargille Series A immersion oil, Certified Refractive Index, n D 25°C 1.5840 ± 0.0002) with a refractive index matching that of the membrane material. We assessed the efficacy of this new method by comparing abundance and biomass estimates for ciliates in settled and filtered samples. Acceptable results were found for the most delicate of samples stored long‐term in acid Lugol's. Some cell shrinkage occurred during the filtration and brief drying steps. Therefore, corrections for ciliate length and width measurements in filtered samples were determined to counteract this effect on total cell biovolume. Overall, the method provides a simple and stable alternative to settling chamber analysis for ciliates preserved in acid Lugol's.

The microtubule (MT) “plus end” constitutes the platform for the accumulation of a structurally and functionally diverse group of proteins, collectively called “MT plus-end tracking proteins” (+TIPs). +TIPs control MT dynamics and link MTs to diverse sub-cellular structures. Neurospora crassa MicroTubule Binding protein-3 (MTB-3) is the homolog of yeast EB1, a highly conserved +TIP. To address the function of MTB-3, we examined strains with mtb-3 deletions, and we tagged MTB-3 with GFP to assess its dynamic behavior. MTB-3-GFP was present as comet-like structures distributed more or less homogeneously within the hyphal cytoplasm, and moving mainly towards the apex at speeds up to 4× faster than the normal hyphal elongation rates. MTB-3-GFP comets were present in all developmental stages, but were most abundant in mature hyphae. MTB-3-GFP comets were observed moving in anterograde and retrograde direction along the hypha. Retrograde movement was also observed as originating from the apical dome. The integrity of the microtubular cytoskeleton affects the presence and dynamics of MTB-3-GFP comets, while actin does not seem to play a role. The size of MTB-3-GFP comets is affected by the absence of dynactin and conventional kinesin. We detected no obvious morphological phenotypes in Δmtb-3 mutants but there were fewer MTs in Δmtb-3, MTs were less bundled and less organized. Compared to WT, both MT polymerization and depolymerization rates were significantly decreased in Δmtb-3. In summary, the lack of MTB-3 affects overall growth and morphological phenotypes of N. crassa only slightly, but deletion of mtb-3 has strong effect on MT dynamics.

We investigated the temporal dynamics of carbon flow through the microbial food web of a coastal upwelling system (ENSENADA station) off northern Baja California during 6 cruises (September 2007 to November 2008). Carbon biomass assessments for major autotrophic size groups (pico- to micro-sized cells) and their microzooplankton grazers were based on analyses using flow cytometry, HPLC pigments and epifluorescence microscopy. Taxon-specific phytoplankton growth and microzooplankton grazing rates were determined from 24 h in situ incubations in the euphotic zone using an abbreviated 3-treatment dilution technique. Carbon biomass and instantaneous growth and grazing rate determinations were used to estimate daily rates of taxon-specific production and losses due to microzooplankton grazing. Overall, microbial biomass showed a close balance between autotrophic and heterotrophic components, except during a period of very strong upwelling (April 2008), which favored large phytoplankters and high primary production. Throughout a wide range of environmental conditions, the community primary production (PP) attributed both to small (mostly picophytoplankton and prasinophytes) and large (mostly diatoms and autotrophic dinoflagellates) autotrophs was significantly grazed (78 +/- 9% of PP) by small (< 20 mu m) and large (> 20 mu m) ciliates and flagellates (including mixotrophic dinoflagellates), respectively, showing complementary temporal shifts in protistan grazer types that matched the dominant phytoplankton. While large diatoms were strongly consumed by large ciliates during the 2 most productive periods (September 2007 and April 2008), pico- and nano-sized phytoplankton were grazed most by nanoflagellates and small ciliates from November 2007 to January 2008. Consequently, biogenic carbon production in this ecosystem is transferred through a multivorous food web.

The temporal variability of the physical and chemical conditions of coastal waters off Ensenada, Baja California (Mexico) was characterized. A historical analysis was made based on 11 years (1998-2008) of temperature and salinity data records measured quarterly by IMECOCAL, along a transect perpendicular to the coast (CalCOFI line 100). Moreover, the physical and chemical conditions at a coastal monitoring observatory called station ENSENADA were described using a 2-year data series (October 2006-November 2008) obtained with improved temporal resolution. The historical analysis of line 100 showed marked seasonal variability in the thermohaline conditions associated with fluctuations in the flow of the equatorward California Current and the poleward California Undercurrent, as well as with coastal upwelling events whose magnitude and frequency increase towards spring-summer. Interannual variability was also observed, related to warm and/or cold ENSO phases that modify the characteristics of the water column in this coastal region. The most striking characteristics of the interannual variability at station ENSENADA were La Nina conditions recorded from summer 2007 to mid 2008. During this cold ENSO phase, temperature, salinity, dissolved oxygen, density, and dissolved inorganic carbon data revealed the anomalous presence of subsurface water at the surface layers in spring 2008. Results suggest that the coastal observatory is sensitive to the temporal variability of hydrographic conditions on shelf coastal waters ( < 50 km) off Ensenada in the northern BC region. Consequently, station ENSENADA would be a good location to high-frequency monitors the oceanographic conditions of the transitional region between tropical/subtropical and subarctic systems of the California Current System. (C) 2010 Elsevier Ltd. All rights reserved.

The ecological dynamics of picoplankton were investigated at a coastal upwelling system of northern Baja California during six cruises (September 2007-November 2008). Populations of Prochlorococcus, Synechococcus, PicoEukaryotes and heterotrophic bacteria were assessed by flow cytometry (FCM). On each sampling date, we used an abbreviated three-treatment dilution technique and C-14-uptake experiments to determine population (FCM) and community (TChl a) rates of growth, grazing and production from 24-h in situ incubations at three to four euphotic depths. Overall, picoplankton comprised an active and important component of the community, with biomass values (2.3-69.8 mu g C L-1) and production rates (0.8-68.4 mu g C L-1 day(-1)) that varied positively with Chl a and community C-14-production. The exception was an intense algal bloom (> 25 mu g Chl a L-1) during La Nina-intensified upwelling conditions in April 2008, during which biomass and production estimates of picophytoplankton were at their lowest levels, suggesting that the smallest primary producers were being replaced by larger cells. Thus, for most of the environmental circumstances encountered during our study, our results supported the recent "rising tide" hypothesis that improved growth (nutrient) conditions benefit all size classes, including picophytoplankton. Under extreme conditions of upwelling, however, the picophytoplankton declined abruptly, despite seemingly strong (average) growth rates. Future studies need to provide a better mechanistic understanding of the physical (advection), physiological (nutrient uptake and temperature) and ecological (food web) factors that result in this dramatic nonlinearity in picophytoplankton response to system forcing and richness.

Zooplankton samples collected during October 1997 (El Nino) and October 1999 (La Nina) were used to study the effects of warm and cold events on the community structure of euphausiids from Punta Abreojos (26.7 degrees N) to Punta Baja (30 degrees N), Baja California, Mexico. A total of 24 species (7 genera) were found. Total abundance was 16% higher during the warm event than during the cold conditions, mainly because of a higher contribution of larvae. The most abundant species in both periods was Nyctiphanes simplex. Other species showed low abundance, and their presence is presumed to be attributable to the particular oceanographic conditions associated with El Nino and La Nina. Differences in abundances of life stages indicated more intense reproductive activity in October 1997 or better larval-survival strategy than during the cold period. Species with tropical and equatorial affinity were more abundant in October 1997, whereas species with subarctic and temperate affinity were more abundant during October 1999.

RESUMEN. Se analizo la distribucion espacio-temporal de los eufausidos colectados en cuatro cruceros estacionales realizados frente a la costa de Concepcion en 1991. Se identificaron ocho especies, Euphausia mucronata, E. vallentini, Nyctiphanes simplex, Nematoscelis megalops, Nematobrachion flexipes, Stylocheiron longicorne, S. affine y S. suhmii. La mayoria de las especies habia sido registrada en aguas del Pacifico suroriental, a excepcion de S. suhmii, que se distribuye comunmente en el Pacifico central. La presencia de S. suhmii constituye su primer registro en el Sistema de la Corriente de Humboldt. Del total de especies, E. mucronata (76%) y N. simplex (23%) fueron las mas abundantes y frecuentes. Los eufausidos se registraron en todos los periodos del ano, aunque con fluctuaciones estacionales y espaciales asociadas a las caracteristicas oceanograficas del area de estudio. Los maximos estacionales se determinaron en otono y los minimos en invierno, presentando diferencias significativas en su distribucion espacial. Se encontro una relacion inversa entre la abundancia de E. mucronata y la concentracion de oxigeno disuelto, asociada a eventos de surgencia costera. Palabras claves: eufausidos, distribucion, abundancia, surgencia, Concepcion, Chile. Spatial and temporal variability of eufausiids off Concepcion coast, Chile