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Mathematical algorithms relate satellite data of ocean color with the surface Chlorophyll-a concentration (Chl-a), a proxy of phytoplankton biomass. These mathematical tools work best when they are adapted to the unique bio-optical properties of a particular oceanic province. Ocean color algorithms should also consider that there are significant differences between datasets derived from different sensors. Common solutions are to provide different parameters for each sensor or use merged satellite data. In this paper, we use satellite data from the Copernicus merged product suite and in situ data from the southernmost part of the California Current System to test two widely used global algorithms, OCx and CI, and a regional algorithm, CalCOFI2. The OCx algorithm yielded the most favorable results. Consequently, we regionalized it and conducted further testing, leading to significant improvements, especially in eutrophic and oligotrophic waters. The database was then separated according to (a) dynamic boundaries in the area, (b) bio-optical properties, and (c) climatic conditions (El Niño/La Niña). Regional algorithms were obtained and tested for each partition. The Chl-a retrievals for each model were tested and compared. The best fit for the data was for the regional algorithms that considered the climatic conditions (El Niño/La Niña). These results will allow for the construction of consistent regionally adapted time series and, therefore, will demonstrate the importance of El Niño/La Niña events on the bio-optical properties of the area.

In the ocean, nitrogen availability is an important control of primary production and influences the amount of energy flowing through food webs. Mesoscale eddies play important roles in modulating the spatial distributions of physical and biogeochemical properties in the Gulf of Mexico (GM), including the availability of nitrate + nitrite (NN). In this study, we explore an oceanographic station classification based on the integrated NN stock that we have named the “nitracentric classification” and a classification based on hydrographic variables that we call the Best Fit Variables (BFVs), such as the depth of the 20°C isotherm and the depth of the 26 kg m-3 isopycnal, to identify stations under the influence of mesoscale eddies. We analyzed hydrographic profiles of CTD data and the NN concentrations in discrete samples collected in June 2016 during the oceanographic campaign XIXIMI-5, which was conducted in the deep-water region of the GM. The best station separation was produced when the NN concentration was integrated between the surface and 200 m depth, which was supported by the station classification based on the BFVs. Our classification system produces a better separation between station groups when compared to other classifications that rely on the use of altimetric variables and hydrographic criteria that have been previously employed to study biogeochemical and physical processes in the GM. We obtained parameterizations that accurately predicted the NN profiles between 100–500 m of stations sampled under stratified conditions in two other XIXIMI cruises in the gulf, although the parameterization has to be adapted to obtain accurate predictions under winter mixing conditions. Our results can be used to predict nitrate stocks and profiles based on a single BFV value obtained from the existing hydrographic databases of the GM as well as from CTD data at the time of sampling. The analysis of the CLIVAR Section A22 in the Caribbean Sea indicates that the nitracentric and hydrographic classification methodology developed in this study can also be applied to other oligotrophic basins where mesoscale eddies play important roles in controlling the distributions of hydrographic and biogeochemical properties.

Habitat characterization provides predictive information about the distribution of species and is useful for as­sessing habitat quality and population stability. Larval abundance of six frequent and abundant demersal species and the relationship of each with the environment were analysed through generalized additive models to determine their preferential distribution and predictive response to the environmental variables in the southern part of the California Current (25-31°N) between two periods of data collection: 1997-2000 and 2006-2010. Essentially, the main associated variables governing the distribution patterns were related to common and oceanographic characteristics of the water column (temperature and salinity at 50 m depth, dynamic height and degree of water column stratification); however, the set of variables and their ranges are usually species-specific. Species of northern distribution, Sebastes sp. and Citharichtys stigmaeus, were recorded mainly in newly emerged, relatively unstratified waters characterized by a shallow mixed layer and low temperatures. Low dynamic height values were the most significant predictor of larval distribution for Merluccius productus. Citharichthys xanthostigma and Symphurus atricaudus were widespread, distributed across the study area mainly in autumn in unstratified or stratified waters and at a shallow mixed layer. Particularly C. Xanthostigma and S. Lucioceps were related to high dynamic height val­ues, likely influenced by a coastal flow towards the pole, as evidenced by counter-currents.

Los remolinos son estructuras transitorias que influyen en gran medida en la circulación promedio del océano. Modifican la distribución de masa y propiedades como calor, sal, clorofila y partículas inertes. La capacidad que tienen los remolinos para transportar propiedades o partículas depende de su capacidad de retención. En este estudio se identificaron y caracterizaron los remolinos de mesoescala del noroeste del golfo de México (NOGM) a través de un método lagrangiano que permite evaluar el tiempo de retención y la fracción de masa que pueden retener y transportar. Para el análisis, se utilizaron datos diarios de altimetría de 1993 a 2016. En el periodo de estudio se detectaron un total de 254 remolinos, 73 anticiclones y 181 ciclones. Se identificó una región (94.75º W, 26.75º N) donde ocurren ~30% del total de los remolinos ciclónicos detectados entre las isóbatas de 1,000 y 2,500 m. En promedio, el radio de los remolinos fue de ~40 km para la isobata 2,500 m. Los remolinos de mesoescala del NOGM pueden trasportar ~60% de la masa que contenían al momento de ser detectados. En promedio, el transporte de masa ocurrió por 33 d para los ciclones y por 26 d para los anticiclones. Rara vez ocurrió por 60 d o más.

This study proposes a method to detect ocean fronts from in situ temperature and density glider measurements. This method is applied to data collected along the CalCOFI Line 90, south of the California Current System (CCS), over the 2006–2013 period. It is based on image-processing techniques commonly applied to sea surface temperature (SST) satellite data. Front detection results using glider data are consistent with those obtained in other studies carried out in the CCS. SST images of the Multi-scale Ultra-high Resolution (MUR) dataset were also used to compare the probability of occurrence or front frequency (FF) obtained with the two datasets. Glider and MUR temperatures are highly correlated. Along Line 90, frontal frequency exhibited the same maxima near the transition zone (~130 km offshore) as derived from MUR and glider datasets. However, marked differences were found in the bimonthly FF probability with high (low) front frequency in spring-summer for glider (MUR) data. Methodological differences explaining these contrasting results are investigated. Thermohaline-compensated fronts are more abundant towards the oceanic zone, although most fronts are detected using both temperature and density criteria, indicating a significant contribution of temperature to density in this region.

Between 1997 and 2016, the Investigaciones Mexicanas de la Corriente de California (IMECOCAL, Mexican Investigations of the California Current) program conducted measurements of chlorophyll a and phytoplankton production off Baja California. Here, we summarize the main results obtained during the survey period. Long-term means indicated that high productivity occurred along the coastal zone where upwelling occurs, and in a semi-permanent cyclonic gyre off Punta Eugenia. At the seasonal scale, the variability of integrated primary production showed differences with respect to integrated chlorophyll a variability. Integrated chlorophyll a exhibited maximum values during the coastal upwelling season in spring and summer, while minimum values occurred during the inflow of tropical-subtropical waters south of Punta Eugenia in autumn and winter. In contrast, the highest values of integrated primary production occurred in spring and summer to the north of Punta Eugenia, whereas to the south of this location, the highest values occurred in winter and autumn. This seasonal variability was associated with phytoplankton size and maximum photosynthetic rates. At the interannual scale, low productivity was associated with increased stratification during the subarctic water intrusion events and the “Blob” and El Niño events in 2015–2016. Productivity time-series were in agreement with the seasonal variability of the north Pacific Gyre Oscillation index. Given that phytoplankton is the first level of the marine food web, this study contributes to the understanding of regional variations in higher trophic levels at the seasonal and interannual scales.

Phytoplankton photosynthetic parameters (maximum light utilization coefficient, alpha*; maximum photosynthetic rate, P*m; maximum quantum yield, phi-max) off Baja California were estimated from samples collected at the 50% light level during winter, spring, summer, and autumn 1999. Chlorophyll concentration was also determined, and in situ experiments were conducted using the 14C method to determine primary production (PP) in the euphotic zone. The highest alpha* and phi-max values were found during the spring survey, mainly at the coastal areas, associated with diatom and dinoflagellate abundances and high chlorophyll concentrations. However, high P*m values were measured during autumn in both inshore and offshore areas due to the presence of smaller phytoplankton cells. The average profiles of the photosynthetic parameters (alpha*(z) and P*m(z)) in the water column were estimated for 2 regions. These parameters were used to estimate PP with semi-analytical models. Modeled PP was contrasted with in situ PP to validate our calculated photosynthetic parameters in the euphotic zone. In general, modeled PP values were similar to in situ values (94 mg C·m–2·h–1), with a high correlation coefficient (r = 0.85). Modeled PP for coastal waters in the northern and central regions was 1.5-fold greater than in situ estimates during spring. In conclusion, when used with remotely sensed phytoplankton pigment and surface irradiance data, the average profiles of alpha* and P*m(z) could be a useful tool to calculate PP in our study area.

The ocean off the Baja California Peninsula (Mexico) has been affected by interannual processes. Some of these processes have produced changes in oceanic circulation and the atmosphere, which have been reflected in the structure of the water column. Stratification, defined as the amount of energy needed to mix water throughout the water column, determines nutrient availability in the euphotic zone for phytoplankton growth. The aim of this study is to describe and relate the interannual variations of phytoplankton production and biomass with changes in the structure of the water column. To this end, we analyzed time series (1997–2016) for integrated Chlorophyll a, integrated primary production, pycnocline depth, mixed layer depth, and stratification indices along line 100 of the Investigaciones Mexicanas de la Corriente de California (IMECOCAL) program. The results showed 2 periods of high stratification and a decrease in phytoplankton production and biomass off Baja California, corresponding to the intrusion of subarctic water and El Niño 2015–2016. Finally, by using generalized additive models, we related 2 photosynthethic parameters-maximum photosynthetic rate and maximum light utilization coefficient-with water stratification. These relationships explained interannual variations in phytoplankton production in terms of water column stratification. The time series and the analysis reported here for IMECOCAL line 100 could be used to extrapolate the effects of interannual physical processes on phytoplankton in other zones off the Baja California peninsula.

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.

A succession of interannual warm events occurred in the Pacific during the 2013–2016 period, when positive thermal anomalies were detected. A broad region with abnormally warm waters in the northeastern Pacific Ocean was observed in 2013, which by 2015 had reached waters off Baja California (Mexico). In addition to the ongoing warming effect, early evidence of an El Niño event was detected in 2015. This paper analyzes hydrographic data measured at Todos Santos Bay and environs, as well as satellite-derived sea-level data, to characterize changes in the hydrographic conditions during the 2013–2016 period. The highest positive temperature anomalies (~5 ºC) were observed in September 2015, with a steep decline to neutral conditions in spring 2016. Records for the bay showed slight variations in hydrographic conditions during spring, highlighting the importance of local over remote forcings during this time of year. In autumn, by contrast, interannual signals prevailed over local processes. Satellite-derived sea-level data revealed an alongshore propagation of remote signals, identified as shallow coastal poleward flows.

The response of primary producers to seasonal and interannual variabilities in the hydrographic conditions observed between 1997 and 2012 is analyzed for the southern portion of the California Current System (CCS). The analysis uses the optimum rate of primary productivity (PP) normalized by units of chlorophyll (Chla) in the water column (PBopt) and Chla concentration. In situ PP estimations using the 14C method were obtained as part of the seasonal cruises conducted by the Investigaciones Mexicanas de la Corriente de California program. Supplementary data included sea surface temperature (SST) as measured by the AVHRR sensor from 1985–2009. We found the mean value of PBopt to be at 5.1 ± 3.3 mg C·(mg Chla)–1·h–1, with maximum ranges of 0.5 and 17.5 mg C·(mg Chla)–1·h–1. The relationship between PBopt and SST suggested a phytoplankton community change at around 19 ºC, which characterized the transitional nature of the southern portion of the CCS. SST data suggested, on the one hand, that on average the 19 ºC isotherm is located in the vicinity of Punta Eugenia and, on the other, that its spatial variability defined the alternating cool–warm conditions. At the seasonal scale, this isotherm showed a marked latitudinal displacement (from 24ºN to 32ºN), which was observed to be even out of this range during interannual events associated with El Niño/La Niña. Under both cool-warm hydrographic conditions, the phytoplanktonic community showed large PBopt rates (~6 mg C·(mg Chla)–1·h–1). At the interannual scale, changes were associated to changes in the abundance and composition of nano-microphytoplankton. Additionally, data suggested that high PP rates during warm periods could be attributed to an enhanced picoplankton contribution.

Due to their sensitivity to climate forcing that may alter their abundance and distribution, small pelagic fish are important ecological indicators of the state of the California Current System. They are schooling, planktivorous fish that provide forage for higher trophic levels. We describe the life cycle histories and patterns of distribution of sardine, anchovy, and mackerel species occupying the waters along the west coast of the Baja California Peninsula (Mexico). The links between the physical and biological forcing and the structure and condition of their habitats, their patterns of movement, productivity, and stock structure (where information is available) are described in relation to the fisheries. We have used the catches as a proxy for the history of the different stocks, while recognizing that fisheries-derived information is biased by the operational scale and only covers areas where fish are available. We have relied mainly on studies of sardines to evaluate the validity of the principal paradigms in fisheries oceanography. We describe how the environment can structure a population by relating an example given by the spatiotemporal variability of the northern and southern stocks of Pacific sardine (Sardinops sagax) and Pacific mackerel (Scomber japonicus) from 2000 to 2014. During this period, the northern stocks showed a marked tendency to contract southwards, as far south as Magdalena Bay, in response to the regional cooling that began in the previous decade. It appears that the combined effects of fishing and climate change may sufficiently alter habitat characteristics so that both the distribution and productivity of a population are shifted.

Fetch-limited wave growth was analyzed using high spatial and temporal resolution measurements obtained with high-frequency (HF) radars. To calculate the wave spectrum, the second-order echo (S2N) was extracted from the Doppler spectrum and mapped to the wave frequency domain. The conversion of S2N to the wave frequency spectrum was carried out using a linear parametric model dependent on wind speed. Wave growth, represented by the dimensionless energy and peak frequency as a function of dimensionless fetch, was determined from the spectra calculated for 25 cells (~400 km2) in the vicinity of a moored Air–Sea Interaction Spar buoy. The buoy data were used as reference and to provide the wind information required for the wave growth analysis. The simultaneous data from the buoy and HF radars showed a suitable agreement with the wave growth curves. The analysis of the 25 cells showed an adequate agreement and certain dispersion around the growth curves. Possible causes of this dispersion are related to errors in the estimation of the frequency spectrum and fetch, and the assumption of homogeneous wind conditions. From the limited data set used, it was not possible to prove any impact of swell on wave growth; however, it is possible that analysis over a wider range of swell heights may provide this evidence. Nevertheless, the results suggest that the linear parametric model is able to reproduce fetch-limited wave growth. This work represents the first effort focused on fetch-limited wave growth using HF radars.

BACKGROUND: Highly productive hotspots in the ocean often occur where complex physical forcing mechanisms lead to aggregation of primary and secondary producers. Understanding how hotspots persist, however, requires combining knowledge of the spatio-temporal linkages between geomorphology, physical forcing, and biological responses with the physiological requirements and movement of top predators. METHODOLOGY/PRINCIPAL FINDINGS: Here we integrate remotely sensed oceanography, ship surveys, and satellite telemetry to show how local geomorphology interacts with physical forcing to create a region with locally enhanced upwelling and an adjacent upwelling shadow that promotes retentive circulation, enhanced year-round primary production, and prey aggregation. These conditions provide an area within the upwelling shadow where physiologically optimal water temperatures can be found adjacent to a region of enhanced prey availability, resulting in a foraging hotspot for loggerhead sea turtles (Caretta caretta) off the Baja California peninsula, Mexico. SIGNIFICANCE/CONCLUSIONS: We have identified the set of conditions that lead to a persistent top predator hotspot, which increases our understanding of how highly migratory species exploit productive regions of the ocean. These results will aid in the development of spatially and environmentally explicit management strategies for marine species of conservation concern.

The effect of Santa Ana winds on bio-optical properties (absorption coefficients of particles, detritus and phytoplankton) off Baja California (Mexico) was evaluated using data obtained during four cruises in January, April, August and October 1999. Meteorological data from a station at Todos Santos Bay and satellite-derived wind data indicated three Santa Ana events during October 1999, with low relative humidity, high air temperature and persistent ENE winds. The absorption coefficient of detritus (adT) in surface samples was ten-fold higher in October because of the inorganic material wind-driven by the Santa Ana events that occurred from 9 to 11 and from 17 to 20 October, 1999. The absorption coefficients of particles (ap) and phytoplankton (aɸ) in the euphotic zone were higher in the April survey as a result of higher phytoplankton abundance. The ap, a and Kd values in the euphotic zone were fitted to chlorophyll using a power function, with an explained variance of 37%, 72% and 16%, respectively. On excluding the October adT surface values, chlorophyll explained 82% of the changes in the absorption coefficients (ap and aɸ), and 87% of the Kd variability. Approximately 80% of the chlorophyll concentration can be explained by the water column spectral reflectance relationship: Rrs(443)/Rrs(555). According to our results, regional chlorophyll estimates from water column bio-optical information were not greatly affected by wind-borne dust transported during the October 1999 Santa Ana events.

Oceanographic conditions in the Pacific waters off Baja California are described for the period 2002–2003. Anomalies in the temperature-salinity relationships were observed from October 2002 to April 2003, with a clear contribution of low salinity and low temperature subarctic water in the upper 100 m of the water column. Below 100 m, physical data suggested weak El Niño conditions. Weakened or poleward winds were not observed. The pelagic ecosystem during the intrusion of subarctic waters showed strong seasonal changes, with localized centers of high chlorophyll and zooplankton in July 2002, followed by a strong decrease in the following autumn and winter seasons. Both chlorophyll and zooplankton rebounded in April 2003, but zooplankton showed a stronger onshore-offshore gradient. There was no strong biological signal. The zooplankton taxa that showed an increase during El Niño 2002–2003 were salps and chaetognaths. The increase of salps did not appear to be related to this event, due to their seasonal occurrence limited to the northern region. In contrast the increase of chaetognaths presented a similar pattern to that observed in El Niño 1997–1998.