Baja California Coast Research Articles

A new species of chimaera, Hydrolagus melanophasma sp. nov. (Chondrichthyes: Chimaeriformes: Chimaeridae), from the eastern North Pacific

A new species of chimaera, Hydrolagus melanophasma sp. nov. (Chimaeridae), is described from the eastern North Pacific. It is distinct from other eastern Pacific chimaeroids by the following characteristics: a large slightly curved dorsal fin spine extending beyond dorsal fin apex, a long second dorsal fin of uniform height throughout, large pectoral fins extending beyond the pelvic fin insertion when laid flat, trifid claspers forked for approximately one-quarter the total clasper length and a uniform black coloration throughout. The new species is compared to other eastern Pacific members of the genus Hydrolagus including H. alphus, H. colliei, H. macrophthalmus, and H. mccoskeri. Remote Operated Vehicle (ROV) video footage has identified and documented Hydrolagus melanophasma from the Gulf of California. ROV observations suggest that individuals typically occur over soft-bottom habitats or cobble patches with minimal vertical relief. This is in contrast to other eastern Pacific Hydrolagus species that tend to occur in areas of high rocky relief. The known distribution of this new species at present extends from southern California, U.S.A., along the Pacific coast of Baja California, Mexico, and into the Gulf of California.

Reproductive biology of the banded guitarfish, Zapteryx exasperata, from the Gulf of California, México

The banded guitarfish, Zapteryx exasperata, is a common species in the artisanal elasmobranch fisheries in the Gulf of California. Fishery-derived specimens were examined to determine critical aspects of the reproductive biology of this poorly known group of rays. Two functional testes and ovaries were found in males and females respectively. Median size at maturity (L50%) for males was 64 cm total length (TL) and for females 69 cm TL. Average length of gravid females was 78.8±3.8 cm TL. Histological analysis showed no evidence of sperm storage in the oviducal gland. Gestation was estimated to occur over a five to six month period (February–July) and was concurrent with vitellogenesis. Mean uterine fecundity was estimated as 7 (range 2–13, SD. = 3). The sex-ratio of embryos was 1:1. The reproductive cycle for Z. exasperata from the Gulf of California was estimated to last one year with parturition, ovulation and copulation occurring during the summer months. Differences found in the reproductive cycle of Z. exasperata between the Gulf of California and the west coast of Baja California could have implications in the future management plans for this fishery in the Gulf of California.

Geographic Variation in Plumage Pattern and Coloration of Savannah Sparrows

Abstract We used principal coordinates analysis to analyze variation in 11 plumage characteristics of 1,722 Savannah Sparrows (Passerculus sandwichensis) breeding at 55 different sites from throughout the species' range in Canada, Mexico, and the United States. There is clinal interpopulational variation in size with considerable overlap among localities. Savannah Sparrows resident along the Pacific coast of southern California and Baja California, and those from along the coast of Sinaloa and Sonora (i.e., saltmarsh Savannah Sparrows) differ from those throughout the rest of their range (i.e., non-saltmarsh). Variation among populations of non-saltmarsh Savannah Sparrows was clinal with the exception of those from Sable Island, Nova Scotia, which are consistently more pallid than birds from the adjacent mainland. Western Savannah Sparrows are more pallid than those in the east. The median crown stripe of eastern Savannah Sparrows is distinct, whereas the median crown stripe of western Savannah Sparrows g...

The artisanal elasmobranch fishery on the east coast of Baja California, Mexico: Characteristics and management considerations

The Mexican elasmobranch fishery ranks among the most important in the world, but details associated with fishing effort and species composition are generally unavailable, particularly for the extensive artisanal fishery. Seasonal surveys were conducted during 1998–1999 in the state of Baja California to determine the extent and activities of the artisanal elasmobranch fishery along its Gulf of California coast. Artisanal fishermen were highly opportunistic, and temporally switched targets depending on the local abundance of teleost, invertebrate, and elasmobranch fisheries resources. At least 17 species of shark, 13 species of ray, and 2 species of skate were identified among the 4495 specimens observed. Small, coastal sharks and rays were typically landed with bottom set gillnets during all surveyed seasons (spring–autumn) and large sharks were targeted with surface set (drift) gillnets during summer and autumn. Sharks and rays contributed similarly to elasmobranch landings, comprising 50.9% and 48.3%, respectively, of the total recorded catch. Overall landings were dominated by mustelid sharks (Mustelus henlei, Mustelus spp.) and the shovelnose guitarfish (Rhinobatos productus), with Alopias pelagicus, Gymnura spp., Heterodontus mexicanus, Mobula munkiana, Myliobatis californica, and Squatina californica also representing important components of the landings. Alopias vulpinus had previously been unreported for the Gulf of California and a new maximum size record was documented for Myliobatis longirostris. Catch rates (mean elasmobranchs/vessel/trip ± standard error) were lowest during autumn (17.4 ± 3.9) and highest in spring (35.3 ± 14.1). Adult, commonly gravid, A. pelagicus, Carcharhinus falciformis, and R. productus females comprised a disproportionately greater component of the landings than did their male counterparts. Given the decades of largely unrestricted exploitation of elasmobranchs in Mexican waters, population declines and shifts in size structure are likely to have occurred among those species with the lowest fecundity and latest ages at maturity.

Growth and mortality of the fish Citharichthys xanthostigma (Pleuronectiformes: Paralichthyidae) off the Western coast of Baja California, México

Longfin sanddab (Citharichthys xanthostigma) represents a very important fishery resource in Southern and Baja California but are not very well known. The purpose of this study is to provide information on the growth and mortality of longfin sanddab population in the Mexican Pacific Ocean at Baja California, México. Data on growth were obtained for longfin sanddab collected with otter trawls during six cruises off the Western coast of Baja California. A total of 1017 longfin sanddab were caught over the sampling period, and from 860 specimens, the male to female ratio was 1:1.8. The relationship between total weight (W) and standard length (SL) is described: W=0.00000743 SL(3.196) for females and W=0.00000764 SL(3.193) for males. Age groups were estimated from length frequency data, and von Bertalanffy annual growth parameters for all fish data combined were the following: L infinity=289.2 mm SL, k=0.20, t0=-0.73; for males, they were L infinity=265.9 mm SL, k=0.21, t0=-0.68, and for females, L infinity=293.6 mm SL, k=0.23, t0=-0.35. Longfin sanddab caught during this study reached a maximum age of 10 years, and at that age, males attained smaller sizes than females. The age groups had a total mortality (Z) rate of 0.82 year(-1), a fishing mortality (F) of 0.52 year(-1), and a natural mortality (M) of 0.3 year(-1). Although the longfin sanddab is not a target species of commercial fisheries, it suffers high mortality as part of the bycatch in the shrimp fishery.

Wind-induced ocean circulation along California and Baja California coasts in June 1999

The circulation along the California Coast in June 1999 was influenced by strong upwelling-favourable winds. To study the ocean circulation and upwelling variability, the Princeton Ocean Model (POM) was setup for the California and Baja California coasts and it was forced with interactively computed wind stress and surface heat fluxes. Atmospheric fields used in the surface flux estimates during the POM runs were obtained from Mesoscale Model 5 (MM5) outputs, while the sea surface temperatures (SST) were prescribed by the ocean model. One-way coupled air–sea modelling was used in the process-oriented approach to asses the influence of the wind stress, wind stress curl, initial density gradients, and nesting procedure with global model for the predicted circulation and accompanied upwelling. Hourly SSTs measured at twelve buoys operated by the National Data Buoy Center (NDBC) were used to evaluate sensitivity modelling studies via root-mean-squared-errors (RMSE). Low-pass filtered currents measured at 15 stations along central and southern Californian coast were also used in the ocean model evaluation procedures. In order to resolve the particular roles and importance of the wind stress vs. wind stress curl in driving coastal upwelling, besides baseline experiment we performed a numerical experiment with curl-free wind forcing. Modelled surface currents obtained in the baseline experiment indicate southward and southwestward flow in the deep ocean areas and alternating equatorward and poleward current near the shore. Wind stress curl appeared to be crucial both for the current field structure and SST distribution. The inshore poleward current obtained in the baseline experiment and evidenced by direct measurements was replaced with an equatorward current in the curl-free experiment. Furthermore, a significant temperature drop occurred along the entire coast in the curl-free experiment, whereas in the baseline the areas of maximum upwelling were confined to the regions with the wind maxima. Climatological temperature profile integrated with the satellite SST appeared to be superior for initialization of the ocean model in comparison with Levitus climatology. Due to too high temperatures predicted by HYbrid Coordinate Ocean Model (HYCOM) along the West Coast, expected improvement of the baseline experiment was not achieved by nesting POM with the Pacific-scale HYCOM. Additionally, a selection of a drag coefficient parameterization appears to have a significant impact on the upwelling evolution.

Sedimentology of Bahía Sebastián Vizcaíno, Baja California, México.

With support from the Oceanographic Ships H05 “Altair” and H03 “Alexander von Humboldt” of the Mexican Navy, 37 samples were collected using a Van Veen sediment dredge in Ranger Bank, Sebastian Vizcaino Bay, and around Punta Canoas of Cedros Island, on the west coast of Baja California. The distribution of sediments, grain size,composition of organic carbon and carbonatecontent, and phosphates were determined.Two major areas of sedimentation were identified: (a) the Ranger Bank, where oxidative anddynamical processes occur within a limestonedominated environment, and coal with highconcentrations of organic matter with mediumsize sand (f = 1.0 to 2.1); b) the northeast portion of Sebastian Vizcaino Bay where the size of the sediments range from very fine to coarse grain (f = 0.2 to 3.7); there a high concentration of phosphates (maximum 22.3 %) occurs, with biogenic material consisting mainly of organic detritus, such as diatomaceous material, phosphorite from fish and other organisms. This can be considered a zone where incipient formation of phophoritic material is generated and formation of future marine phosphorite occurs. The prevailing material in Keller Canal, located between Cedros and Nativity islands consists of coarse grains because of the strong currents that remove the finematerial, whereas in the central-western portion of the bay abundant and very fine sand and mud occur, which are associated with low speed near the center of rotation of an anticyclonic semipermanent flow that characterizes water movement in the Bay.

Region-wide Trends of Nesting Ospreys in Northwestern Mexico: A Three-decade Perspective

Abstract We used a double-sampling technique (air plus ground survey) in 2006, with partial double coverage, to estimate the present size of the Osprey (Pandion haliaetus) nesting population in northwestern Mexico (coastal Baja California, islands in the Gulf of California, and coastal Sonora and Sinaloa). With the exception of Natividad, Cedros, and San Benitos islands along the Pacific coast of Baja California (all three excluded from our coverage in 2006 due to fog), this survey was a repeat of previous surveys conducted by us with the same protocol in 1977 and 1992/1993, allowing for estimates of regional population trends. The minimum population estimate for the area we surveyed in 2006 was 1343 nesting pairs, an 81% increase since 1977, but only a 3% increase since 1992/1993. The population on the Gulf side of Baja California generally remained stable during the three surveys (255, 236, and 252 pairs, respectively). The population of the Midriff Islands (Gulf of California in the vicinity of 29°N la...

DDT in sediments from the northwest coast of Baja California (Mexico) and its biotransformation by Vibrio sp.

During December 2003, sediment samples were collected at 20 stations along the northwest coast of Baja California (Mexico). Analyses of DDT were performed by gas chromatography, percentage of particles <63 μm in diameter, organic carbon, and sulphate-reducing bacteria. A bacterium from station 49, which showed the highest concentration of DDT and its metabolites DDE and DDD, was isolated and identified biochemically and molecularly to belong to the genus Vibrio. In the laboratory this bacterium was tested in minimal salts medium plus DDT and incubated at 28ºC under aerobic and anaerobic conditions for 90 days. In general, the sediments showed low concentrations of DDT, ranging from undetected to 4.78 ng g–1, and the highest concentrations occurred at the deepest stations containing higher percentages of particles <63 μm and organic carbon concentrations. The isolated Vibrio strain showed an ability to biotransform DDT under aerobic and anaerobic conditions, presenting the o,p′-DDE and o,p′-DDD metabolites after 60 and 90 days of incubation in both conditions.

Interactions between seabirds and endemic deer mouse populations on Santa Barbara Island, California

Nesting seabirds alter habitat and food availability for insular rodent populations; in turn, rodents can reduce seabird nest success by consuming eggs and chicks. Predation by deer mice ( Peromyscus maniculatus elusus Nelson and Goldman, 1931) is considered a significant threat to reproductive success of Xantus’ Murrelet ( Synthliboramphus hypoleucus (Xantus de Vesey, 1860)), a small, burrow-nesting seabird that breeds off the coast of southern California and Baja California. We live-trapped mice in and out of seabird colonies on Santa Barbara Island, California, USA, to determine the effects of seabirds on mouse populations. We used stable isotope analysis to determine if mice fed on murrelet eggs and chicks. Mouse densities increased significantly on all sites from winter to summer, but there were no significant differences in densities between areas with and without seabirds. Although mice were abundant in murrelet colonies, mouse populations appeared to be affected more by habitat factors than seabird populations: areas with greater rock cover supported higher densities, fewer juveniles, and larger adults in winter and spring, whereas grassland sites had high densities and more reproductive adults in summer. We found no evidence of consumption of murrelet chicks or eggs, suggesting that eggs are not a major component of the diet of most mice. However, mice can still have a significant impact on local murrelet productivity because few eggs are laid each season relative to the high numbers of mice present.

Ecosystem energetic implications of parasite and free-living biomass in three estuaries

Parasites can have strong impacts but are thought to contribute little biomass to ecosystems. We quantified the biomass of free-living and parasitic species in three estuaries on the Pacific coast of California and Baja California. Here we show that parasites have substantial biomass in these ecosystems. We found that parasite biomass exceeded that of top predators. The biomass of trematodes was particularly high, being comparable to that of the abundant birds, fishes, burrowing shrimps and polychaetes. Trophically transmitted parasites and parasitic castrators subsumed more biomass than did other parasitic functional groups. The extended phenotype biomass controlled by parasitic castrators sometimes exceeded that of their uninfected hosts. The annual production of free-swimming trematode transmission stages was greater than the combined biomass of all quantified parasites and was also greater than bird biomass. This biomass and productivity of parasites implies a profound role for infectious processes in these estuaries.

Mitochondrial and Nuclear Genetic Variation across Calving Lagoons in Eastern North Pacific Gray Whales (Eschrichtius robustus)

Accurate knowledge of population structure in cetaceans is critical for preserving and managing breeding habitat, particularly when habitat is not uniformly protected. Most eastern gray whales return to their major breeding range each winter along the Pacific coast of Baja California, Mexico, concentrating in 3 major calving lagoons, but it is unknown whether genetic differences exist between lagoons. Previous photo-identification studies and genetic studies suggest that gray whales may return to their natal lagoons to breed, potentially resulting in the buildup of genetic differences. However, an earlier genetic study used only one genetic marker and did not include samples from Bahia Magdalena, a major calving lagoon not currently designated as a wildlife refuge. To expand on this previous study, we collected genetic data from the mitochondrial control region (442 bp) and 9 microsatellite markers from 112 individuals across all 3 major calving lagoons. Our data suggest that migration rates between calving lagoons are high but that a small but significant departure from panmixia exists between Bahia Magdalena and Laguna San Ignacio (Fisher's Exact test, P < 0.0001; F(ST) = 0.006, P = 0.025). Coalescent simulations show that the lack of extensive population structure may result from the disruption of structure due to whaling. Another possibility is that rates of migration have always been high (>10% per generation). In addition, microsatellite data showed evidence of a severe population bottleneck. Eastern gray whales are still recovering from the impacts of whaling on their breeding grounds, and these populations should be protected and monitored for future genetic changes.

Trace Metals in Sediments and Zostera marina of San Ignacio and Ojo de Liebre Lagoons in the Central Pacific Coast of Baja California, Mexico

San Ignacio and Ojo de Liebre lagoons in central Baja California, Mexico are nursery and grazing grounds for whales and turtles. Ojo de Liebre Lagoon also supports a salt mine operation. By concentrating trace metals via evaporation, this activity might harm biota. Consequently, salt mining might be incompatible with the lagoon's ecological role. Eelgrass can incorporate these elements and reroute them to other organisms. Trace metals in sediments (Cd, Co, Cu, Mn, Ni, Pb, Zn, and Fe) were measured at both lagoons. Some (Cu, Mn, Pb, and Zn) were also measured in Zostera marina patches at both lagoons. The results did not show elevated metal concentration at any lagoon, either for sediments or eelgrass. No statistically significant differences between lagoons were found. However, eelgrass at both lagoons showed larger concentration ranges than in sediments. Also, a correlation exists between sediment metal concentration and its concentration in eelgrass. Surprisingly, several sediment metal concentrations are higher than those considered as elevated for the Southern California Bight.

The Rediscovery and Status of Dissanthelium californicum (Poaceae) on Santa Catalina Island, California

Dissanthelium californicum (Nutt.) Benth. (Poaceae) is an annual grass known only from Santa Catalina and San Clemente Islands, off the coast of Southern California, USA, and Guadalupe Island, off the coast of Baja California, Mexico. It had not been recorded since Blanche Trask collected it in 1903 on San Clemente Island, and was therefore considered to be extinct, possibly as a result of overgrazing by introduced goats on these islands. During monitoring by the Catalina Island Conservancy from March through July 2005, D. californicum was discovered growing in seven diverse and widely spread locations on Santa Catalina Island. The rediscovery may be a result of the removal of feral goats and pigs from the island, record-breaking rainfall, and increased exploration of remote areas. A revised description of the species and a summary of its habitat preferences are provided. Thorough monitoring and surveying of known and potential locations will be necessary to better determine the conservation status of the species.

Effects of ENSO 1997 1998 on the distribution of small pelagic fish off the west coast of Baja California

Changes in the Small Pelagic Fish (SPF) densities and distribution during ENSO 1997-1998 were found from observations made between 1994 and 2001 along the northwestern coast of Baja California, Mexico. Results showed that temperature and variables associated with the surface mixing processes had an effect on the SPF shoal behaviour and distribution. Between September and December 1997, acoustic density showed maxima that were accentuated in the neritic and slope zones and with a wide depth interval. The results suggest that in the presence of climate anomalies like ENSO 1997-1998, SPF are concentrated in specific areas not influenced directly by these anomalies and therefore available to fishing in considerable abundance and vulnerable to a high fishing scheme. Our recommendation for SPF-fishery management is to regulate the fishing during these anomalously warm periods. Finally, it is important to investigate more about fishing scenarios during climate anomalies to contribute to an improved concept for a sustainable fishery.

Bio-optical characteristics of a phytoplankton bloom event off Baja California Peninsula (30–31°N)

A phytoplankton bloom was detected in the Southern California Current System, off the Baja California Peninsula (Mexico) on June 2003 with chlorophyll- a concentration (TChl a) of 10.13 mg m −3. Two stations (D1 and D2) were sampled on June 24, and D2 was resampled 6 days later; chlorophyll- a concentration had decreased by about one half. LAC MODIS-Chl a images were obtained and showed the spread of the bloom on the day after sampling. The phytoplankton community consisted primarily of dinoflagellate temporary cysts, mainly at the surface and at 5 m in station D1. Two Pseudo-nitzschia species ( P. australis, P. seriata) were also very abundant. Samples from the bloom had a specific phytoplankton absorption coefficient ( a ph * ( λ ) ) lower than the rest of the samples. Values varied from 0.0186 to 0.0455 m 2 mg −1 for a ph * (440) and from 0.0092 to 0.0294 m 2 mg −1 for a ph * (675), with ratios a ph * (440): a ph * (675) ranging from 0.99 to 2.20. These low ratios were associated with the combined effect of packaging, and with the relatively high ratios of fucoxanthin, peridinin, diadinoxanthin and chlorophyll- c2 to TChl a. Samples from the surface and 5 m depth at station D1 had higher ratios of Perid:TChl a (0.12–0.32) than the rest of the samples, suggesting that cysts have similar Perid:TChl a as free-living dinoflagellates. An unusual absorption spectrum with a broad maximum around 480–500 nm was associated with the high proportion of cysts and diatoms. The slope of the spectra between 443 and 488 nm was a good index to differentiate bloom samples containing high proportions of dinoflagellate temporary cysts. Further investigation of the absorption properties of dinoflagellate cysts is needed in order to detect these waters by remote sensing. Although much work is still necessary to understand and explain the bio-optical properties of a bloom, the present study is the first assessment off the Baja California coast to simultaneously consider aspects such as absorption properties, pigment composition and to include a spatial evaluation of the extension of a bloom with satellite images.

Spatial distribution and enrichment of mercury in surface sediments off the northwest coast of Baja California, Mexico

An evaluation was made of the distribution and enrichment of Hg in surface sediments collected in August 1998 at 37 stations along the northwest coast of Baja California (NCBC). Mercury was analyzed by cold-vapour atomic absorption spectrophotometry. The percentages of mud ( &lt; 63 µm) and organic matter were also measured. Mercury concentrations showed a homogeneous spatial distribution in the study area. The mean concentration of Hg (58.2 ng g–1) was similar to that found in the Southern California Bight. Mercury enrichment was observed at four stations located in the northern and central zones of the study area. In general, the Hg concentrations (30.4–96.5 ng g–1) were relatively low and pose no environmental concerns according to North American marine sediment quality guidelines. No correlations were found between Hg and mud or organic matter, suggesting that these variables do not influence the concentration and spatial distribution of this element in sediments from NCBC.

Eelgrass Meadows in the California Channel Islands and Adjacent Coast Reveal a Mosaic of Two Species, Evidence for Introgression and Variable Clonality

Seagrasses are important facilitator species in shallow, soft-bottom marine environments worldwide and, in many places, are threatened by coastal development and eutrophication. One narrow-leaved species (Zostera marina) and one wide-leaved species, variously designated as Z. marina, Z. pacifica or Z. asiatica, are found off the California Channel Islands and adjacent California-Mexico coast. The aim of the present study was to confirm species identification genetically and to link patterns of genetic diversity, connectivity and hybridization among and within the populations with historical sea levels (Ice Age) or the contemporary environment. Samples (n = 11-100) were collected from 28 sites off five California Channel Islands and six sites off the adjacent coast of southern California and Baja California, Mexico. DNA polymorphisms of the rDNA-ITS (internal transcribed spacer) cistron (nuclear), the matK intron (chloroplast) and nine microsatellite loci (nuclear) were examined in a population genetic and phylogeographic context. All wide-leaved individuals were Z. pacifica, whereas narrow-leaved forms were Z. marina. Microsatellite genotypes were consistent with hybridization between the two species in three populations. The present distribution of Z. pacifica follows a glacial age land mass rather than present oceanographic regimes, but no link was observed between the present distribution of Z. marina and past or present environments. Island populations of Z. marina often were clonal and characterized by low genotypic diversity compared with populations along the Baja California coast. The high level of clonal connectivity around Santa Catalina Island indicated the importance of dispersal and subsequent re-establishment of vegetative fragments. The pristine environmental conditions of offshore islands do not guarantee maximum genetic diversity. Future restoration and transplantation efforts of seagrasses must recognize cryptic species and consider the degree of both genetic and genotypic variation in candidate donor populations.

Norrisiella sphaerica gen. et sp. nov., a new coccoid chlorarachniophyte from Baja California, Mexico

A new chlorarachniophyte, Norrisiella sphaerica S. Ota et K. Ishida gen. et sp. nov., from the coast of Baja California, Mexico is described. We examined its morphology, ultrastructure, and life cycle in detail, using light microscopy, transmission electron microscopy, and time-lapse videomicroscopy. We found that this chlorarachniophyte possessed the following characteristics: (1) vegetative cells were coccoid and possessed a cell wall, (2) a pyrenoid was slightly invaded by plate-like periplastidial compartment from the tip of the pyrenoid, (3) a nucleomorph was located near the pyrenoid base in the periplastidial compartment, (4) cells reproduced vegetatively via autospores, and (5) a flagellate stage was present in the life cycle. This combination of characteristics differs from any of the described chlorarachniophyte genera, and therefore a new genus is established. Fluorescent microscopic observations suggested that the alga formed multinucleate cells prior to forming autospores. Time-lapse observations during autospore formation showed that cytokinesis occurred simultaneously in the multinucleate cells. Zoospores were also produced, and video sequences captured the release of zoospores from coccoid cells.

Phosphate balance and spatial variability on the continental shelf off the western US-Mexico border region

A cruise was conducted along the northwestern coast of Baja California, off the US-Mexico border region, to determine the physical, biological and anthropogenic factors affecting the concentration and distribution of phosphates (PO43–). The vertical distribution of temperature and PO43– showed isograms (13ºC and 0.75 µM, respectively) rising towards the shore. Although this phenomenon was observed in all the study area, it was more abrupt in the southern part than in the northern, resulting in lower phosphate concentrations in the north (0.50 µM) associated with high chlorophyll a concentrations (10.0 mg m–3). A principal components analysis indicated that the PO43– concentrations were related to upwelling (51%) and phytoplankton biomass (23%). A PO43– mass balance indicated that sewage discharge had a very local effect, contributing 3.0 ± 0.12 t d–1 (5%), while horizontal advection contributed 20.0 ± 2.2 t d–1 (32%) and upwelling supply was 41.1 ± 2.3 t d–1 (63%). If the PO43– sewage contribution trend does not change, we predict that in 2035 the anthropogenic contribution will be as high as the amount delivered to the area by the Californian Current. Conversely, if Mexico´ s sewage treatment systems improve to a level similar to that in the USA, we estimate that by 2012 the anthropogenic sources will be insignificant.