REGISTRO DE LA VARIABILIDAD OCEANOGRÁFICA EN PECES DEMERSALES Y DEPREDADORES TOPE DE LA ZONA OCEÁNICA FRONTAL DE BAHÍA MAGDALENA, MÉXICO

Abstract

ABSTRACT The oceanic region in front of Bahía Magdalena has a clear seasonal variability in physicochemical and biological properties as a consequence of the intermittent presence of the California Current (CC) and the North Equatorial Current (CNE), and has therefore been denominated as the Temperate-Tropical Transition Zone (ZTTT). Despite the variability caused by the oceanographic forcing and the seasonal change in primary production of as much as an order of magnitude between March and November, the region sustains communities of crustaceans, fishes and marine mammals over the whole year. This suggests that the energy flow, in terms of carbon and nitrogen, could use alternative routes during times when productivity is low. To determine if the oceanographic forcing cause temporal variability of trophic dynamics, and if this variability is registered in the resident species, stable isotopes (δ13C and δ15N) and stomach content of demersal fishes and stranded marine mammals were analyzed and the oceanographic characteristics of the study area were established. Samples were collected during March and November of 2006 and 2007. The criteria for the selection of the collected biological species was based on their low mobility and food fidelity in the case of the demersal fishes, and in feeding, breeding and reproduction site fidelity for the marine mammals. For each station demersal fishes were collected using a bongo net (600 μm), sediments were collected using a Smith- McIntyre grab, water samples were collected at different depths using Niskin bottles, and CTD casts were done for the determination of conductivity, temperature and density at the time of sampling. The demersal fishes were identified and their stomach content was analyzed using numerical, gravimetrical and occurrence frequency methods, from which the percentages of the index of relative importance (%IRI) were calculated. Samples of collected organisms, superficial sediments and particulate organic material were analyzed to determine their elemental and isotopic (δ13C and δ15N) composition. The concentration of nitrogen-containing nutrients (NO3 -, NH4 +) and the isotopic signal of nitrate (δ15-NO3 -) were determined in water samples. The representative average value for the study area of the isotopic enrichment between predatordetermined using a linear trophic route from particulate organic material (MOP) to the top predator of the area (Zalophus californianus). In addition satellite images of sea surface temperature (TSM), chlorophyll a (Chla) were analyzed and the net primary production (PPN) was estimated using a model. The MEI index (the multivariate index of the El Niño phenomenon) and the Bakun upwelling index (ISB) were calculated. Analysis of the information generated from the CTD casts, the TSM, the ISB and the PPN model indicated that the variability between spring and fall for the years 2006 and 2007 in the oceanic area in front of Bahía Magdalena were associated with a relaxation of upwelling and the southern flow of the CC. However, during March 2007 a positive 5 anomaly was detected (~ 4 ˚C) in the TSM, which together with the presence of subsuperficial equatorial waters (ASSE) and positive values in the MEI index (>1) indicated moderate El Niño conditions. The impact of the temporal oceanographic forcing was registered in the number of captured demersal species and in the Shannon diversity index (Hˈ), with a higher Hˈ registered during the March sampling compared to November for both years. A principal component analysis indicated that the specific richness was positively related to the ISB and negatively with the TSM (factor 1 vs factor 2 = 95%). The stomach content of the captured species of demersal fishes was dominated by the presence of crustaceans, of which Nyctiphanes simplex and Pleuroncodes planipes were the most prevalent. During the March 2007 season the biomass of P. planipes decreased considerably in the study area which was also associated with a lower presence of this specie in the stomach content. Although the stomach content of Kathetostoma averruncus was dominated by P. planipes (% IRI > 90) in both seasons of 2006 and 2007, there were significant isotopic differences in δ13C and δ15N, which suggests that the isotopic signal of its diet preserves the temporal characteristics of the ZTTT and its associated water masses. Thus the abundance, diversity, stomach content and the isotopic signal of the demersal fishes reflect the variability of water masses in the ZTTT. The process of denitrification in the area seems have a larger importance during fall conditions with the intrusion of tropical water masses. The δ15N signal of the collected species was significantly higher compared to species from the same trophic levels in other regions where denitrification is less intense. The average value of the predator-prey isotopic enrichment (Δ) was dynamic (2.5-2.9) and had a negative relation to the seasonal oceanographic forcing. The Δ-value was 1 part per mil lower than the Δ- values reported for latitudes over 45 ˚N. In addition, it was detected that the variability of the trophic level estimation obtained from δ15N and stomach content increased in species that feed off small pelagic fishes and zooplankton. This variability was larger at the top predator level, as evidenced by an increase in the standard deviation. The anomalous climatic variation detected during March 2007 was registered in P. planipes, demersal fishes and top predators in the area, which shows the impact a major, in scale and duration, El Niño event could have on the ecosystem in front of Bahía Magdalena. The enrichment in the nitrogen-isotope signal of NO3 -, a consequence of the intense denitrification in the oceanic region, which is transferred to the following trophic levels, is highlighted.