TODOS SANTOS ( MAYO , 1983 ) DISTRIBUTION AND ABUNDANCE OF THE PHYTOPLANKTON IN TODOS SANTOS BAY ( MAY , 1983 )

From 20 to 22 May, 1983, 32 stations were sampled in Todos Santos Bay at three depths (surface, 20 and 30 m) in order to determine the distribution and abundance of the phyto­ plankton. The distribution of the total phytoplankton was patchy with different abundance values: maximum values close to the coast and minimum values in the centre of the bay. By major taxa, the dinoflagellates had a density higher than 50% of the organisms in 60% of the samples. This was probably due to the fact that northward currents during the El Nino phenomenon carry typical tropical communities and/or to the succession of the phytoplanktonic species.


INTRODUCTION
Studies on the phytoplankton of the western coast of Baja California have mainly been carried out in coastal lagoons (Al varez-Borrego et al., 1977;Alvarez-Borrego and Najera de Munoz, 1979), and to a lesser extent in the adjacent seas (Walsh et al., 1974;Estrada and Blasco, 1979).Information on the phytoplankton in Todos Santos Bay is practically nonexistent (Gaxiola-Castro and Alvarez-Borrego, 1984;Millan-Nunez and Loya-Salinas, in preparation).The typical paración).
Se contaron las diatomeas céntricas y penadas, dinoflagelados y nanoplancton.Para el análisis se siguió el método Ciencias Marinas, Vol. 17, No. 1, 1991 composition of the phytoplankton in the California Current is mainly dominated by diatoms and dinoflagellates (Eppley et al., 1979).In 1983, an El Niño phenomenon occurred along the California Current up to Alaska (Simpson, 1983).During this event, for the summer of 1983, Torres-Moye and Alvarez-Borrego (1985) reported higher abundances of nanoplankton, high temperatures, greater concentration of nutrients and chlorophyll a in San Quintín Bay.These authors reported that the abundance of diatoms and dinoflagellates was one or two orders of magnitude lower, whereas that of the nanoplankton was one order of magnitude higher than the values reported by Lara Lara et al. (1980)

STUDY AREA
Todos Santos Bay is located between parallels 31043' and 31054' N and meridians 116036' and 116049 W. It is a shallow bay.Approximately 90% of the area has a depth that varies between 10 and 50 m and che rest forms part of the narrow submarine canyon of Todos Santos, between the islands of the same name and Punta Banda (Secretaria de Marina, 1974) (Fig. 1).

METHODOLOGY
A cruise was conducted on board the vessel Matiano Matamoros of the Secretarla de Marina, on 20, 21 and 22 May, 1983.Thirty-two statidhs (Fig. 1) were sampled at three levels (surface, 10 and 30 m), the latter when the depth allowed.The samples of sea water were collected with Van Dorn bottles.From each sample, subsamples were obtained in 125 ml plastic bottles; they were fixed with lugo1 in order to be later analysed in the laboratory.
The centric and pennate diatoms, dinoflagellates and nanoplankton were counted.For the analysis, the Utermöhl method was followed (Hasle, 1974), using 10 CC chambers with a sedimentation time of four hours for each centimetre of elevation of the chamber.Figura 1. Localización del área de estudio y estaciones de muestreo.Figure 1.Location of the study area and sampling stations.
Ciencias MMnaî, Vol. 17, No. 1,1991 andRivas-Lozano, 1988).The abundance of the phytoplankton in surface samples fluctuated behveen 5 x 103 and 3.49 x 105 cells/l (Fig. 2a).For the 10 m samples, the same patchy density distribution was observed, with a lower range of abundance than the surface samples, behveen 14x l@ and 60x 103 cells/l, except for some stations close to the coast and between the islands and Punta Banda that reached between 3.55 x 105 and 5.2x 106 cells/l (Figs.2a, 3a).Tl-re density of the phytoplankton at 30 m was lower than at the other two depths, with a range between 2 x 103 and 3.4 x 105 ce&/1 between the islands and Punta Banda (Fig. 4a).The total phytoplankton abundance was probably high at some stations because of the blooms of some species.For example, the surface and 10 m samples from station 12 and the surface samples from station 13 (Figs.2b, 3b) were dominated by Trichodesmiun sp.Station 30, at 10 m (Fig. 3b), was dominated by Exuvielfa sp.The relative abundance distribution (%) by major taxa at the surface and 10 m, was dominated by the dinoflagellates that had, in 60% of the samples, a density higher than 50% of the organisms.At 30 m, the percentage decreased slightly and in some samples the diatoms exceeded the dinoflagellates (Figs. 2b,3b,4b).The most abundant genera of dinoflagellates were Exuviella sp., Gonyaulax sp., Prorocentnrm sp. and Petidinium sp.The most abundant centric diatoms were Bacteriasttum sp., Melossira sp., Leptocylindtus sp. and Chaetoceros sp.Among the pennate diatoms, the most abundant were Navicula sp. and Nitzschia sp.Of the blue-green group, the most abundant was Ttichodesmiun sp., and of the chlorophytes, the most abundant was Pediastnun sp., even though the latter group is not very abundant and has little variety among the unicellular algae.They are most numerous in fresh water (Margalef, 1969), and it is therefore possible that the presente of this species can be attriiuted to the inflow of drainage affluents into the bay.
For a station in Todos Santos Bay, Gaxiola-Castro and Alvarez-Borrego (1984) reported an abundance of phytoplankton by major tasa of 70 to 110 cells/ml for diatoms, of 14 to 25 cells/ml for dinoflagellates and of 202 to 526 ce.lls/ml for the nanoplankton in July 1981, during a period when the El Niño phenomenon did not occur.They concluded that, in'all the samples, the nanoplankton was Tabla 1. Abundancia total y por grandes grupos taxonómicos del fitoplancton en las estaciones y profundidades muestreadas.Gaxiola-Castro Alvarez-Borrego (1984) reportaron para Una estación en la bahía de Todos Santos, una abundancia de fitoplancton por grandes grupos taxonómicos de 70 a 110 cel/ml para diatomeas, de 14 a 25 cel/ml para dinoflagelados y de 202 a 526 cel/ml para el nanoplancton en el mes de julio de 1981, en un periodo de no ocurrencia de El Niño, concluyendo que en todas las muestras el nanoplancton fue mayor que los otros grupos y las diatomeas fueron mayores que los dinoflagelados.Sin embargo, para este trabajo, en la mayoria de las muestras la abundancia de los dinoflagelados fue mayor que la de las diatomeas.Esto pudo ser debido a que el muestreo se llevó a cabo a finales de primavera, siendo típico para estas latitudes la sucesión de especies fitoplanctónicas (Margalef, 1969).Millán-Núñez et al. (1982) reportaron en una serie de tiempo de 10 días en la boca de la bahía de San Quintín, que la abundancia de los dinoflagelados sobrepasó la de las diatomeas, después del relajamiento de la surgencia en verano de 1979.De igual manera, se reportó para una estación al sur y afuera de la bahía de Todos Santos (González- Morales, 1988); o a que las aguas calientes que fueron transportadas por las corrientes hacia el norte durante el fenómeno de El Niño, traen consigo comunidades del fitoplancton típicas de zonas tropicales, constituidas en un porcentaje mayor de dinoflagelados (Raymont, 1980).De igual manera, esta ocurrencia fue reportada por Torres-Moye y Alvarez-Borrego (1985) para el inicio de la serie de tiempo en bahía San Quintín en junio de 1983.
Por lo anterior, se concluye que la comunidad fitoplanctónica de la bahía de Todos Santos fue afectada durante el periodo de El Niño 1983.Se recomienda llevar a cabo muestreos sistemáticos en toda la bahía, e inclusive generando series de tiempo, para llegar a conocer la variación del fitoplancton en la bahía de Todos Santos.
However, in the present work, the dinoflagellates were more abundant than the diatoms in most of the samples.This could be due to the fact that the sampling was carried out at the end of spring, when the succession of phytoplanktonic species is typical for these latitudes (Margalef, 1969).Millán-Núñez et al. (1982), in a ten-day time series at the mouth of San Quintín Bay, reported that the abundance of the dinoflagellates exceeded that of the diatoms, after the relaxation of the upwelling in summer 1979.This was also reported for a station to the South and outside of Todos Santos Bay (González- Morales, 1988).This could also be due to the warm waters that were transported by northward currents during the El Niño phenomenon which carry typical phytoplankton communities of tropical zones, mainly composed of dinoflagellates (Raymont, 1980).This occurrence was also reported by Torres-Moye and Alvarez-Borrego  (1984).Relación fotosíntesis-irradiancia en el fitoplancton de aguas costeras del noroeste de Baja California.Ciencias Marinas, lO(3): 5366.
and Millán-Núñez el al. (1983) for 1977 and 1979, when the El Niño event did not occur.The objective of the present study is to determine the distribution and abundance by major taxa of the phytoplankton in Todos Santos Bay in May 1983.
Bay was affected during the 1983 El Niño event.Further systematic studies throughout the bay, including time series, are recommended in order to determine the variation of the phytoplankton in Todos Santos Bay.English translation by Christine Harris.Blasco, D. (1979).Two phases of the phytoplankton community in the Baja California upwelling.Limnol.Oceanogr., 24(6): 1065-1080.Gaxiola Castro, G. y Alvarez Borrego, S.

Table 1 .
Total abundance and by major taxa of the phytoplankton at the stations and depths sampled.