Hydrology of winter–spring “red tides” in Bahía de Mazatlán, Sinaloa, México

“Red tide” events are frequent and periodical in Bahía de Mazatlán, Sinaloa, México. Yet, the ones observed from 4 February to 4 June 2000, showed some distinctive features: First, the dinoflagellates Prorocentrum balticum (85%), P. mexicanum (5%), and Ceratium furca (5%), dominated the composition of the blooms; Second, the average cell abundance by date was 1.3×10⁶ cells l⁻¹, with a range of 3.5×10³ to 24,500 × 10³ cells l⁻¹, well above previous records; Third, the temperature registered at 10–20 m deep was unusually cold (19 °C), below the normal average of 21.5 °C observed over the last 10 years. Salinity was high (35.9 psu) and showed very little influence on the water density gradient. A mean thermal stratification index (TSI), of 3.4, with a maximum of 7 °C, was observed throughout the period, in spite of a weak upwelling activity and intermittent strong NW winds which were unable to break up water column stratification. Temperature fluctuations at the surface and at the bottom layers showed a 30-day periodicity, suggesting some association with the lunar cycle. To explain the characteristics of the “red tides” registered in Bahía de Mazatlán during the winter–spring period of year 2000, it is proposed that the temperature and density stratification, stabilized further by internal waves that compensated for the weak upwelling activity and provided the necessary nutrients to the surface layer, favored the persistence and intensity of the harmful algal bloom events then observed.     

Vegetation and soil seed bank of successional stages in tropical lowland deciduous forest

Abstract. We studied vegetation structure and soil seed bank composition in different successional stages of secondary lowland tropical deciduous forest in Yucatán, Mexico. The series of study sites includes: slashed (S), slashed-and-burned (SB), and regenerating for 1, 6, 10, 15, 30, 40 and 100 yr. Species richness (S = 42 - 65), evenness (E = 0.32 - 0.38), and diversity (H' = 1.2 - 1.6) do not vary much as the forest grows older. 20 species of shrubs and trees were present in at least six of the seven regrowth years studied; 10 of these account for more than 50% of the total density values per regrowth year. These species dominate the vegetation due to their capacity to withstand repeated fire and felling. One third of the individuals sampled had regenerated from coppiced shoots. Species composition little resembles that in earlier accounts. The area is now largely covered by young regrowth stages (1 - 20 yr). Species constituting the original woody structure of the mature forest are rare or absent due to the lack of seed sources and failure of dispersal (which is due to limited dispersal capacities), lack of dispersal agents, or long distances. Herbs were the most important life form in the soil seed banks; only one tree species was found. The number of viable seeds varied between sampled areas: 70/m² in the 40 yr-old, to 1 815/m² in the slashed-and-burned (SB). The vegetation of S- and SB-areas was the same, but the number of viable seeds germinating in SB was twice the number in S; the number of species in the seed bank is the same for both areas. We speculate that fire modifies species dominance early in succession, allowing seeds of some species to germinate in great numbers.