The dynamics of Rhizophora mangle litter production and decomposition were studied in a tropical coastal lagoon on the Gulf of Mexico in Veracruz, Mexico over a year (October 2002-October 2003. This region is characterized by three seasons: northerly winds (called 'nortes'), dry, and rainy. Annual litter production (1116 g m )2 ) followed a seasonal pattern with leaf litter as the main fraction (70%) with two peaks in the dry and one in the rainy season. Leaf decomposition was evaluated with two types of litter bag in each season: fine mesh (1Â1 mm) and coarse mesh (3Â7 mm). Decomposition data were adjusted to a single negative exponential model. The results indicated faster decomposition rates in the coarse litter bag and significant differences among seasons. However these differences occurred after the 60th day of decomposition, indicating that leaching and microbial action were responsible for more than 50% of mass loss. After this period, the effects of aquatic invertebrates were evident but depended on climatic conditions. In the rainy season, the gastropod Neritina reclivata was associated with increasing leaf decomposition rate. In the 'nortes' season, the effect of aquatic invertebrates was smaller, and there were no differences in the decay constants calculated for the two litter bag types. High litter production represents an important input of organic matter which, through decomposition, may represent an important source of C, N, and P in this aquatic system.

This study analyzes the relationship between physical and chemical factors and the algal communities in tropical streams in micro-watersheds where [70% of their area has different land uses, specifically, cloud mountain forest, coffee plantations, and livestock pastures. Physical, chemical, and biological variables were measured monthly in each stream over a 1-year period. The concentrations of nitrates ? nitrites, total suspended solids (TSS), and silica in the streams were found to differ during the dry and rainy seasons. Coffee-plantation streams showed the highest levels of suspended solids, nitrates ? nitrites, and sulfates. Based on chlorophyll a concentration, the forest and coffee-plantation streams are oligo-mesotrophic, while pasture streams are meso-eutrophic. Forest streams displayed the lowest levels of richness and algal diversity, followed by coffee-plantation streams, whereas pasture streams were the most diverse. Chlorophyll a concentration and species richness depended on land use and season. Forest coverage was positively correlated with acidophilous and oligo-eutraphentic diatom species. Coffee coverage displayed a significant positive correlation with motile species and a significant negative correlation with pollution-sensitive diatom taxa. The results show that diatom assemblages responded to micro-watershed conditions and can be used to monitor the effects of land use on streams in tropical regions.

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Ciclo anual del fitoplancton con énfasis en las especies potencialmente nocivas en bancos ostrícolas de la laguna de Términos, sureste del Golfo Resumen.-Para definir la composición de la comunidad fitoplanctónica con énfasis en las especies nocivas en los bancos ostrícolas de la laguna de Términos, SE del Golfo de México, 6 sitios de muestreo fueron monitoreados mensualmente desde agosto 2012 a septiembre 2013. Se midió la temperatura del agua, salinidad, potencial de hidró geno, saturación de oxígeno, nutrientes inorgánicos y la abundancia de fitoplancton. La temperatura y la salinidad se caracterizaron por diferencias estacionales marcadas. Los valores de pH y saturación de oxígeno sugirieron un predominio de la actividad fotosintética. La comunidad fitoplanctónica se caracter izó por el predominio de nanoflagelados y dia tomeas. La abundancia de fitoplancton y su variación estacional presentaron los valores mínimos (10 3 células L -1 ) durante la época de secas (febrero-mayo) y valores al tos (10 6 célul as L -1 ) dur ante la temporada de lluvias (junio-septiembre). Otra característica importante de la comunidad fitoplanctónica fue la presencia de especies de dinoflagel ados nocivos: Akashiwo sanguinea, Karenia cf. mikimotoi, Pyrodinium bahamense var. bahamense, Prorocentrum mexicanum y P. minimum. Las cianobacterias Anabaena y Cylindrospermopsis cuspis alcanzaron abundancias de 1.9x10 6 y 1.3x10 6 células L -1 , respecti vamente. Los g éneros Alexandrium y Pseudo-ni tzschia estuvi eron presentes, pero los taxones no fueron identificados a nivel de especie. En conclusión, la comunidad fitoplanctónica se somete a cambios en la composición de especies y en la estructura de la comunidad durante cada temporada climática, en respuesta a la variación ambiental, que permite el desarrollo de la comunidad fitoplanctónica de acuerdo a las condiciones imperantes. Palabras clave: Cambios estacionales, cianobacterias, diatomeas, dinoflagelados, lagunas costerasAbstract.-To define the composition of the phytoplankton community, with an emphasis on harmful species, 6 stations were monitored monthly in the oyster beds of Términos Lagoon, SE Gulf of Mexico, from August 2012 to September 2013. Water temperature, salinity, hydrogen potential, oxygen saturation, inorganic nutrients and abundance of phytoplankton were determined. Temperature and salinity were characterized by marked seasonal differences. The pH values and the oxygen saturation suggest a predominance of photosynthetic activity. The phytoplankton community was characterized by the dominance of nanoflagellates and diatoms. , respectively. The genera Alexandrium and Pseudo-nitzschia were present, but the taxa were not identified to the species level. In conclusion, the phytoplankton community undergoes changes in both species composition and structure of the community during each climatic season, in response to environmental variation, which allows the development of the phytoplankton community according to the conditions. Key words: Seasonal changes, cyanobacteria,...