ABSTRACT We studied the effects of particulate and dissolved optically active components on the attenuation of photosynthetic active radiation (PAR) in a tropical lake. The temporal and spatial distribution of tripton, Chl-a and aCDOM(440) and their relative contribution to the diffuse PAR attenuation coefficient (Kd) was investigated at 21 sites (dry and wet seasons and two intermediate periods) and at monthly interval at 1 pelagic site. Higher values of Kd were observed during the mixing period, characterized by a higher concentration of tripton and Chl-a compared to the stratified rainy season. In the spatial sampling PAR attenuation was dominated by tripton absorption/scattering (average relative contribution of 79%), followed by Chl-a (average 11.6%). In the monthly sampling tripton and Chl-a accounted for most of the Kd with relative contributions of 47.8% and 35.6%, respectively. Multiple linear regression analysis showed that Chl-a and tripton in combination explained 97% of the monthly variation in Kd (p<0.001), but Chl-a had more influence (higher regression coefficient). Thus, although most of light attenuation was due to tripton, seasonal variations in phytoplankton abundance were responsible for most of the temporal fluctuations in Kd.

RESUMO Nós determinamos pela primeira vez os parâmetros reprodutivos e a estrutura populacional de Hypostomus francisci no rio Itapecerica, bacia do São Francisco. Um total de 250 espécimes foi capturado trimestralmente entre março de 2010 e fevereiro de 2012. O peso corporal, comprimento total e peso das gônadas foram obtidos em laboratório. Amostras de gônadas foram submetidas à técnicas histológicas e histoquímicas. Fêmeas com ovários capazes de desovar foram utilizadas para determinar a fecundidade e a fecundidade relativa. Observou-se uma proporção sexual de 1:1,01 (Machos:Fêmeas). Os machos foram mais numerosos do que fêmeas na classe de indivíduos menores que 170 mm, no entanto, o número de fêmeas foi significativamente maior para os espécimes acima de 330 mm. A relação peso comprimento estimada para H. francisci indica crescimento alométrico negativo. Fêmeas capazes de desovar foram registradas principalmente em Novembro-Dezembro-Janeiro. Duas populações de ovócitos em um momento determinado do ciclo comprovam desenvolvimento ovocitário do tipo grupo-sincrônico. Os ovos atingiram 3,4 mm e a fecundidade variou de 312-1,460 ovócitos com uma média de 585,81 ± 337,43 ovócitos por fêmea. Os parâmetros reprodutivos e a estrutura populacional de H. francisci do rio Itapecerica sugerem que esta espécie possui táticas reprodutivas singulares entre congêneres.

ABSTRACT Herein we determine for the first time the reproduction parameters and population structure of Hypostomus francisci in the Itapecerica River, São Francisco Basin. A total of 250 specimens was captured quarterly between March 2010 and February 2012. Body weight, total length and weight of the gonads were obtained in the laboratory. Gonad samples were submitted to histological and histochemical techniques. Females with spawning capable ovaries were used to determine the fecundity and relative fecundity. Sex ratio with 1:1.01 (female:male) was observed. Males were more numerous than females for individuals smaller than 170 mm, however the number of females was significantly greater for specimens larger than 330 mm. The length-weight relationship estimated for H. francisci indicates negative-allometric growth. Females spawning capable were observed mostly in November-December-January. Two cohorts of oocytes at a determined time evidencing the development type group-synchronic. The eggs reaching 3.4 mm and the fecundity ranged from 312-1,460 oocytes with an average of 585.81 ± 337.43 oocytes per female. The reproductive parameters and population structure of H. francisci from Itapecerica River suggested that this species showed singular reproductive tactics among congeners.