| Resumo: |
Oithona nana (Copepoda: Cyclopoida)is a widely-distributed neritic species, which often comprises up to 90 percentage of the zooplankton in many coastal areas. This small copepod plays an important role as a dominant prey in the diet of Engraulis anchoita larvae, which is a main component of major ecological relevance in the pelagic food webs of the Argentine Sea (SW Atlantic Ocean). The objective of this study was to estimate, for the first time, the seasonal variation of O. nana secondary production in a temperate region: coastal waters off Buenos Aires province, Argentina, at the EPEA Station (Estación Permanente de Estudios Ambientales, (38°28' S-57°41' W). Zooplankton samples (N=14)were taken by oblique tows of a Minibongo net (mouth diameter: 0.18 m; mesh opening: 67 km; with a flowmeter), from October 2003 to December 2004, on board the R/V Capitán Cánepa (INIDEP). The annual variations of O. nana abundance, biomass and production were investigated, focusing on the relationship between those parameters and the environmental conditions such as temperature, salinity and chlorophyll a. Also the succession of generations and size structure were studied. O. nana annual production was calculated from population fecundity rates estimated for each sampling date, combining field data (egg to female ratio)and experimental data (embryonic time and inter-clutch period). This rates, expressed as the proportion of female biomass (C)that is daily converted into eggs (specific egg production rate SEPR, dsup-1/sup), and biomass of each stage were used to calculate the daily production rates (secondary production SP, mg C msup-3/sup dsup-1/sup)for each sampling date, and were integrated to estimate annual population production and turnover rate. Population density was lower in late fall and winter than in early spring and summer. Sex ratio was highly skewed for females. Throughout the year, 6 generations were estimated (generation time: 22.5 - 60 days). Adults and stages total length changed inversely with temperature. Individuals were smaller in summer and fall and bigger in winter and spring. The number of eggs contained in a pair of sac (mean = 24 c 3.6 eggs sacsup-1/sup)was higher during the spring. Embryonic time was inversely related to the temperature (rsup2/sup= 0.98; p= 0.125), while the interclutch time was independent of it (mean = 0.68 c 0.07 days). Mean population SEPR (0.129 c 0.10 day sup-1/sup)did not show a clear seasonal pattern. However, slightly higher values were observed in the warmest months. It was not significantly correlated with temperature (rsup2/sup=0.08; p=0.37)or chlorophyll a (rsup2/sup=0.12; p=0.27). Copepodites made the most contribution to the total production (67 percentage), while females only constituted a 33 percentage. The 66 percentage of the total production was attained during spring months. These results increased the growing body of literature that highlights the relevance of the small cyclopoids, historically under sampled due to the use of coarse meshes ( 200 km). Also, this research contributes to a better understanding of the population dynamics and reproductive biology of one of the key (and less known)copepod species in the pelagic food webs in coastal waters off Argentina
Universidad Nacional de Mar del Plata. Facultad de Ciencias Exactas y Naturales
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