Diurnal rhythm of biodiversity in a zooplankton community of a macroscale anticyclonic gyre

Zooplankton sampling in the upper 200 m layer (at two hour intervals throughout the day) was carried out at a long-term drift station (11°S 22°W; December 1985) within the macroscale convergence zone in the southern tropical Atlantic Ocean. The distinct diurnal changes of total copepod abundance and single species-abundances are compared to less pronounced species: and genus-diversity changes in the upper 200 m. The coefficient of variance for the diurnal changes of the total abundance is 33%, while it is 26% for species-diversity, and 22% for genus-diversity. The increase of the total abundance of copepods is associated with an increase of the slope of the size spectrum. Peaks of species-diversity and genus-diversity do not coincide. They shift in time, with a lag of four hours approximately. The diurnal patterns of individual copepod species-abundance are quite diverse. Some species exhibit well pronounced uni-modal rhythms, others show bi-modal and multi-modal diurnal rhythms of abundance. The abundant species have bimodal diurnal rhythms as a rule. The diurnal rhythm of total copepod abundance corresponds, in general, to the fluctuation of the phytoplankton pico-fraction (less than 2.5 μm) abundance, sampled at the same stations. From copepod population structure it is obvious that within the convergence zone of the macroscale anticyclonic gyre a mature zooplankton community, typical for the final phase of the spatio-temporal succession in epipelagic planktonic ecosystems is found. Species-diversity is high, and abundance values are low (4000-16 000 ind m^-3). In general, 11 species compose 75% of total copepod abundance, and only three species contribute for slightly more than 10% each. The population structure of the species is also quite different. In 92 species, females dominate the population by forming more than 50% of it. In five species copepodite stages I to III form more than 50% of the population. Male dominance has not been found.