Effects of turbulence on the development of phytoplankton biomass and copepod populations in marine microcosms

Abstract

Effects of turbulence on the dynamics of copepods as well as on the development of phytoplankton biomass were studied in 30 dm3 laboratory microcosms. Two experiments were carried out, with different successional stages of the plankton populations, using seawater from the Masnou nautical harbour, 20 km north of Barcelona. Two experimental conditions were established (stirred microcosms with copepods, and unstirred microcosms with copepods). Enclosure of plankton populations was followed by a phytoplankton bloom, which differed in intensity in stirred and unstirred microcosms. Time changes in the abundance of the dfferent developmental stages of copepods (from eggs to adults), sex ratios (d / P+d), fecundty (no. eggs/no. females), total copepod biornass, and the ratio consumers/producers, also differed in stirred and unstirred microcosms. Turbulence seems to reduce consumer biomass through changes in the demographic composition (lower proportion of males and higher development rates) and probably by increasing the metabolic activity of copepods (feeding pressure and excretion rates). The ultimate effect of turbulence would be a reduction of the trophic efficiency of the system, and accelerated turnover rates.