Contribution of planktonic and detritic fractions to the natural diet of mesozooplankton in Bahía Blanca estuary

Abstract

The relative importance of phytoplankton and microzooplankton in the natural diet of mesozooplankton was assessed in Bahía Blanca Estuary, Argentina, in December 2005. Grazing experiments were performed using 200-to-2,000 µm grazers and natural food <100 µm. Individual and community filtration and ingestion rates were estimated for each food fraction after 24 h incubation. Abundance and carbon data of prey and grazers were qualitatively and quantitatively analyzed. Phytoplankton was mainly composed of diatoms and microzooplankton mainly of tintinnids. Both fractions were less abundant than detritus. Most of the grazers belonged to the copepod Acartia tonsa. Mean filtration and ingestion rates on phytoplankton + microzooplankton were 6.44 ml grazer-1 day-1 and 0.03 µg C grazer-1 day-1, respectively. This figure increased to 6.95 ml grazer-1 day-1 and 1.65 µg C grazer-1 day-1 when detritus was included. Mean carbon-specific ingestion rates on phytoplankton and microzooplankton were 0.006 and 0.005 µg C µg C-1 day-1, respectively, whereas after the addition of detritus, the overall rate increased to 0.588 µg C µg C-1 day-1. Highly significant differences were found between grazing rates on detritus and planktonic fractions. Consumers showed higher filtration rates on microzooplankton than on phytoplankton although 78% of the cells ingested (54.7% µg C) came from the latter. The results point to a higher contribution of detritus to the natural diet of mesozooplankton in late spring. The omnivory of A. tonsa and the high turbidity of Bahía Blanca estuary may explain the differences observed among food fractions in terms of carbon intake.