Seasonal variability in bacterial abundance, production and protistan bacterivory in the lower Urdaibai estuary, Bay of Biscay

Abstract

Seasonal variations in abundance of heterotrophic bacteria, small (1-8 µm) heterotro- phic nanoflagellates (HNF), and ciliates, as well as bacterial production (BP) and bacterivory (PB) rates were investigated from May 1998 to May 1999 in the lower Urdaibai estuary (Bay of Biscay). Abundances ranged from 2.6 × 10 8 to 1.3 × 10 9 cells l -1 , 0.65 × 10 6 to 3.00 × 10 6 cells l -1 , and 12 to 436 cells l -1 for bacteria, HNF and ciliates, respectively. BP varied between 0.9 and 18.5 × 10 8 cells l -1 d -1 . Bacteria seemed to be more limited by food supply than by temperature. The major bacterivores were small HNF, but they could not satisfy their carbon demand only through bacterivory. Bacterivory rate per small HNF varied between 2.7 and 28.2 bacteria flagellate -1 h -1 . This rate showed seasonal vari- ations and was higher during late winter/early spring than during summer/early autumn. This pat- tern is probably due to the greater dependence of small HNF on bacteria during late winter/early spring, when the availability of food sources other than bacteria, e.g. autotrophic picoplankton, was low. This led to seasonal variations in the degree of grazing control on the bacterial standing stock, this control being tighter during late winter/early spring. Bacterivory rate was substantially higher than BP in many months, i.e. bacteria showed a negative growth, and this was paralleled by reduc- tion in bacterial abundance. The relationship between log bacterial abundance and log small HNF abundance suggested that small HNF were largely under a substrate supply (bottom-up) control.