Bacterial and phytoplankton dynamics along a trophic gradient in a shallowtemperate estuary

Heterotrophic bacterial and phytoplankton biomass, production, specific gro wth rates and growth efficiencies were studied from June to October in 1996 and from February to May in 1997 along the longitudinal axis of the Urdaib ai Estuary, a shallow temperate tidal estuary. Both phytoplankton and bacte rial biomass showed a distinct lower to upper estuary increasing gradient. Longitudinal variations of phytoplankton biomass were controlled mainly by inorganic nutrient availability and tidal flushing. Longitudinal variations of bacterial biomass were further influenced by allochthonous inputs of or ganic carbon from a wastewater treatment plant located at the upper estuary . On a temporal scale, the dilution and washout effect of river discharge w as a major controlling factor of both bacterial and phytoplankton biomass i n the upper estuary. In this zone there were no indications that bacteria w ere resource limited, and temperature was the variable that explained most of the variability in bacterial specific growth rates. In the lower estuary , however, data suggested bacteria were nitrogen limited. Likewise, phytopl ankton growth rates also showed a positive correlation with inorganic nutri ents in this area. Free-living bacteria were generally more abundant than attached ones all al ong the estuary. However, following the longitudinal gradient of turbidity, the proportion of attached bacteria increased from the lower to the upper estuary. Rates of thymidine incorporation per cell were generally higher fo r particle-attached bacteria than for free-living bacteria. On average, bacteria contributed only 23% of the phytoplankton plus bacteri al carbon biomass. Bacterial contribution to total (bacterial plus phytopla nkton) net production of organic carbon was on average 17% for surface samp les and 39% for bottom samples. Average phytoplankton,growth efficiency in the euphotic zone was estimated to be higher (85%) than bacterial growth ef ficiency (46%). (C) 2000 Academic Press.