Bacterial and Phytoplankton Dynamics along a Trophic Gradient in a Shallow Temperate Estuary

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Heterotrophic bacterial and phytoplankton biomass, production, specific growth 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 Urdaibai Estuary, a shallow temperate tidal estuary. Both phytoplankton and bacterial 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 organic carbon from a wastewater treatment plant located at the upper estuary. On a temporal scale, the dilution and washout effect of river discharge was a major controlling factor of both bacterial and phytoplankton biomass in the upper estuary. In this zone there were no indications that bacteria were 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, phytoplankton growth rates also showed a positive correlation with inorganic nutrients in this area.Free-living bacteria were generally more abundant than attached ones all along 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 for particle-attached bacteria than for free-living bacteria.On average, bacteria contributed only 23% of the phytoplankton plus bacterial carbon biomass. Bacterial contribution to total (bacterial plus phytoplankton) net production of organic carbon was on average 17% for surface samples and 39% for bottom samples. Average phytoplankton growth efficiency in the euphotic zone was estimated to be higher (85%) than bacterial growth efficiency (46%).