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Short-term variability in microbial food web dynamics was investigated from 3 to 17 May 2001 at 2 contrasting sites in the Urdaibai estuary (Bay of Biscay). The lower site is located in an area where tidal advection exerts a major influence, whereas the upper site is located in area where the influence of the tide is much lower and that of river runoff is markedly higher than at the lower site. The biomass of microbial plankton (phytoplankton, bacteria, non-pigmented flagellates [NPF] and ciliates) and their rates of activities (primary production, bacterial production, herbivory, bacterivory and microbial community respiration) were markedly higher at the upper site, thus showing that tidal flushing is a key factor controlling these spatial differences along the longitudinal axis of the estuary, rendering the upper channel a high productivity zone and the lower euhaline area a less productive one. Water residence time also exerted an overriding effect on chlorophyll a distribution on a temporal basis, with drastic increases in river runoff, due to heavy rain pulses, being responsible for immediate bloom dispersion in the upper channel. Bacterial density was less affected than phytoplankton biomass by increased river flow. It is hypothesised that detrital material increases caused by increases in river runoff provided an extra source of carbon for bacterial growth. On average, microbial herbivory was responsible for the fate of ca. 50% of chlorophyll a standing stock. Herbivory rates were not significantly different on phytoplankton < 8/> 0.2 µm and on those <100/> 8 µm in size and grazing pressure on phytoplankton was lower in the more productive site. Protists 1 to 8 µm in size were the main bacterivores. No synchrony was observed between bacterial and NPF abundances. Instead a lag phase was observed between the peak densities of bacteria and NPF, which was larger at the more productive upper site than in the lower estuary and allowed bacteria in the former zone to exploit substrate availability without significant grazing control for several days. Microbial community rate of respiration was significantly correlated with bacterivory rates and NPF abundance in the upper estuary, but not in the lower zone. The median of the proportion of bacterial production removed by bacterivory was 64% at the lower estuary station and 17% in the upper estuary. A possible explanation for this difference could be that attached bacteria (abundant in the upper reaches of the estuary) are less accessible for protistan grazing. Bacterivory by protists accounted for a mean of 14 and 7% of protistan herbivory at the lower and upper sites respectively.

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The evolution of a Phaeocystis bloom off Plymouth in spring 1990 is described. Data were obtained (1) during weekly visits to station El to measure physical parameters, nutrient concentrations, primary production and grazing pressure and (2) by regular tows of an Undulating Oceanographic Recorder around a circuit of five stations including El to detect stratification, monitor the geographical distribution of chlorophyll and determine vertical profiles at each station.The data show that the Phaeocystis bloom emanated from near-shore, its spread towards the south-east correlating with the wind direction prevailing at the time. Due to bad weather, seasonal stratification had not been established at the time of the bloom so that a major diatom outburst had not occurred and nutrients remained high as the Phaeocystis moved offshore.Zooplankton populations at El, averaged over 50 m, decreased whilst Phaeocystis was present, resulting in very low grazing pressure. Termination of the bloom was not, therefore, due to consumption by herbivores. It was, instead, caused by Phaeocystis colonies becoming nutrient-depleted and releasing single cells which were transported downwards. As a result of this, chlorophyll concentrations near the bottom remained high for some time after the Phaeocystis had disappeared from the surface water.A scenario is proposed to explain the observations in terms of our current understanding of the ecophysiology of Phaeocystis.

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