Short-term variability in microbial food web dynamics in a shallow tidal estuary

Abstract

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.

Nutrient stimulation of lytic phage production in bacterial populations of the Gulf of Mexico

“…When nutrient pulses were observed from riverine input into the Bay of Biscay, there was a lag phase between peak abundance of bacteria and nonpigmented flagellates (Iriarte et al 2003). This lag phase enabled bacteria to exploit substrate availability without grazing control for several days.…”

“…Under non-steady-state conditions (i.e. nutrient amendment) bacterial mortality caused by viruses is generally greater than that caused by bacterivory (Guixa-Boixereu et al 1999).When nutrient pulses were observed from riverine input into the Bay of Biscay, there was a lag phase between peak abundance of bacteria and nonpigmented flagellates (Iriarte et al 2003). This lag phase enabled bacteria to exploit substrate availability without grazing control for several days.…”

Environmental factors shaping microbial community structure in salt marsh sediments

“…, respectively (Calbet et al 2001, Iriarte et al 2003. Thus, bacteria are proportionally more abundant than heterotrophic protists in these sediments compared to the overlying water column.…”

“…Porewater protist concentrations were not several orders of magnitude greater than in the water column, as is the case with bacteria (Rublee 1982, Schmidt et al 1998). Rather, the , respectively (Calbet et al 2001, Iriarte et al 2003. Thus, bacteria are proportionally more abundant than heterotrophic protists in these sediments compared to the overlying water column.…”

Daily bacterioplankton dynamics in a sub-Saharan estuary (Senegal River, West Africa): a mesocosm study

“…Information on the timescales of the variability of bacterial development is rather scarce, especially for shallow estuaries, which are very dynamic environments, subject to frequent drastic changes in hydrodynamic conditions on a timescale of days (Iriarte et al 2003). One central question in aquatic microbial ecology involves the relative importance of resource limitation, grazing and viral mortality on both the functional and taxonomic diversity of bacterial communities (Jürgens et al 1999, Jürgens & Matz 2002.…”