Microscale gradients of planktonic microbial communities above the sediment surface in a mangrove estuary

The microscale (1 and 4 cm sampling resolution) distributions of chemical (O2, NH3, NO3 , NO2 , PO4 3 ) and biological (Chl a, phytoplankton, bacterioplankton, viruses) parameters were measured in the 16 cm of water immediately overlaying the sediment-water interface (SWI) within a temperate mangrove estuary in South Australia during December 2003 and March 2004. Shear velocities (u ) during the time of sampling were very low (<0.1 cm s 1), and we consequently predict that resuspension of organisms and materials was negligible. In December 2003, profiles were often characterised by strong gradients in nutrients and organisms, with the highest concentrations often observed within 0.5 cm of the SWI. Microscale patterns in O2, NH3, NO3 and NO2 indicated that a variety of anaerobic and aerobic transformation processes probably occurred at the SWI and within profiles. Strong gradients in PO4 3 were indicative of nutrient flux across the SWI as a consequence of degradation processes in the sediments. Pico- and nanophytoplankton concentrations were strongly correlated ( p < 0.01) to PO4 3 , and exhibited 12- and 68-fold changes in abundance, respectively, with highest concentrations observed nearest to the SWI. Several bacterial subpopulations were discriminated using flow cytometry and significant shifts in the ‘cytometric structure’ of the bacterial community were observed within microscale profiles. Two populations of viruses were correlated to the phytoplankton and low DNA (LDNA) bacteria, and each exhibited elevated concentrations within 0.5 cm of the SWI. In March 2004, microscale distributions of O2 and nutrients were more homogenous than in December 2003, and dissimilar microbial community structure and patterns were observed above the SWI. The patterns observed here support the prediction that benthic processes can strongly influence the ecology of planktonic communities in the overlaying water, and provide further evidence for the existence of microscale variability amongst communities of aquatic microorganisms.