Microbial dynamics in a tropical monsoon influenced estuary: Elucidation through field observations and microcosm experiments on biofilms

- Elucidation of estuarine microbial dynamics through changes in biofilms.
- Bacteria and periphyton dominance in biofilms influenced by salinity.
- Marine and brackish biofilms were reservoirs of Vibrio spp. and faecal indicator bacteria.
- Biofilm translocation leads to community alteration.
- Role of grazers in biofilm modulation.

The changes that occur in biofilms reflect the influence of the environment. In a case study, the biofilms developed along a salinity gradient from marine to freshwater in a tropical monsoon influenced estuary (Zuari, west coast of India) were evaluated through in situ observations, and effect of translocation through microcosm experiments. The bacterial abundance was tide driven and high at the mouth of the estuary, whereas periphyton biomass, in terms of chlorophyll a was higher in the freshwater to brackish zone. These results point out decoupling of bacteria and phototrophic organisms in estuarine early stage biofilms. Vibrio spp. and faecal indicator bacteria were abundant in marine and brackish biofilms. The translocation experiments revealed that at times when marine biofilms with high total bacterial abundance were transferred to either brackish or freshwater, the abundance decreased. However, a significant increase in the bacterial abundance was observed when freshwater biofilms were transferred to either brackish or marine conditions. The microcosm experiments revealed that the influence of grazers differed with the type of biofilm and played an important role in modulating the composition of biofilms. The diagnostic pigments provided considerable insights into spatial and seasonal dynamics of periphyton groups in different biofilms and were determined by the surrounding water column communities and the availability of nutrients. Decay of freshwater periphyton in biofilms when translocated to marine and brackish conditions lead to remineralisation and higher bacterial population.