Planktonic metabolism and microbial activity in the Macquarie Marshes, Australia

Planktonic metabolism, the gross primary productivity of phytoplankton (GPP) and planktonic respiration (PR) in an aquatic system, is affected by inundation, nutrients, and microbial activity. Variations in planktonic metabolism and microbial communities in the water column were investigated in three hydro-geomorphologically distinct zones of the Macquarie Marshes with different historical inundation frequency (IF). Soil samples were inundated in a mesocosm, GPP and PR were determined using incubation methods, and Biolog EcoplatesTM were used to assess functional microbial diversity. Zone 1 (moderate-IF) had the highest mean GPP (18.62±6.58 mg C m-3 h-1) and lowest PR (3.30±8.07 mg C m-3 h-1) compared with zone 2 (low-IF) and zone 3 (very low-IF), the latter having the lowest GPP (13.72±8.07 mg C m-3 h-1) and highest PR (30.07±7.26 mg C m-3 h-1). GPP/PR showed zones 1 and 2 to be autotrophic, while zone 3 was heterotrophic. Ecoplates showed the highest microbial activity in zone 3 (AWCD=1.77±0.25) and lowest activity in zone 1 (AWCD=1.31±0.16), indicating the clear linkage of bacterial communities with PR in the rarely inundated area which leads to heterotrophic conditions. Thus, environmental water decision-making should take into account the antecedent ecological conditions of the targeted floodplains to inform the delivery of environmental flows.