Planktonic metabolism and microbial carbon substrate utilization in response to inundation in semi-arid floodplain wetlands

In semi‐arid floodplain wetlands, planktonic metabolism, a balance of gross primary productivity of phytoplankton (GPP) and planktonic respiration (PR), may be affected by hydrology, geomorphology, carbon, and microbial communities. Understanding relationships between these factors helps develop effective water and land management plans for these wetlands. In this study, variations in planktonic metabolism and microbial carbon substrate utilization were investigated in the semi‐arid floodplain wetlands of the Macquarie Marshes, south‐eastern Australia, by comparing three geomorphologically distinct zones with different historical inundation frequencies (zones 1: moderate; 2: low and; 3: very low). Floodplain soil samples collected from each zone were inundated artificially in controlled laboratory conditions. GPP and PR were determined using biological oxygen demand methods. Biolog EcoPlates^TM were used to assess microbial utilization of carbon substrate. Among the zones examined, zone 1 had the highest GPP and lowest PR, while zone 3 had the lowest GPP and highest PR. GPP/PR showed zones 1 and 2 autotrophic, and zone 3 heterotrophic. EcoPlates showed the highest and lowest microbial activity in zone 3 and zone 1 respectively, indicating a strong link between microbial carbon substrate utilization and PR for heterotrophy in the rarely inundated zone. Inundation of drier wetland zones may lead to a large pulse in PR, at least for a short period of time, potentially leading to anoxic water events. When making decisions for environmental water allocations, the antecedent hydro‐geomorphological and microbial condition of the targeted wetlands should be considered to help predict expected aquatic ecosystem responses.