Algae are dominant primary producers and an important component of riverine food webs; algal succession and biomass can determine the structure of higher trophic levels within an aquatic community. Changes to flow can affect algal communities, which in turn can affect the higher trophic organism’s dependent on algal communities. Increased flows or flooding in river systems can temporarily alter the composition of algal assemblages and result in both increases and decreases in algal biofilm abundance. In turn, alterations to algal biomass and assemblage structure have been correlated with changes in taxonomic richness, biomass and density of invertebrate communities. We investigated the effects of environmental watering actions on algal biofilm structure and production as part of a long-term intervention monitoring program evaluating the broad effects of Commonwealth Environmental Flows (CEW) to ecosystem health in the Lower Goulburn River in northern Victoria.
The impacts of CEW on biofilm structure and abundance was assessed using artificial substrates deployed at two sites in the Lower Goulburn River during winter and summer. Artificial substrates, comprised of compact disks, were deployed at two depths, representing the photic and nonphotic zones, for 4, 8 and 12 weeks, over which time CEW flows occurred. Upon retrieval, biofilms were scraped from artificial substrates and biomass as chlorophyll-a, total organic plus inorganics dry mass (DM) and total organic content as ash free dry mass (AFDM) were determined. Biofilm community structure was assessed using multiwavelength-excitation Pulse Amplitude Modulated fluorometry, which differentiated biofilm communities into chlorophytes, diatoms and cyanobacteria.
These environmental flows significantly differed between winter and summer sampling events. Significantly greater variations were observed during winter, with up to 3 m differences in water levels, compared to summer where water levels varied by up to 1m. This resulted in different responses in biofilm structure and abundance between seasons. The impacts of environmental watering actions on biomass and biofilm structure endpoints and potential implications for higher trophic levels will be discussed.