title: Impacts of macrophyte beds on nutrient dynamics and phytoplankton biomass during water level fluctuations
Creators: name: Lu, Jing
Publisher: name: Griffith University
Year: 2017
Subjects: subject: Phytoplankton
subject: Ecosystems
subject: Macrophytes
subject: Cabomba
subject: Hydrilla
subject: Water level fluctuations
subject: Nutrient budgets
subject: Stable isotope

description: Phytoplankton blooms have been regarded as a major threat to freshwater ecosystems. Macrophytes play a key role in maintaining a clear water state by competing with phytoplankton for nutrients and light, and/or releasing allelopathic chemicals to inhibit phytoplankton growth. However, macrophytes can become desiccated after severe water level drawdown, subsequently releasing substantial nutrients that stored in their biomass after rewetting. Previous studies have mainly focused on the nutrient uptake by macrophytes, with little attention paid to the potential negative implications of macrophyte loss during water level fluctuations (WLFs). This thesis compared the role of macrophytes as a nutrient sink versus a source during WLFs, and its implications for phytoplankton blooms during WLFs. The first data chapter of this thesis has examined the nutrient storage capacity of a floating-leaved macrophyte, Nymphoides indica in a subtropical reservoir. The nutrient stored in N. indica biomass in this reservoir was 30% of total nitrogen (TN) and 150% of total phosphorus (TP) stored in the water column. This species was subsequently used in glasshouse experiments to investigate the impact of different durations of water level drawdown, followed by rewetting on nutrient release from both macrophyte litter and their associated sediment. This study showed that macrophyte beds can shift from a nutrient sink to a source after desiccation, followed by rewetting. This occurred when minimum soil moisture content was reached after 10 weeks of desiccation. This shift also caused an increase in phytoplankton biomass, as nutrients were released. The effect of WLFs on two submerged macrophyte species, the invasive species Cabomba caroliniana (Cabomba) and the native species Hydrilla verticillata (Hydrilla) was also examined. Macrophytes were enriched with stable nitrogen isotope (15N), then used to examine the fate of released 15N from desiccated, and subsequently rewetted macrophytes. The study showed that 15N released from macrophytes was assimilated by phytoplankton faster than being transferred to other N pools, such as to living macrophytes and sediments. Desiccation of the associated sediment, followed by rewetting, resulted in a greater proportion of the macrophyte-derived 15N remaining in the water column and assimilated by phytoplankton, compared with the treatments without sediment desiccation (i.e. constantly wet conditions). Regrowth of macrophytes also changed the fate of nutrients released from macrophyte litter, following rewetting the desiccated sediment. Less 15N was transferred into the water column, sediments and phytoplankton when macrophytes regrew, compared to the treatment without macrophyte regrowth. As such, 28 d after rewetting, regrown Cabomba and Hydrilla assimilated 19–100% of released N and 6–42% of released P from macrophyte litter when the biomass ratio of macrophyte litter to regrown macrophytes was 9:1. In addition, regrown Hydrilla had significantly higher nutrient uptake rates than Cabomba. Despite this, Cabomba was more efficient at reducing water column N concentrations, and neither Cabomba or Hydrilla significantly reduced water column P concentrations. This indicates regrown Cabomba and Hydrilla differ in their use of water column versus sediment nutrients. This has important implications for the capacity of each species to reduce water column nutrients and control phytoplankton biomass. This study also showed that litter of the invasive species Cabomba decomposed faster and had a higher N and P content, compared with the same amount of Hydrilla litter. This resulted in higher water column nutrient and Chlorophyll a concentrations but lower dissolved oxygen concentrations in the Cabomba treatments. The Cabomba litter also had significantly higher N and P content compared with N. indica, indicating Cabomba could also have a greater impact on water column nutrients than N. indica during decomposition. Overall, both the submerged and floating-leaved macrophyte species in this study had a greater impact on water column P than N, after drying then rewetting. This changed the water column N:P ratio with potential flow-on effects to phytoplankton biomass and species assemblages. In summary, these findings of the thesis indicate that effective water level management is essential to protect macrophyte beds and reduce the risk of phytoplankton blooms, during drying then rewetting cycles. Management options could include reducing the frequency, amplitude, or duration of water level drawdown to minimize the loss of macrophytes and maximize their regrowth. Since invasive submerged macrophytes have greater potential to increase the phytoplankton biomass during WLFs, more attention should be paid to reservoirs or lakes dominated by invasive submerged macrophytes. Keywords: macrophytes; water level fluctuations; nutrient budgets; stable isotope; phytoplankton blooms; invasive species