Kemp, Hannah Rose
(2025)
Blooming blanket weed: occurrence, impacts and environmental drivers of nuisance filamentous green algal blooms in shallow freshwaters.
PhD thesis, University of Nottingham.
Abstract
Blooms of filamentous green algae (FGA) form dense mats at the surface of shallow freshwaters and have multiple negative impacts on aquatic ecosystem functions, services and aesthetics. Effective management and control of FGA blooms is therefore a pressing issue. However, their spatial and temporal dynamics and associated environmental conditions remain under researched, especially in shallow freshwater ecosystems. This thesis aimed to address this knowledge gap by: 1) exploring the occurrence, impacts and current management of FGA blooms at the national scale using survey and citizen science data; 2) investigating the spatial and temporal dynamics of FGA growth and bloom presence and associated environmental conditions over a 19-month monitoring period in Clumber Lake, Nottinghamshire, UK, a shallow ornamental lake managed by the National Trust, 3) examining how light and temperature affect FGA surface bloom formation in nutrient-replete conditions in a series of large-scale mesocosm experiments using underwater and surface photography; and 4) assessing whether FGA function as ecological engineers in Clumber Lake.
The survey and citizen science data revealed that FGA blooms are occurring across the UK in shallow standing freshwater bodies, are generally found at lower elevations, and have a wide range of impacts on both ecology and people. These impacts depends on the waterbody’s uses and functions, as people place different values on different waterbodies and the ecosystem services they provide. Successful management of FGA blooms requires a collaborative approach at the catchment scale to reduce nutrient loading into freshwater systems, but short-term measures, such as FGA removal, are sometimes necessary to enable ecosystem service provision.
Monthly limnological monitoring at Clumber Lake revealed Cladophora spp. was the dominant FGA taxon, which grew all year round in the benthic zone. Surface blooms occurred between April – September, generally coinciding with increases in benthic FGA biomass. They were dominated by Ulva sp. early in the season in the shallowest areas of the lake, followed by Cladophora later in the season across the whole lake. High loads of phosphorus and nitrogen enter Clumber Lake from the surrounding Poulter catchment (127km2), and nutrient addition bioassay experiments confirmed that FGA growth was never limited by nutrient availability over a seasonal cycle. Nitrate and total nitrogen concentrations significantly affected the likelihood of surface bloom presence and benthic FGA biomass, respectively, whilst daylength and monthly sunshine hours were both significant predictors of surface bloom presence. We established that Clumber Lake exists in an FGA-dominated clear-water state, most likely enabled by rapid lake flushing rates that prevent the accumulation of phytoplankton biomass, limiting their ability to compete with FGA which remain in the lake and utilise incoming nutrients. The FGA also function as ecological engineers in Clumber Lake by altering nutrient cycling and environmental conditions in the water column, including light, dissolved oxygen and pH. They also providing refuge, substrate and/or food for macroinvertebrates, functionally compensating for the lack of macrophytes.
At the experimental scale, a minimum threshold of daily light dose of ~55.0 mol m-2 (a combination of photoperiod and irradiance) was required for bloom formation and substantial FGA growth. Surface blooms did not occur during shorter photoperiods (i.e. 8 hours), suggesting daylength is a limiting factor. A longer daylength allowed more time for photosynthetically-derived gas bubbles to accrue in the FGA masses, making them buoyant and float to the water surface. Temperatures between 16-22 °C were also optimal for FGA to form surface blooms. With the increasing impact of climate change on freshwater ecosystems, these results shed new light on the drivers of surface blooms and helps predict when waterbodies may be at risk of FGA blooms in the future.
Overall, this research demonstrates that FGA blooms are common in shallow freshwater bodies, and whilst their growth is primarily driven by nutrient pollution, surface bloom formation is dependent on specific light and daylength requirements within certain temperature ranges. Management strategies should focus on reducing nutrient loading by working collaboratively at the catchment scale. These results can also be used to help predict FGA bloom occurrence in order to focus management efforts and resources. It is vital that the impacts of management are monitored and quantified to build a greater evidence base that can be used to inform successful FGA management in the future.
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