Abstract
Cyanobacterial blooms are an increasing threat to water quality and global water security caused by the nutrient enrichment of freshwaters. There is also a broad consensus that blooms are increasing with global warming, but the impacts of other concomitant environmental changes, such as an increase in extreme rainfall events, may affect this response. One of the potential effects of high rainfall events on phytoplankton communities is greater loss of biomass through hydraulic flushing. Here we used a shallow lake mesocosm experiment to test the combined effects of: warming (ambient vs. +4°C increase), high rainfall (flushing) events (no events vs. seasonal events) and nutrient loading (eutrophic vs. hypertrophic) on total phytoplankton chlorophyll‐a and cyanobacterial abundance and composition. Our hypotheses were that: (a) total phytoplankton and cyanobacterial abundance would be higher in heated mesocosms; (b) the stimulatory effects of warming on cyanobacterial abundance would be enhanced in higher nutrient mesocosms, resulting in a synergistic interaction; (c) the recovery of biomass from flushing induced losses would be quicker in heated and nutrient‐enriched treatments, and during the growing season. The results supported the first and, in part, the third hypotheses: total phytoplankton and cyanobacterial abundance increased in heated mesocosms with an increase in common bloom‐forming taxa—Microcystis spp. and Dolichospermum spp. Recovery from flushing was slowest in the winter, but unaffected by warming or higher nutrient loading. Contrary to the second hypothesis, an antagonistic interaction between warming and nutrient enrichment was detected for both cyanobacteria and chlorophyll‐a demonstrating that ecological surprises can occur, dependent on the environmental context. While this study highlights the clear need to mitigate against global warming, oversimplification of global change effects on cyanobacteria should be avoided; stressor gradients and seasonal effects should be considered as important factors shaping the response.
Highlights
Blooms of cyanobacteria are a major threat to freshwater qual‐ ity and global water security (Codd, Morrison, & Metcalf, 2005; Steffensen, 2008), driven primarily by the anthropogenic over‐ enrichment of freshwaters (Taranu et al, 2015)
A fully factorial experiment combining two temperature treatments, two nutrient treatments and two extreme rainfall treatments was performed in 32 outdoor mesocosms from July 2014 to August 2015 at the Centre for Ecology & Hydrology's Aquatic Mesocosm Facility located in the North West of England (54°1′N, 2°46′W)
Flushing events resulted in statistically significant reductions in total phytoplankton and cyanobacterial abundance (Table S3); these losses did not have long‐term effects on total phyto‐ plankton or cyanobacterial abundance, that is flushing as a treatment did not explain additional variation in the abundance of total phyto‐ plankton or cyanobacteria sampled between May and August, indi‐ cating that recovery was rapid during the spring/summer months
Summary
Blooms of cyanobacteria are a major threat to freshwater qual‐ ity and global water security (Codd, Morrison, & Metcalf, 2005; Steffensen, 2008), driven primarily by the anthropogenic over‐ enrichment of freshwaters (Taranu et al, 2015). The response of cyanobacteria to warming and nutrient enrichment may, be complicated by other large‐scale environmental changes which can alter phytoplankton growth and community structure This includes the predicted increase in extreme stormy weather (IPCC, 2013). Nutrient loading in turn depends on the source of nutrients, that is point or diffuse (Elliott, Jones & Page, 2009), catchment geology and antecedent weather (Perga et al, 2018; Reichtwaldt & Ghadouani, 2012) This complexity results in a wide range of environmental change scenarios which can impact phytoplankton dynamics in different ways. We hy‐ pothesised (e) that cyanobacteria may be more sensitive to flush‐ ing (slower to recover) because of slower growth rates compared to other competing phytoplankton taxa
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