Environmental factors that promote Phormidium blooms in Canterbury rivers

Abstract

During the past ten years there has been an apparent increase in the prevalence of benthic cyanobacterial blooms in New Zealand rivers (Heath et al., 2011). Phormidium appears to be the dominant genus (Heath et al., 2010). Phormidium can produce powerful neurotoxins, which pose a risk to human and animal health, and animal toxicosis associated with benthic cyanobacteria has become increasingly prevalent in New Zealand (Hamil, 2001; Wood et al., 2007). Despite this health risk, little is known about the environmental factors that are important in regulating the occurrence of riverine benthic cyanobacterial blooms. In comparison, a breadth of knowledge exists regarding the environmental factors that lead to planktonic algal blooms (Oliver et al., 2012). Hydrological regime (Heath et al., 2011), water temperature (Heath et al., 2013) and nutrient concentrations (Biggs, 2000; Wood & Young, 2012) are all implicated as important environmental factors in regulating benthic cyanobacterial blooms. In this study, we monitored three sites along the Ashley River/Rakahuri for 10 weeks and also analysed historical data for 10 sites (five with regular Phormidium blooms and five without) from the wider Canterbury region. Environmental factors investigated included nutrient concentrations (DRP, DIN, TP, TN, and nitrate), water temperature, depth, river flow, point velocity and substrate composition. The aim of this study was to identify environmental factors that correlate with Phormidium percentage cover. Sites with regular Phormidium blooms, with the exception of Temuka at Manse Bridge were dominated by larger substrate (boulder and cobble). Comparatively, sites without Phormidium blooms were dominated by smaller substrate (sand/silt, fine gravel and gravel). All sites had low DRP concentrations. There were differences in DIN concentrations but these did not relate to probability of bloom formation. Heath et al., (2011) highlights temperature as an important factor in determining whether Phormidium is present or absent. In this study, we found no correlative relationship between Phormidium percentage cover and water temperature. Furthermore, Phormidium was observed in a range of water temperatures, between 4–20°C. A distinct pattern existed at some sites between flushing flows (3 times median flow) and Phormidium percentage cover, with more frequent flushing flows resulting in decreased Phormidium percentage cover. However, the general flushing flow rule that three times the median flow is sufficient to remove all Phormidium mats was not applicable in all of the Canterbury rivers studied. For example, a large flushing flow of 22 times the median occurred at Pareora at the huts on the 28.01.2011 and did not remove all the Phormidium. Like Heath et al., (2013), we found that Phormidium had no specific preference for water velocity and depth, but occurred at a range of depths (0.03–0.59 m) and point velocities (0–1.4 ms⁻¹). At present, it appears that water quality is a weak predictor of Phormidium blooms. Rather substrate stability and flow may be the most important factors controlling the dynamics of Phormidium in Canterbury rivers.