Spatial and temporal variability in Phormidium mats and associated anatoxin-a and homoanatoxin-a in two New Zealand rivers

Toxic benthic cyanobacterial proliferations in freshwater are becoming increasingly prevalent, and associated animal poisonings are being reported with greater regularity. Despite this, few studies have investigated spatial and temporal variations in freshwater mat-forming Cyanobacteria and their associated toxin production. Some sections of the Hutt and Wainuiomata rivers (lower North Island, New Zealand) contain extensive Phormidium sp. proliferations that sometimes produce anatoxin-a (ATX) and homoanatoxin-a (HTX). The percentage coverage of Phormidium sp., ATX and/or HTX concentrations and a suite of physicochemical parameters were monitored at 8 sites for 12 mo. The percentage coverage of Phormidium mats was greater in the summer months and these correlated with warmer water temperatures and stable river flows. Flows in excess of 3 times the mean resulted in the removal of Phormidium mats. There was no correlation between the presence/absence of Phormidium mats and water-soluble nutrients. The presence and concentration of ATX and/or HTX and their degradation products, dihydroanatoxin-a and dihydrohomoanatoxin-a, was highly variable across all sites and over time. Anatoxin-a and HTX occurrence was restricted to periods of warm water temperatures (above 13.4 degrees C) and below average river flows.