Patch dynamics of Phormidium in Canterbury rivers

Abstract

In recent decades there has been an apparent increase in the prevalence of blooms of Phormidium in medium-sized rivers in Canterbury. Given the propensity for this alga to be toxic, this has implications for the management of these rivers, and considerable research has gone into attempts to understand what is driving the increase in blooms, and hence how it may be controlled. To date these investigations have been largely statistical, using broad surveys of bloom frequency, intensity and river attributes to attempt to identify correlates, which may then be investigated as potential causative agents. This statistical approach has implicated a number of possible variables, but as yet no “smoking gun” leading to a “silver bullet” has emerged. In this study we attempt to develop a new approach to understanding Phormidium blooms in Canterbury Rivers by studying the dynamics of the patches of algae that make up river bed cover, rather than just measuring cover. To this end, individual patches were followed for four weeks in four rivers (Opihi, Ashley, Selwyn and Tengawai), by repetitive photography. We also attempted to map the dynamics of patches at river scale using an unmanned aerial vehicle. The study was coordinated with other ongoing work on this river, and at a later date it will be possible to assess results against water quality variables from each river (still under analysis).