In situ river experiments to explore variability in Microcoleus autumnalis mat expansion

Proliferations of benthic mats dominated by anatoxin-producing Microcoleus autumnalis (basionym Phormidium autumnale) pose an increasing health risk to human and animals worldwide. This study assessed M. autumnalis mats in two in situ experiments undertaken at three spatial scales: (1) reach-scale along a river; (2) reach-scale, focusing on two sites upstream and downstream of a tributary with substantial differences in naturally occurring M. autumnalis; and (3) patch-scale differences in light intensity. Most studies exploring drivers of proliferation have surveyed natural populations with correlative analyses; these in situ experiments aimed to validate previously hypothesised drivers. In experiment one, M. autumnalis mat area and expansion tended to increase downstream. This longitudinal variation was partially associated with increases in dissolved inorganic nitrogen, fine sediment and associated biologically available phosphorus. M. autumnalis mat area differed markedly at sites above and below the most downstream tributary. A second experiment with additional parameters (metals, grazing and light) was undertaken in this reach. None of the parameters conclusively explained differences in mat area, although numerous metals were identified for further investigation with a controlled experimental approach. These results posit that there are multiple non-linear drivers of M. autumnalis proliferation and their hierarchy of importance varies amongst sites.