Effects of increases in salinity on phytoplankton in the Broadwater of the Myall Lakes, NSW, Australia

The Broadwater of the Myall Lakes system is highly susceptible to cyanobacterial bloom formation after heavy rain events. During prolonged low flow periods, saline intrusion from the lower Myall River increases salinity levels and effectively controls some bloom forming algal taxa. To assess the effect of low-to-moderate increases in salinity (up to 4 ppt) on phytoplankton chlorophyll a, cell abundance, diversity and assemblage structure, salinity enhancement experiments were conducted on Broadwater samples collected in June 2005 (salinity 1.5 ppt), October 2005 (4 ppt) and January 2006 (12 ppt). Natural phytoplankton assemblages were incubated in the laboratory for 10 days, under different treatments of salinity (no addition, +2 ppt, + 4 ppt) and nutrient conditions (no addition, excess N+P). The greatest impact of salinity enhancement in N+P enriched samples was observed in June (1.5-5.5 ppt); chlorophyll a was significantly reduced in samples with the highest salinity treatment, and the taxon most negatively affected by an elevation in salinity to 5.5 ppt was Anabaena circinalis. Taxonomic richness and diversity (Shannon-Wiener index) were unexpectedly significantly higher at 5.5 ppt than at 1.5 ppt. This result, in part, explains the observed significant differences in phytoplankton assemblage structure over this salinity range. In October, the main effect of elevating salinity levels from 4 ppt to 8 ppt was a reduction in the abundance of chlorophytes, particularly Scenedesmus. Phytoplankton samples that were collected when the lake salinity level was 12 ppt were little affected by salinity increases of 2 ppt and 4 ppt, most likely because field samples were already relatively high in salt content. We suggest that further investigations focus on phytoplankton responses to salinity under a range of nutrient regimes that are common to coastal lakes.