Developing a strategy for monitoring phytoplankton community structure in Lake Mulwala

"October 2007".

Project Number: Blue-green algae assessment - Lake Mulwala - M/BUS/224.

MDFRC item.

This report describes the spatial distribution of both total phytoplankton community structure and blue-green algal biovolume in Lake Mulwala during a period of high algal abundance. The distribution of phytoplankton was then used to design monitoring strategies to determine both changes in overall phytoplankton community structure over time, as well as fulfil statutory monitoring requirements for determining blue-green algal biovolume in the lake. There was a statistically significant difference in both overall algal community structure and abundance as well as blue-green algal biovolume in different regions of Lake Mulwala, with the lowest diversity found towards the western end of the reservoir. Therefore: 1.It is recommended that any phytoplankton sampling program takes samples randomly from throughout the lake. Although there were slightly more taxa found in depth-integrated samples, there was no statistically significant difference in overall phytoplankton community structure or abundance between surface and depth-integrated samples taken throughout the lake; however, there was a difference in the biovolume of blue-green algae. Therefore: 2. It is recommended that if the sole purpose of the phytoplankton monitoring program is to determine overall phytoplankton community structure depth integrated samples should be considered. If the purpose of the monitoring program is to determine public health risks associated with Cyanophyte abundance, then surface samples should be taken. Comparison of sampling effort with the number of new taxa found was used to estimate the optimum number of samples needed to be taken throughout Lake Mulwala. It was estimated the number of new taxa found per sample rapidly declined after five or six samples. The number of samples necessary to determine whether or not the biovolume of Cyanobacteria exceed statutory trigger levels was estimated using posthoc power analysis. Between three (total biovolume) and six (biovolume of toxic bluegreen algal) samples would give sufficient power to differentiate between the population of blue-green algae in Lake Mulwala at the time of sampling and a hypothetical community with a mean concentration the same as the statutory trigger levels. Therefore: 3. It is recommended that at least six phytoplankton samples be taken during each sampling event in Lake Mulwala. Although not specifically addressed in the current sampling program, rates of change in chlorophyll-a and Anabaena circinalis determined from historical data sets were used to estimate frequency of sampling. 4. At least monthly sampling is required within the lake to determine shifts in overall algal biomass as well as blue-green algal biovolume. The frequency of sampling should increase as the biovolume of toxic blue-green algae approaches statutory risk-based notification trigger levels.