Algal blooms are a topic of high research interest due to the impact harmful species can have on human health and ecosystems. However, algal bloom research has focused primarily on toxic phytoplankton strains. Periphyton blooms are less common compared to their phytoplankton counterparts, but benthic blooms such as the didymo (Didymosphenia geminata) are becoming more common and invasive. Torch Lake, located in the northern part of Michigan, has experienced periphyton blooms in the last 2 years, with blooms occurring in different places around the coastline. We investigated the possible sources and causes of these blooms and specifically addressed the following questions: (a) what is the current limiting nutrient in the lake? (b)Are the species growing in these blooms known to cause health hazard concerns? (c)What is the current community composition of the periphyton? (d)And is ground water a possible source of excess nutrients promoting the observed blooms? We prepared nutrient diffusing assays with 4 different experimental treatments: nitrogen only, phosphorous only, nitrogen plus phosphorous, and negative control (no nutrient added). We measured chlorophyll a rates, cell densities and community composition of each nutrient treatment. Additionally, we collected water samples from both the water column and ground water sources.
Results/Conclusions
The periphyton was dominated by diatoms. There was a trend of higher chlorophyll a rates in the nitrogen only treatment. Cell densities were highest in the nitrogen plus phosphorous treatment followed by the nitrogen only treatment. Preliminary results from community composition show similar communities in the nitrogen and nitrogen plus phosphorous treatment and similarity between the phosphorous and control treatment. Water samples revealed higher levels of phosphorous compared to nitrogen from ground water sources. No known toxic species was encountered. Achnantidium spp. of diatoms appeared to be the most dominant species within the bloom, phosphorous and control samples. These results suggest phosphorous coming from ground water is promoting the observed periphyton blooms. Torch Lake is known for its turquoise waters usually an indicator of an oligotrophic system. We believe that the lake went from phosphorous-limited to nitrogen-limited, however it is still not understood what could have led to the recent influx of phosphorous from ground water. These blooms allow us the particular opportunity to understand how ecosystem changes can influence benthic blooms. Further research will reveal the impact these blooms have in the system and higher trophic levels.