COS 41-2 - Light color and nutrients alter freshwater phytoplankton communities and trophic transfer to zooplankton

Background/Question/Methods

Lakes and ponds have different light and nutrient environments, which can affect phytoplankton community structure. Shifts in light can alter phytoplankton photosynthesis, while differences in nutrient availability can either limit or increase phytoplankton growth. Past work has focused on how changes in light intensity can alter community structure, while usually ignoring light color despite the natural variation in the color of available light in aquatic ecosystems. Light color may interact with nutrient availability to influence community structure; these effects on phytoplankton communities may then affect higher trophic levels. We conducted a laboratory microcosm experiment to assess the impact of differences in light color and phosphorus availability on natural phytoplankton communities, and whether these differences impact trophic transfer to zooplankton grazers. We collected water samples from low-phosphorus Lake Joccassee, SC, and high-phosphorus Lake Murray, SC. Samples were combined to generate a regional species pool, and then inoculated into 24 experimental microcosms randomly assigned to a light color (Red, Blue, Green, Broad) and phosphorus level (High or Low). Phytoplankton communities were enumerated via light microscopy. The indirect effects of light color and phosphorus availability on trophic transfer were tested with a juvenile specific growth rate bioassay using Daphnia pulex neonates.

Results/Conclusions

Light color and phosphorus availability interacted to influence phytoplankton community composition and trophic transfer to zooplankton grazers. High phosphorus treatments led to more abundant phytoplankton communities relative to low phosphorus treatments, while red light led to the least abundant communities, regardless of phosphorus treatment. In terms of trophic transfer, light color and phosphorus availability had both main and interactive effects on Daphnia growth rate. Surprisingly, treatment had a significant effect on Daphnia survivorship. Most notably, all individuals in the Broad/High treatment, across all replicates, died. High phosphorus treatments led to higher juvenile specific growth rates relative to low phosphorus treatments, while Blue and Green light treatments resulted in higher growth rates relative to other light colors. The Blue/High and Green/High treatments resulted in the highest juvenile specific growth rates when compared to all other treatments, The rank order of juvenile specific growth rate changed depending on the phosphorus treatment, indicating an interaction between light color and phosphorus availability. Our results demonstrate that light color and phosphorus availability interact to affect freshwater phytoplankton community composition and trophic transfer from phytoplankton to zooplankton grazers.