PS 50-176 - Effects of light color and phosphorus availability on absorption of natural phytoplankton communities

Background/Question/Methods

Light spectrum and nutrient levels are major determinants of aquatic phytoplankton community structure. Shifts in available light spectrum influence phytoplankton abundance and interactions, while phosphorus limitations can hinder growth and cell division. Understanding the interaction of phosphorus and light spectrum on natural phytoplankton communities is vital to predict how shifting environments may impact aquatic communities over time. Here, we conducted laboratory experiments to assess the influence and potential interaction of light spectrum and phosphorus levels on phytoplankton communities using absorption spectra. We collected 16 1L water samples at three sampling depths in Lake Jocassee and Lake Murray, SC. They were combined into a mixed community representing the regional species pool, then inoculated into WH growth medium in 24 experimental microcosms randomly assigned to a light spectrum (Red, Blue, Green, Full) and nutrient level (high P (1 mg/L) or low P (1 µg/L)). The experiment continued for 54 days with media periodically replenished; 20mL were sampled from individual microcosms every 10 days for preservation for cell counts and analysis via absorption spectra. The absorption spectra of microcosms across the six sampling periods were compared within and between light treatments and P levels.

Results/Conclusions

Light spectrum and phosphorus level influenced community structure with different taxa dominating across treatments, and all but one microcosm exhibited an increase in density over the 54-day span. By Day 30, microcosms in Blue/High-P had peak absorbance at 480-490nm, indicating diatom and/or green algae dominance in the communities. On Day 54 across both phosphorus treatments, Red microcosms had the lowest overall absorbance while Green had the greatest. Absorbance in Green/High P and Green/Low P is of particular interest due to the relationship between phosphorus availability and phytoplankton acclimation. High P microcosms underwent rapid growth from Day 30 to Day 54 with a large increase in absorption of green wavelengths (570-580nm). It is likely cryptophytes dominated these communities by optimizing photon capture of green light over time, potentially via chromatic acclimation. Significant absorption of green wavelengths within green microcosms was not detected in Low-P microcosms, suggesting nutrient availability may play a role in photoacclimation efficiency of phytoplankton. These results indicate that the interaction of light color and phosphorus affects phytoplankton community structure and the ability for communities to optimize the use of available light.