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We performed a mesocosm experiment in buckets to examine the effects of leaf litter input on phytoplankton, using chlorophyll a as a proxy. We followed this up with a second experiment examining the effects of tannins and water color on chlorophyll a to better understand potential mechanisms of leaf litter affecting chlorophyll a. 

Leaf litter is an important input to freshwater systems. Leaves provide carbon, nutrients, and secondary compounds. We examined the effects of tree leaf species on chlorophyll a concentration—a proxy for phytoplankton biomass. We found that an input of Chinese tallow (Triadica sebiferum, invasive in the southeastern USA) and red maple (Acer rubrum) leaves resulted in lower chlorophyll concentrations than controls and other native species. These leaf species also leached tannins, resulting in a darker water color, and either may have caused the patterns observed. To separate these potential mechanisms (darker water leading to light limitation and tannin toxicity), we conducted a second experiment with a fully factorial design manipulating tannins and water color. We found that darker water resulted in the lowest chlorophyll concentration, suggesting light limitation. In the clear-water treatment, the addition of tannic acid lowered chlorophyll concentrations but also resulted in moderately darker water by the end of the experiment. The tannic acid may have been toxic to the algae, or there may have been some light limitation. Our results suggest that tannins that darken water color may substantially suppress phytoplankton and that tree species composition may influence both phytoplankton and the brownification of freshwater.