P2.57 Saturday, Jan. 5 Nutrient stoichiometry, species traits, and regime shifts in freshwater ponds MCCANN, MJ; Stony Brook University email@example.com
Alternative community regimes can occur in freshwater ponds, which are either dominated by submerged aquatic vegetation (SAV) or by unrooted, free-floating plants (FFP). Shifts between regimes are driven by changes in nutrients (nitrogen and phosphorous). At low nutrient levels, SAV species dominate because they uptake scarce nutrients directly from the sediment via roots. At high nutrient levels, FFP are not nutrient-limited and are the superior competitors because of their primacy for light. In any waterbody, both plant groups are represented by a diversity of species. The species richness of each plant group may affect the likelihood of a regime shift depending on the degree of redundancy or complimentarity in relevant species traits such as growth rate, nutrient uptake rate, shade tolerance, or resistance to herbivory. If there is high trait complimentarity, then greater species richness within a plant group may increase the likelihood that a pond is found in that regime. Through both natural and anthropogenic mechanisms, ponds vary in the dissolved nitrogen (ammonia and nitrate) to phosphorus ratio (N:P). To test the effects of N:P stoichiometry on a FFP growth rate (a key trait), I measured relative growth rates of three species of FFP – Lemna minor, Spirodela polyrhiza, and Wolffia borealis - grown in laboratory monocultures for 17 days in a fully-crossed factorial design of 3 nitrogen levels (0.5, 5.0, and 10 mg N / l, as a 1:1 ammonia: nitrate mix) and 3 phosphate levels (0.083, 0.83, and 1.66 mg P / l) for a total of nine combinations. Also, species-specific uptake rates of nitrate, ammonia, and phosphate were measured after 48 and 96 hrs. Identifying, measuring, and quantifying the complimentarity or redundancy of relevant species traits will help inform the effect of species richness on the likelihood of community regime shifts.