Using aquatic plants to understand community regime shifts

Background/Question/Methods

Community regime shifts are nonlinear transitions between distinct community and ecosystem states. These changes occur in many types of ecological systems and are typically depicted as the change in dominance of species or functional groups in response to an environmental driver. Many small, freshwater ponds can be in one of two states – dominated by unrooted, floating plants (FP) or dominated by submerged aquatic vegetation (SAV) – and can be shifted between these two states by changes in dissolved nutrient levels. These ponds are an ideal system for studying how species-, community-, and ecosystem-level properties can modify the likelihood of a regime shift. To investigate the likely factors that change the relationship between an environmental driver (e.g., nutrients) and system state (e.g., dominant vegetation type), I surveyed 13 Long Island (LI) ponds in summer 2011 and analyzed a published dataset of 185 Connecticut (CT) lakes. I quantified FP abundance, species composition, water chemistry, and lake morphometry. Relationships between these variables were analyzed with both univariate and multivariate statistics (principal components analysis, PCA).    

Results/Conclusions

FP were present in ~27% of CT ponds. Across both datasets, FP species richness within a pond ranged from zero to three, including the genera Lemna, Wolffia, and Spirodela. In both regions, FP dominance was rare (7 of 61 ponds with FP had >65% cover) and limited to small waterbodies (< 3 ha surface area). For LI ponds, Lemna abundance and dissolved phosphate had the highest positive scores, while pond surface area and SAV richness had the most negative scores on PC1. Spirodela and Wolffia abundance and dissolved nitrate had high scores on PC2 (PC1 + PC2 = 58.4% of total variation). CT ponds had similar PC1 scores, but data on nitrogen were not available to determine if there was a similar relationship with Spirodela and Wolffia and dissolved nitrogen as in LI. Although the casual relationship is uncertain, Lemna dominance occurs with high phosphate concentrations, but pond size and SAV richness limit the occurrence of this regime shift. Wolffia and Spirodela are more dominant with high nitrogen, and do not become dominant without Lemna present. The likelihood of this community regime shift depends on species composition, lake morphometry, and the particular types of dissolved nutrients.