Assessment of vegetation changes between 2007 and 2017 in permanent freshwater ponds of the Province Lands, Cape Cod National Seashore

Smith, Stephen

Aquatic vegetation in the permanent ponds within the Province Lands region of Cape Cod National Seashore was surveyed from July-August of 2017 based on the previous sampling design and methods of Smith et al. (2008). This long-term monitoring protocol was designed for repeated sampling along permanent transects and 10 x 10 m plots within 15 ponds. Vegetation cover, soil organic matter surface water pH, conductivity, temperature, and water depths were measured and analyzed from these locations. Two state-threatened species noted in 2007, Orontium aquaticum (golden club) and Carex oligosperma), were also present in 2017. Three exotic species, Phragmites australis (common reed), Lythrum salicaria (purple loosestrife), and Salix cinerea (gray willow), were also detected. Both Phragmites and Salix increased in cover, while Lythrum much was reduced – likely due to the introduction of Galerucella spp. beetles in 2005 as a method of biological control for this species. More generally, plant community composition among ponds differed primarily based on the relative abundances and distributions of three dominant species – Nymphaea odorata (white waterlily, increased), Juncus militaris (military rush, increased), and Decodon verticillatus (water willow, decreased), although on a broader scale most species (66%) exhibited an increase in abundance. Although highly variable among ponds, species richness increased substantially between 2007 and 2017 and was correlated with the standard error values of water depth means (i.e., higher species richness associated with larger variations in water depths along transects). The large difference (25 cm) in average water levels between 2007 (wetter) and 2017 (drier) is likely one of, if not the, most important factors driving observed changes in plant communities. However, other processes such as natural succession are also continuously forcing vegetation change. One other aspect of the Province Lands ponds that was investigated was the potential impact of the Provincetown landfill. In this regard, two of the ponds in close proximity and hydrologically down-gradient from the landfill had the two highest concentrations of soil nitrogen (Figures 50, 58). The same pattern was observed for soil TP (Figure 51). The other pond (Bennett) is the closest site to the landfill, but it is unclear how water may be moving in/out of this pond since it appears to largely in the groundwater flow path from a natural, open area to the north (Figure 58). Consequently, whether the landfill has leeched nutrients into adjacent ponds is still questionable, but the elevated concentrations of TN and TP in Little Bennett and Duck ponds seem to indicate that there may be at least some nutrient enrichment, and this warrants further investigation.

Assessment of vegetation changes between 2007 and 2017 in permanent freshwater ponds of the Province Lands, Cape Cod National Seashore

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