The spatial movement of resources or predators among different ecosystems (e.g., from rivers to adjacent terrestrial habitat) has been well studied.  However, the potential for different phases to interact within an ecosystem has been seldom considered.  Phases can be generated by temporal heterogeneity (e.g., climatic variation) resulting in distinct communities (i.e. aquatic and terrestrial) at a single location, examples include seasonal wetlands and temporary ponds.  The California Vernal Pool Ecosystem (hereafter ‘CVPE’) consists of two distinct phases created by annual variation in precipitation and temperature.  During winter, the “aquatic phase” develops as a temporary pond, and as the pools dry in spring, the “flower phase” is dominated by terrestrial plants.  The purpose of this study was to test the hypothesis that resource addition during the “aquatic phase” increases the algal crust when pools dry, which decreases biomass and species composition of the “flower phase”.  We used mesocosms layered with CVPE soil and allowed them to fill with rainwater.  The treatments consisted of five levels of nitrogen and phosphorous addition added every other week during the wet phase from January 2008 to April 2008.   As pools dried into the “flower phase”, we measured total plant species richness, percent cover, and biomass. 
Results/Conclusions

Aquatic nutrient addition increased phosphorus and algae cover during the “aquatic phase” and decreased terrestrial plant species richness and cover during the “flower phase”.  Total plant biomass was not affected by treatments. The increase in aquatic algae created an extensive algal crust layer that likely inhibited particular plant species’ germination and growth.  Several alternative explanations exist, including the changing of abiotic conditions by resources.  Few significant patterns were observed with percent cover of individual plant species, but most exhibited negative trends with resource addition.  CVPE provides habitat for numerous threatened and endangered endemic species, and therefore, these results have several conservation implications.  Management and restoration of CVPE habitat needs to fully consider (1) the effects of increased nitrogen and phosphorous input from agricultural and urban development on ecosystem functioning and species composition and (2) the interactions among these phases.   Overall, our results suggest that phase interaction in temporally heterogeneous environments is an important feature that has implications for conservation, ecosystem function, and species diversity.