The effects of system closure on the dynamics of productivity and nutrient cycling are examined in four wetlands that differ in plant growth form and magnitudes and sources of water input and nutrient loading. Dynamics in relatively closed ombrotrophic Carex marsh and Taxodium swamp systems from Okefenokee Swamp are compared to those in open, rheotrophic riparian systems. The riparian systems examined include Zizaniopsis marshes along the tidal freshwater portion of the Altamaha River in Georgia and a mature Taxodium-Nyssa swamp along the Cache River in Illinois. Water budgets in the ombrotrophic systems are dominated by precipitation inputs while in the riparian wetland they are dominated by overbank flooding. Nutrient loading to the open and closed systems differs by only two orders of magnitude, the former depending on atmospheric inputs and the latter depending on tidal and riverine inputs. Comparisons of nutrient import, export, and retention indicate that greater than 90% of inorganic nutrients are retained in the closed systems while less than 5% are retained in the open systems. Nutrient budgets for wetland vegetation, including aboveground uptake, root uptake, leaching, death, and translocation, are constructed. Strong differences in nutrient conservation within plant communities are found between marsh and forested closed systems and between open and closed systems as a whole. There is the indication that nutrients turn over more rapidly and nutrient cycles are less retentive and conservative as systems become more open and nutrient inputs increase. Nutrients turn over more rapidly in marshes with nonwoody vegetation that in swamp forests. This phenomena is partially attributable to the growth form of the vegetation as trees store vast amounts of high C:nutrient ratio biomass in boles. Substituting space for time and marsh and swamp wetlands for young and mature ecosystems enables patterns of productivity and nutrient cycling for these wetlands to be compared with Odum's (1969) prediction of ecosystem development. Patterns of ecosystem development in wetlands agree with those predicted for terrestrial systems in general, but there are many areas of contradiction. The degree of system closure appeared to be a major factor controlling nutrient retention and cycling in wetland ecosystems. System closure is also likely to be important in determining the response of wetland systems to global increases in CO2 levels.