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Nutrient enrichment is a major driver of environmental change in mangrove ecosystems. Yet, nutrient enrichment impacts on physiological processes that regulate CO2 and water fluxes between mangrove vegetation and the atmosphere remain unclear. We measured peak growing season photosynthesis (A) and respiration (R) in black mangrove (Avicennia germinans) leaves that had been subjected to long-term (8-year) nutrient enrichment (added N, added P, control) in north Florida. Previous results from this site indicated that Avicennia productivity was N-limited, but not P-limited. Thus, we expected that N addition would increase light saturated net photosynthesis at ambient CO2 (Anet), intrinsic water-use efficiency (iWUE), maximum rate of Rubisco carboxylation (Vcmax), and leaf dark respiration (R), while P addition would have little effect on any aspect of photosynthesis or respiration. We expected that increased photosynthesis and respiration would be most apparent immediately after N addition and in newly formed leaves. Indeed, Anet and Vcmax increased just after N addition in the N addition treatment; these increases were limited to leaves formed just after N addition. Nonetheless, over time, photosynthetic parameters and iWUE were similar across treatments. Interestingly, R measured at 25 °C increased with N addition; this effect was consistent across time points. P addition had little effect on R. Across treatments and time points, Vcmax,25 (Vcmax standardized to 25 °C) showed no relationship with R at 25 °C, but the maximum rate of electron transport for RuBP regeneration standardized to 25 °C (Jmax,25) increased with R at 25 °C. We conclude that N addition may have small, short-lived effects on photosynthetic processes, but sustained effects on leaf R in N-limited mangrove ecosystems.