Ecological Effects of Nutrient Enrichment on a Coastal Fringe Mangrove Forest

Mangrove ecosystems are highly productive coastal wetlands that connect tropical terrestrial and marine systems. Increasing human populations and land use changes are enhancing nutrient loading to tropical coastlines, potentially leading to eutrophic conditions for marine systems. The carbon (C) cycle is critical for mangrove forest structure, function, and resilience under environmental change. Increasing trends of nutrient enrichment in tropical coastal waters may have an effect on organic C mineralization by alleviating nutrient limitation of the heterotrophic microbial community, which may lead to a loss of within-stand carbon sequestration. To better understand and predict the consequences of increased nutrient input, a fertilization study within a fringe mangrove system in southeast Puerto Rico was established to mimic high-nitrogen agriculture runoff (N:P ratio of 50:1) or urban runoff with higher phosphorus (N:P ratio of 16:1) for an annual loading rate of 70 g N m-2 y-1 and 3.1 or 9.7 g P m-2 y-1. Bi-weekly pulses of fertilization began in October 2011 and continued for two years. Chapter 1 investigates nutrient enrichment effect on organic carbon mineralization by sediment microbial respiration rates and above- and below-ground litter decomposition. Neither of the fertilization scenarios had an effect on any of the C mineralization processes compared with ambient conditions in this two-year study. Sediment respiration rates ranged from 0.54 to 2.63 μmol CO2 m-2 s-1. Above-ground litter was calculated to completely decompose within 38 to 220 days by microbial activity alone. This work agrees with previous studies of nutrient enrichment and organic C decomposition rates in mangrove systems, yet differences in the reported values of C mineralization between sites suggest that forest type, environmental conditions, and location may play a substantial role in mangrove C dynamics. Chapter 2 evaluates above-ground production (tree biomass and litter production) in response to nutrient enrichment. Mangrove tree response was characterized by above-ground wood production and litterfall input. Basal area ranged from 17.1 to 32.7 m-2 m-2. Above-ground biomass (wood and foliage) ranged from 3.9 to 15.4 kg dry mass m-2. Mangrove above-ground growth was highly variable and was not affected by nutrient treatment. Litterfall was strongly seasonal, with higher rates associated with the start of the rainy season in late summer. Total litterfall for this fringe mangrove ranged from 1061 to 1217 g dry mass m-2 y-1, which was on the higher range of neo-tropical fringe mangroves. Total litter production was not affected by nutrient treatment, but agriculture treatment (high N:P) enhanced leaf and stipule production (p = 0.04 and 2.2e-06, respectively). These results suggest that two years of high N:P nutrient addition results in a small positive effect for above-ground production within a fringe mangrove, and that litterfall may be a more sensitive response than above-ground biomass for a short-term (two years) assessment. Considering the different effects to mangrove production by the nutrient enrichment regimes (high vs. intermediate N:P loading), further work is needed to assess N and P interactions that may affect above-ground mangrove production. In Chapter 3, a one-time spike of 15N stable isotope tracer was added to ambient (receiving local channel water) and chronically nutrient enriched (receiving 70 g N m-2 y-1 and 9.7 g P m-2 y-1 for 21 to 24 months) sites within a coastal fringe mangrove forest in southeastern Puerto Rico to determine if nutrient enrichment was being retained in the mangrove ecosystem, and to identify mangrove N sinks. Measurements for 15N retention occurred at 1 month, 3 months, and 10 months after the initial spike. 15N tracer was recovered in all components (mangrove foliage, standing litter, surface sediment, and fine roots) within ambient and fertilized sites. The highest proportion of tracer retention (28 to 59% under ambient conditions and 14 to 28% under nutrient enriched conditions) occurred in the organic layer of the forest floor (standing litter, surface sediment, and surficial fine roots). 15N retention within the mangrove canopy represented the smallest N sink, although foliar δ15N continued to increase throughout the measurement period. Tracer retention within the organic layer of the forest floor indicates strong recycling and retention of nutrients that supports a highly-productive system in an oligotrophic marine environment. Chronically nutrient enriched sites retained more 15N tracer than ambient sites, but a smaller proportion of the applied 15N fertilizer, suggesting the capacity of nutrient retention had been exceeded by the added nutrient load. The culmination of this work suggests resilience of mangrove ecosystem function within the first two years of nutrient enrichment