| Summary: | Mummichog (Fundulus heteroclitus) responses to chronic whole-ecosystem nutrient enrichment were examined near Plum Island Sound, Massachusetts. Dissolved fertilizer was released into replicate salt marsh creeks over 6 growing seasons to simulate agricultural run-off (bottom-up effects). Density, biomass, secondary production, growth rate, and condition factor were estimated in fertilized and reference creeks. Mummichog densities were also used to determine if mummichog growth or health varied with density. Over 7,600 mummichogs were marked and released into the treatment and control areas to measure responses. Over 900 mummichogs were recovered. Mummichog abundance was higher (p = 0.055) in nutrient-enriched creeks than reference creeks (0.81 ± 0.04 fish m-2 and 0.59 ± 0.07 fish m-2 respectively). Nutrient enriched-creek biomass of 522.9 ± 36.1 mg dw m-2 was significantly higher (p=0.028) than control-creek biomass of 338.5 ± 26.7 mg dw m-2. However, reference-creek growth rates of 0.105 ± 0.091 were significantly higher (p=0.04) than the nutrient enriched-creek growth rates of 0.073 ± 0.065 mm d-1. Secondary production and condition factor of mummichogs did not differ with nutrient enrichment. Nutrient enrichment likely stimulated primary production causing bottom-up effects in the food web, which increased mummichog abundance and biomass. However, as abundance increased, mummichog growth rates decreased, suggesting a density-dependent response, likely caused by either intraspecific competition or behavioral changes causing dietary shifts.
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