EFFECTS OF NITROGEN AND PHOSPHORUS ADDITIONS ON PRIMARY PRODUCTION AND INVERTEBRATE DENSITIES IN A GEORGIA (USA) TIDAL FRESHWATER MARSH

We added nitrogen (N), phosphorus (P), and N+ Pt o aZizaniopsis miliacea (Giant Cutgrass) dominated tidal freshwater marsh in Georgia USA to investigate nutrient limitation of tidal freshwater marsh primary production and invertebrate densities. After two years, aboveground biomass was significantly greater in the plots receiving N (2130 g m 22 ), and N+ P( 2066 gm 22 ) than in the control (886 g m 22 ) and P (971 g m 22 ) only treatments. We observed no enrichment of leaf N or P in response to nutrient additions. Rather leaf N decreased and C:N increased in plots receiving N, suggesting that leaf N was diluted by increased production of carbon laden structural components used to support increased plant height. N:P ratios (mol:mol) of the plant tissue were consistently , 30 (14-27) in the treatment plots, also suggesting N limitation of primary production. Total densities of benthic invertebrates and oligochaete worms (primarily Tubificidae and Lumbriculidae) were significantly greater in the N+P plots than in the other treatments after two years of nutrient additions. There were no clear differences in diversity of benthic invertebrates among treatments. Our results suggest that tidal freshwater marsh primary and secondary production is limited or co-limited by N, and thus, like estuaries and salt marshes, are susceptible to N enrichment and eutrophication.