Responses of epiphytes on eelgrass, Zostera marina L, to separate and combined nitrogen and phosphorus enrichment

We examined the effects of separate and combined nitrogen (N) and phosphorus (P) enrichment on the development of epiphytes on eelgrass (Zostera marina L.). The experiment used ten mesocosms as living models of shallow coastal lagoons along the southern shore of Rhode Island, USA. The mesocosms comprised, in duplicate, controls with no anthropogenic nutrient additions and four nutrient treatments: daily water column enrichment with phosphate alone (PO4), nitrate alone (NO3), nitrate plus phosphate (NO3 + PO4), or ammonium plus phosphate (NH4 + PO4). In the mesocosms enriched with PO4 alone, the epiphyte biomass, organic content, chlorophyll a content and community composition were as in the controls, but the nutrient content of the epiphytes (N and P) increased. Enrichment with NO3 alone increased chlorophyll a content, nutrient content, and relative abundance of cyanobacteria during the summer. Enrichment with NO3 + PO4 and with NH4 + PO4 produced intense and long lasting phytoplankton blooms that lowered epiphyte biomass and chlorophyll a content, but increased the N and P content of the epiphytes and the relative abundance of diatoms. There were no significant differences between the mesocosms enriched with NO3 + PO4 and those enriched with NH4 + PO4. Epiphyte biomass appears not to be a good indicator of nutrient loading or eutrophication in shallow coastal lagoons.