RESPONSES OF EPIPHYTES ON EELGRASS, ZOSTERA-MARINA L, TO SEPARATE ANDCOMBINED NITROGEN AND PHOSPHORUS ENRICHMENT

We examined the effects of separate and combined nitrogen (N) and phos phorus (P) enrichment on the development of epiphytes on eelgrass (Zos tera 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 anthropogeni c nutrient additions and four nutrient treatments: daily water column enrichment with phosphate alone (PO4), nitrate alone (NO3), nitrate pl us phosphate (NO3 + PO4), or ammonium plus phosphate (NH4 + PO4). In t he mesocosms enriched with PO4 alone, the epiphyte biomass, organic co ntent, chlorophyll a content and community composition were as in the controls, but the nutrient content of the epiphytes (N and P) increase d. Enrichment with NO3 alone increased chlorophyll a content, nutrient content, and relative abundance of cyanobacteria during the summer. E nrichment with NO3 + PO4 and with NH4 + PO4 produced intense and long lasting phytoplankton blooms that lowered epiphyte biomass and chlorop hyll a content, but increased the N and P content of the epiphytes and the relative abundance of diatoms. There were no significant differen ces between the mesocosms enriched with NO3 + PO4 and those enriched w ith NH4 + PO4. Epiphyte biomass appears not to be a good indicator of nutrient loading or eutrophication in shallow coastal lagoons.