MEPS 194:113-122 (2000)  -  doi:10.3354/meps194113

ABSTRACT: Algal Œgardening¹ by marine grazers has been described for several species but the possibility that grazers influence their gardens through nutrient enhancement has received less attention. This study examined the hypothesis that nutrient regeneration by the territorial intertidal limpet Patella cochlear enhances the productivity of its algal gardens. Nitrogenous excretions by P. cochlear were quantified by (1) measuring urea and ammonium concentrations under limpet shells at low tide; (2) conducting a laboratory experiment to monitor changes in urea, ammonium and nitrate concentrations in the presence of limpets; and (3) using an isotope dilution technique in a field experiment to obtain direct estimates of limpet excretion and algal uptake rates. During low-tide exposure, nitrogenous excretions in the form of urea and ammonium (urea: 1.15 µmol ml^-1; ammonium: 0.07 µmol ml^-1) accumulated under P. cochlear shells, in close contact with the surrounding algal garden. The average quantity of ammonium excreted by a 30 mm limpet was estimated as 458.9 µg NH₄-N d^-1, which is similar to the estimated nitrogen growth requirements of an algal garden (427.8 µg N-growth garden^-1 d^-1). Algal uptake rates, estimated in an experimental chamber on the basis of ¹⁵N incorporation by the algae, indicated that ammonium excretions by P. cochlear supplied approximately 30% of the algal garden¹s daily nitrogen growth requirements. Moreover, the algae demonstrated a potential for Œsurge¹ uptake, an adaptation which would enable them to exploit nitrogenous excretions before these are dissipated by water movement in the field. Our findings show that P. cochlear, and potentially other marine gardening species, can enhance the productivity of their algal gardens through the regeneration of limiting nutrients.

KEY WORDS: Algal gardening · Marine grazer · Plant-herbivore interaction