Does the limpet Patella cochlear fertilize its own algal garden?

Algal 'gardening' by marine grazers has been described for several species but the possibility that grazers influence their gardens through nutrient e nhancement has received less attention. This study examined the hypothesis that nutrient regeneration by the territorial intertidal limpet Patella coc hlear enhances the productivity of its algal gardens. Nitrogenous excretion s by P, cochlear were quantified by (1) measuring urea and ammonium concent rations under limpet shells at low tide; (2) conducting a laboratory experi ment to monitor changes in urea, ammonium and nitrate concentrations in the presence of Limpets; and (3) using an isotope dilution technique in a fiel d experiment to obtain direct estimates of limpet excretion and algal uptak e rates. During low-tide exposure, nitrogenous excretions in the form of ur ea and ammonium (urea: 1.15 mu mol ml(-1); ammonium: 0.07 mu mol ml(-1)) ac cumulated under P, cochlear shells, in dose contact with the surrounding al gal garden. The average quantity of ammonium excreted by a 30 mm limpet was estimated as 458.9 mu g NH4-N d(-1), which is similar to the estimated nit rogen growth requirements of an algal garden (427.8 mu g N-growth garden d( -1)). Algal uptake rates, estimated in an experimental chamber on the basis of N-15 incorporation by the algae, indicated that ammonium excretions by P. cochlear supplied approximately 30% of the algal garden's daily nitrogen growth requirements. Moreover, the algae demonstrated a potential for 'sur ge' uptake, an adaptation which would enable them to exploit nitrogenous ex cretions before these are dissipated by water movement in the field. Our fi ndings show that P. cochlear, and potentially other marine gardening specie s, can enhance the productivity of their algal gardens through the regenera tion of Limiting nutrients.