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.