Eutrophication is the most acute environmental problem in the Archipel
ago Sea, SW Finland. When analysing the factors behind this escalating
eutrophication the determination of limiting nutrient at a given time
is essential. Besides experimentations, nutrient limitation of plankt
on has been extensively studied by direct chemical analyses. We used t
he latter approach in this work. Nutrient limitation was studied by ca
lculating different nutrient ratios - total nitrogen:phosphorus, inorg
anic nitrogen:phosphorus, and nutrient balance ratio. Results showed t
hat phosphorus usually limited primary production only near the coast
line. In the middle zone of the Archipelago Sea the limiting factor va
ried temporally. Outer in the open sea nitrogen limited primary produc
tion during most of the year. Phosphorus limited phytoplankton growth
especially in spring and in summer and nitrogen in late summer and in
autumn. Our results suggested that nitrogen is an important limiting n
utrient in the Archipelago Sea. In recent years when the eutrophicatio
n has proceeded there has been a shift from.production limitation by b
oth nutrients to limitation by nitrogen alone. But if we want to defin
e and characterize the nutrient limitation of the entire ecosystem of
the Archipelago Sea, budgets have to be calculated for both N and P an
d internal recycling must be taken into account as well as external su
pply of nutrients and loss processes.