Nutrients potentially limiting the maximal biomass of natural assemblages were studied on the basis of samples collected in late July 1997, along a transect from the mouth of the river Daugava to the central part of the Gulf of Riga. The inorganic nutrient concentrations decreased strongly between the river mouth and the central part of the gulf from 17.3 to <1 mu M NO sub(3) super(-)+NO sub(2) super(-)+NH sub(4) super(+) (DIN), and from 3.5 to 0.03 mu M PO sub(4) super(3-). DIN : PO sub(4) super(3-) increased from 5 to 20. Similar trends occurred for SiO sub(3) super(2-) whose concentration decreased from 72 to 1.6 mu M, but DIN : SiO sub(3) super(2-) only halved. Comparison of DIN : P-PO sub(4) super(3-) ratios (atom : atom) with the Redfield's value indicated nitrogen to be the nutrient limiting the algal growth potential (AGP) in central waters, and phosphorus in near-shore waters; bioassays showed that nitrogen was the first limiting and phosphorus the second limiting nutrient in all samples tested, and the limitation index (growth in absence of one nutrient versus that in the unenriched control) indicated that in nearshore samples phosphorus was as limiting, or almost so, as nitrogen. Altogether, it is inferred that, in summer, there is a weak gradient in potential nutrient limitation from the estuary to the central part of the gulf. Nearshore, in the plume influenced by freshwater discharge, nitrogen and phosphorus are equally limiting, while in the central part of the gulf, AGP is clearly nitrogen-limited. Silicon and vitamins could become limiting if concentrations of nitrogen and phosphorus supplies should greatly increased. Iron appeared to be potentially limiting only in the central part of the gulf.