The largest natural lake in France, Grand-Lieu, has suffered eutrophication. The objective of the study was to estimate the annual input of nutrients (N, P) resulting from avian excrement, deposited by birds feeding out of the lake and returning to its waters for breeding or roosting, as compared to the input by the rivers that enter in the lake. Two years are compared: 1981-82 and 1990-91. About 1600-2000 breeding herons and cormorants, 20 000-33 000 wintering ducks, gulls and cormorants and 1-2.4 million starlings deposited about 5800 kg total N in 1981-82 and 7640 kg in 1990-91. Respectively, 2000 and 2530 kg total P were deposited over the same time periods.These represent 0.7% and 0.4% of the total N input of the lake and 2.4 and 6.6% of the total P input in 1981-82 and 1990-91. Starlings account for 74% of the N and mallards most of the rest. P input by starlings (36% in 1981-82, 41% in 1990-91), and by mallards and herons (35% and 27% in 1981-82 and 22% and 24% in 1990-91 respectively) plays an appreciable role among birds. During the plant growing period (April-September), the contribution by birds can increase to 37% of total P input of the lake. Piscivorous bird colonies concentrate Phosphorus 42 times more within the colony than outside the colony. Overall, the role birds play in total N and P input is relatively small due to very high inputs from human sewage and agriculture run off. The monthly mean concentration of the water of the two rivers reaches currently 10 mg 1of N (to 23 mg during peak floods) and 394 mg m-3 of P (to 468 mg during peak floods). Earlier, for example in the 1960's, water in Brittany only contained 0.1 to 1.1 mg 1-1 of N and 1 to 5 mg m-3 of P during the maximum flow period. At this time, birds could probably have represented annually up to 37% of the N input and up to 95% of the P input to the lake.

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