A total of four birds were removed from the lake at the East Worthing park by animal rescue charity WADARS last weekend.
One of the birds was already dead in the water while the second died after being taken to Grove Lodge veterinary hospital.
The two other birds were also taken to Grove Lodge and were said to be recovering.
Walter James Smith, 68, who lives nearby and walks around the pond regulary saw the birds being removed.
He said: “I saw a man removing four birds, which looked like seagulls, from the pond at Brooklands and I thought about what else could be dying in the pond.
“If it is a bacterial problem then there are sure to be more dead animals in the water.
“It seems clear to me that there is something wrong with the water there and something needs to be done about it before it gets out of hand.
“I know the pond has had a few problems with algae in the past and it makes me think that it is probably something to do with that.
“There are a few signs up around the pond warning about bacteria in the water but there is nothing to protect people.”
WADARS operations manager Tracy Cadman said: “We can confirm that we sent a man to pick up one dead bird from the pond and three live birds over the weekend.
“We spoke with Worthing Borough Council and removed the birds accordingly.
“Unfortunately, one of the birds died on arrival at the Grove Lodge vets.
“It is too early for post-mortems to determine the reason for the deaths but it could be because of the algae.
“We think the algae may have cleared now due to rain because it builds up when the water is not moving around.
Andy Edwards, Adur and Worthing council parks and foreshore manager, said: “WADARS confirmed with me that they did remove a small number of dead birds from the lake at the weekend during a routine welfare visit.
“The deaths are thought to be as a result of natural mortality and not linked to the water quality of the lake.”
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