Plague One - River of Blood

“And the Lord spake unto Moses, Say unto Aaron, Take thy rod, and stretch out thine hand upon the waters of Egypt, upon their streams, upon their rivers, and upon their ponds, and upon all their pools of water, that they may become blood; and that there may be blood throughout all the land of Egypt, both in vessels of wood, and in vessels of stone.

And Moses and Aaron did so, as the Lord commanded; and he lifted up the rod and smote the waters that were in the river, in the site of Pharaoh, and in the site of his servants; and all the waters that were in the river were turned to blood.

And the fish that was in the river died; and the river stank; and the Egyptians could not drink of the water of the river; and there was blood throughout all the land of Egypt.

Exodus 7:19-21

The Egyptians must have feared for their lives and for the land of Egypt itself when the Nile turned red with blood. Even in modern Egypt the Nile is still classed as an integral part of daily life, with many Egyptians living off the fertile land up and down the great river, and anything that would threaten to take away this source could prove to be fatal.
The fish within the great river are an essential part of the Egyptian diet, as well as the waterway being the main method for transportation from the North to the South. If anything were to happen to this life source, the effects would be devastating.

Pharaoh was warned of this event and was watching as Moses apparently used his staff to turn the water of the Nile to blood, a blow not only on Pharaoh and his land, but a direct attack on Hapi, the Egyptian god of the Nile.

‘Red Tides’, such as the one that hit Hong Kong in 1998, are caused by forms of alga that can be associated with fish lesions and kills. These algae release toxins that cause water to be undrinkable and, containing red pigments, can help to turn masses of water to red - aided by the blood of the fish that are contaminated as their skin is eaten away.
An incident such as this also occurred in North Carolina in 1997 where a form of alga called Pfiesteria inflicted deep sores on fish causing them to die and the water to turn red, an incident that could have been described as a biblical plague.

It is possible that a freshwater dinoflagellate had flourished in the waters of the Nile, causing a mass infection of the fish which would cause the waters to turn red with the blood and undrinkable with the toxins. The Egyptians would not have the expertise to study such a phenomenon as microscopic alga and would have attributed this catastrophic incident to the hand of God.