In 1981, the most abundant algae species was the Green algae species, Ulva sp. In 1983 and 1984, the most abundant algae species were the Brown algae species, Enteromorpha sp. and Giffordia sp.

The marine iguana became shorter by as much as 20% (6.8 cm) within two years. This shrinking coincided with low availability of food that resulted from El Niño events.

Body length increased again during subsequent La Niña conditions, when algal food was abundant.

A study found that lizards that shrank more survived longer than larger iguanas during harsh periods because their foraging efficiency increased and their energy expenditure decreased.

The correlation between snout-vent length (SVL) and stomach content is the larger SVL of marine iguana, the higher stomach contents can be found. Meanwhile, the smaller SVL of marine iguana, the lower stomach contents can be found. This phenomenon is influenced by the availability of algal food.

Different algae species have different content of cellulose and digestibility. The appearance of algae depends on the cellulose contents.

Marine iguana feeding times varied daily according to low tide, and it fed in different intertidal areas. Animals that fed longer and in more productive foraging areas had higher algal intake per day and intake of algae per unit feeding time was higher in the subtidal area than in the intertidal area.

Subtidal areas are areas that are always submerged, so the marine iguana needs more cellulose for energy to stay in depth water.

When the food intake is high, then the stomach content will be high. Vice versa, intertidal areas are border for subtidal and will be exposed during low tide, so the marine iguana needs less cellulose to convert as energy when they can be exposed to sunlight to absorb the light using their dark scale. When the food intake is low, the stomach content will be low.

Marine iguanas enter the water to feed. They primarily consume red and green algae in both subtidal and deeper, cooler water.

Adult females have lower mortality rate than adult males during the food shortage, as their size differs.

Cellulose reduced the apparent digestibility of energy in all diets. Thus, high digestibility implies a lower cellulose content.

Their blunt noses and sharp teeth allow them to easily graze on the algae that grows on rocks.

They have also been observed eating grasshoppers, crustaceans, and sometimes on land plants.

Females are more likely to be able to forage for limited food resources more efficiently and with less energy, as they only need to reach a body size enough to produce surviving offspring.

Sexual size dimorphism was caused by the fact that females matured at a younger age and size than males, after which they constantly allocated resources into eggs, slowing their growth.

As body size is highly heritable in general, these selective processes should result in genetic differences in body size between the sexes while growing up.

For example, warmer water causes the disappearance of the red and green algae that marine iguanas prefer.

The amount of brown algae increases, but it is difficult to digest due to the cellulose content. This may even be toxic to them, resulting in reduction in food, causing the animals to die of starvation.

As marine iguana's preferred body temperature is from 35 to 39°C, it will spend a long period of time basking in the sun to regulate its body temperature after foraging in the relatively cold waters around the Galápagos Islands, which typically are between 11 and 23°C.

As an ectothermic animal, the marine iguana can spend only a limited time in cold water diving for algae. Its body temperature can drop as much as 10°C when it is in the ocean.

Marine iguana will move more slowly when its body temperature is low which will make it more vulnerable to predators. So, the marine iguana acts in a highly aggressive way to bluff its way out of danger to survive.

When the temperature is low, the survival rate of marine iguana will decrease.

There are several factors that can influence the growth rate of algal such as, temperature, light, and pH balance. The temperature for different algal growth varied from certain species.

The intertidal zone is the area where the ocean meets the land between high and low tides.

The intertidal zone is physically divided into four divisions, which are splash zone, high intertidal zone, middle intertidal zone, and low intertidal zone.

The splash zone, also known as supratidal or spray zone is the area above the spring high tide line, on coastlines and estuaries, that is regularly splashed but not submerged by ocean water.

The high tide zone is only submerged at high tide and is hotter and drier.

The low tide zone is only exposed during low tide and has the greatest biodiversity of the four zones because it provides more favorable conditions for those organisms that cannot tolerate exposure for a long period of time.

The optimal temperature range for algal growth is 20-30°C. If the temperature is higher than 35°C, it can be lethal for a number of algal species, especially green algae. However, if the temperature is lower than 16°C, the growth of algal will slow down.