Solar energy is required for converting carbon dioxide and water into organic matter, otherwise known as photosynthesis. As with temperature and nutrients, different algae have different rates of metabolic response to increasing light and also different optimal light conditions for growth, as some algae are more adapted for darker environments while others can outcompete in lighter conditions. Light can also interact with nutrients to affect species-specific algal productivity response to increasing light. These different responses at the organismal level propagate up to influence metabolism at the ecosystem level. Even in low-nutrient lakes where nutrients would be expected to be the limiting resource for primary productivity, light can still be the limiting resource, which has cascading negative effects on higher trophic levels such as fish productivity. Variability in light in different lake zones and within a lake through time creates patchiness in productivity both spatially and temporally.
In addition to controlling primary productivity, sunlight can also influence rates of respiration by partially oxidizing organic matter which can make it easier for bacteria to break down and convert into carbon dioxide. This partial photooxidation essentially increases the amount of organic matter that is available for mineralization. In some lakes, complete photooxidation or partial photooxidation can account for a majority of the conversion from organic to inorganic matter, however, the proportion to bacterial respiration varies greatly among lakes.