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Algae
Most streams contain some form of algae. Algae are microorganisms that act like plants and reside at the bottom of the food chain. Algae produce oxygen when the sun is out and consume oxygen when there's no sunlight. If there are too few hours of sunlight during the day, there's often a net loss of oxygen within a stream; the algae will consume more oxygen than they release, and there will be a smaller supply of oxygen for every other living creature in the water. This kind of oxygen shortage can wreak havoc on stream organisms. For instance, fish will often grow weaker as their immune systems cannot function at full capacity without sufficient oxygen. Oxygen shortages in streams often lead to mass deaths of fish and other forms of life.
Photosynthesis
Most streams teem with autotrophs---microscopic plants (including algae), and various kinds of fungi and bacteria. Autotrophs are organisms that produce their own food through a process called photosynthesis. They take in light and convert it into the energy they need to sustain life. These autotrophs serve as food for heterotrophs---organisms whose bodies can't make their own food. Less sunlight means fewer and smaller autotrophs, and therefore less food for every other creature, which could induce some stream organisms to leave a stream to find shelter elsewhere.
The Hydrologic Cycle
A reduced number of daylight hours also affects the hydrologic cycle, which is crucial to stream organisms. "Hydrologic cycle" is the scientific term for the process of evaporation, condensation and precipitation that allows all bodies of water to maintain a consistent amount of water. This cycle also keeps the pH level and salinity of stream water at consistent levels as well. Fewer daylight hours, however, means slower evaporation, which can lead to salinity and pH levels that aren't suitable to the organisms living within a particular stream. Again, these conditions can lead many of those organisms to move away from that stream in order to find better water conditions elsewhere.
Not Every Organism is Greatly Affected
Some organisms living within a stream, however, are not drastically affected when the hours of daylight are shortened. Mainly, these are organisms that fall into the category of benthic macroinvertebrates. "Benthic" means that a creature lives on the very bottom of a river or stream; "macro-" means it can be seen without a microscope, and "invertebrate" means it does not have a spinal column or vertebrae. The category of benthic macroinvertebrates is a large one, and includes certain kinds of crustaceans (including some crayfish), as well as some snails, worms and insect larvae. Because these organisms spend almost their entire lives in isolation, shielded from the light of the sun---hiding in logs or under rocks---a lack of sunlight has little direct effect on their daily lives. Still, lingering indirect effects from a reduction in daylight hours, such as a pH level of water that's too acidic, can in inhibit their activities and, over time, even shorten their lifespans.