<P> Algae, including both phytoplankton and periphyton are the principle photosynthesizers in ponds and lakes . Phytoplankton are found drifting in the water column of the pelagic zone . Many species have a higher density than water which should make them sink and end up in the benthos . To combat this, phytoplankton have developed density changing mechanisms, by forming vacuoles and gas vesicles or by changing their shapes to induce drag, slowing their descent . A very sophisticated adaptation utilized by a small number of species is a tail - like flagellum that can adjust vertical position and allow movement in any direction . Phytoplankton can also maintain their presence in the water column by being circulated in Langmuir rotations . Periphytic algae, on the other hand, are attached to a substrate . In lakes and ponds, they can cover all benthic surfaces . Both types of plankton are important as food sources and as oxygen providers . </P> <P> Aquatic plants live in both the benthic and pelagic zones and can be grouped according to their manner of growth: 1) emergent = rooted in the substrate but with leaves and flowers extending into the air, 2) floating - leaved = rooted in the substrate but with floating leaves, 3) submersed = growing beneath the surface and 4) free - floating macrophytes = not rooted in the substrate and floating on the surface . These various forms of macrophytes generally occur in different areas of the benthic zone, with emergent vegetation nearest the shoreline, then floating - leaved macrophytes, followed by submersed vegetation . Free - floating macrophytes can occur anywhere on the system's surface . </P> <P> Aquatic plants are more buoyant than their terrestrial counterparts because freshwater has a higher density than air . This makes structural rigidity unimportant in lakes and ponds (except in the aerial stems and leaves). Thus, the leaves and stems of most aquatic plants use less energy to construct and maintain woody tissue, investing that energy into fast growth instead . In order to contend with stresses induced by wind and waves, plants must be both flexible and tough . Light, water depth and substrate types are the most important factors controlling the distribution of submerged aquatic plants . Macrophytes are sources of food, oxygen, and habitat structure in the benthic zone, but cannot penetrate the depths of the euphotic zone and hence are not found there . </P> <P> Zooplankton are tiny animals suspended in the water column . Like phytoplankton, these species have developed mechanisms that keep them from sinking to deeper waters, including drag - inducing body forms and the active flicking of appendages such as antennae or spines . Remaining in the water column may have its advantages in terms of feeding, but this zone's lack of refugia leaves zooplankton vulnerable to predation . In response, some species, especially Daphnia sp., make daily vertical migrations in the water column by passively sinking to the darker lower depths during the day and actively moving towards the surface during the night . Also, because conditions in a lentic system can be quite variable across seasons, zooplankton have the ability to switch from laying regular eggs to resting eggs when there is a lack of food, temperatures fall below 2 ° C, or if predator abundance is high . These resting eggs have a diapause, or dormancy period that should allow the zooplankton to encounter conditions that are more favorable to survival when they finally hatch . The invertebrates that inhabit the benthic zone are numerically dominated by small species and are species rich compared to the zooplankton of the open water . They include Crustaceans (e.g. crabs, crayfish, and shrimp), molluscs (e.g. clams and snails), and numerous types of insects . These organisms are mostly found in the areas of macrophyte growth, where the richest resources, highly oxygenated water, and warmest portion of the ecosystem are found . The structurally diverse macrophyte beds are important sites for the accumulation of organic matter, and provide an ideal area for colonization . The sediments and plants also offer a great deal of protection from predatory fishes . </P>

What kind of water is found in the littoral zone of a lake apex