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===Primary producers===
[[File:Nelumbo nucifera LOTUS bud.jpg|thumb|upright|''[[Nelumbo nucifera]]'', an aquatic plant.]]
Algae, including both [[phytoplankton]] and [[periphyton]], are the principle photosynthesizers in ponds and lakes.<ref>{{Cite journal |lastlast1=Cael |firstfirst1=B. B. |last2=Seekell |first2=David A. |date=2023 |title=How does lake primary production scale with lake size? |url=https://www.frontiersin.org/articles/10.3389/fenvs.2023.1103068 |journal=Frontiers in Environmental Science |volume=11 |doi=10.3389/fenvs.2023.1103068/full |issn=2296-665X |doi-access=free }}</ref> Phytoplankton are found drifting in the water column of the pelagic zone. Many species have a higher density than water, which should cause them to sink inadvertently down into the [[benthos]]. To combat this, phytoplankton have developed density-changing mechanisms, by forming [[vacuole]]s and [[gas vesicle]]s, or by changing their shapes to induce drag, thus slowing their descent.<ref>{{Cite web |last=Smriti |first=Saifun Nahar |date=2023-10-05 |title=Adaptation of Phytoplankton to Float in Water |url=https://greenleen.com/adaptation-of-phytoplankton-to-float-in-water/ |access-date=2023-10-05 |website=GreenLeen.Com |language=en-US}}</ref> 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.<ref name="bronmark:2005"/> Phytoplankton can also maintain their presence in the water column by being circulated in [[Langmuir circulation|Langmuir rotations]].<ref name="kalff:2002"/> 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.<ref name="bronmark:2005"/>
[[Aquatic plants]] live in both the benthic and pelagic zones, and can be grouped according to their manner of growth: ⑴ '''emergent''' = rooted in the substrate, but with leaves and flowers extending into the air; ⑵ '''floating-leaved''' = rooted in the substrate, but with floating leaves; ⑶ '''submersed''' = growing beneath the surface; ⑷ '''free-floating macrophytes''' = not rooted in the substrate, and floating on the surface.<ref name="brown:1987"/> 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.<ref name="bronmark:2005"/>
===Eutrophication===
[[Eutrophication|Eutrophic]] systems contain a high concentration of phosphorus (~30 µg/L), nitrogen (~1500 µg/L), or both.<ref name="bronmark:2005"/> Phosphorus enters lentic waters from [[sewage treatment]] effluents, discharge from raw sewage, or from runoff of farmland. Nitrogen mostly comes from [[Fertilizer|agricultural fertilizers]] from runoff or leaching and subsequent groundwater flow. This increase in nutrients required for primary producers results in a massive increase of phytoplankton growth, termed a "[[Algal bloom|plankton bloom]]." This bloom decreases water transparency, leading to the loss of submerged plants.<ref>{{Cite journal |lastlast1=Birk |firstfirst1=Sapriya |last2=Miller |first2=J. David |last3=MacMullin |first3=Aidan |last4=Patterson |first4=R. Timothy |last5=Villeneuve |first5=Paul J. |date=February 2023 |title=Perceptions of Freshwater Algal Blooms, Causes and Health among New Brunswick Lakefront Property Owners |url=https://link.springer.com/10.1007/s00267-022-01736-2 |journal=Environmental Management |language=en |volume=71 |issue=2 |pages=249–259 |doi=10.1007/s00267-022-01736-2 |issn=0364-152X}}</ref> The resultant reduction in habitat structure has negative impacts on the species that utilize it for spawning, maturation, and general survival. Additionally, the large number of short-lived phytoplankton result in a massive amount of dead biomass settling into the sediment.<ref name="moss:1998"/> Bacteria need large amounts of oxygen to decompose this material, thus reducing the oxygen concentration of the water. This is especially pronounced in [[Lake stratification|stratified lakes]], when the [[thermocline]] prevents oxygen-rich water from the surface to mix with lower levels. Low or anoxic conditions preclude the existence of many taxa that are not physiologically tolerant of these conditions.<ref name="bronmark:2005"/>
===Invasive species===
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