Editing Freshwater ecosystem (section)

== Threats  ==
{{Further|Lake ecosystem#Human impacts|River ecosystem#Human impacts|Ecosystem#Human interactions with ecosystems}}

=== Biodiversity ===
Five broad threats to freshwater [[biodiversity]] include [[overexploitation]], [[water pollution]], flow modification, [[Habitat destruction|destruction or degradation of habitat]], and invasion by [[Introduced species|exotic species]].<ref name="Dudgeon">{{Cite journal|last1=Dudgeon|first1=David|last2=Arthington|first2=Angela H.|last3=Gessner|first3=Mark O.|last4=Kawabata|first4=Zen-Ichiro|last5=Knowler|first5=Duncan J.|last6=Lévêque|first6=Christian|last7=Naiman|first7=Robert J.|last8=Prieur-Richard|first8=Anne-Hélène|last9=Soto|first9=Doris|date=2005-12-12|title=Freshwater biodiversity: importance, threats, status and conservation challenges|journal=Biological Reviews|volume=81|issue=2|pages=163–82|doi=10.1017/s1464793105006950|pmid=16336747|issn=1464-7931|citeseerx=10.1.1.568.4047|s2cid=15921269}}</ref> Recent extinction trends can be attributed largely to sedimentation, stream fragmentation, chemical and organic pollutants, dams, and invasive species.<ref name="Ricciardi" /> Common chemical stresses on freshwater [[ecosystem health]] include acidification, [[eutrophication]] and copper and pesticide contamination.<ref>{{Cite journal|last=Xu|first=F|date=September 2001|title=Lake Ecosystem Health Assessment: Indicators and Methods|journal=Water Research|volume=35|issue=13|pages=3157–3167|doi=10.1016/s0043-1354(01)00040-9|pmid=11487113|bibcode=2001WatRe..35.3157X|issn=0043-1354}}</ref>

Freshwater biodiversity faces many threats.<ref name="Reid">{{Cite journal|last=Reid|first=AJ|display-authors=etal|date=2019|title=Emerging threats and persistent conservation challenges for freshwater biodiversity|journal=Biological Reviews|volume=94|issue=3|pages=849–873|doi=10.1111/brv.12480|pmid=30467930|doi-access=free}}</ref> The World Wide Fund for Nature's [[Living Planet Index]] noted an 83% decline in the populations of freshwater vertebrates between 1970 and 2014.<ref>{{Cite web|title=Living Planet Report 2018 {{!}} WWF|url=https://wwf.panda.org/knowledge_hub/all_publications/living_planet_report_2018/|access-date=2019-04-09|website=wwf.panda.org}}</ref> These declines continue to outpace contemporaneous declines in marine or terrestrial systems. The causes of these declines are related to:<ref>{{Cite web|last1=Reid|first1=Andrea Jane|last2=Cooke|first2=Steven J.|title=Freshwater wildlife face an uncertain future|url=http://theconversation.com/freshwater-wildlife-face-an-uncertain-future-108863|access-date=2019-04-09|website=The Conversation|date=22 January 2019 |language=en}}</ref><ref name="Reid" /> 

# A rapidly [[Climate change|changing climate]]
# Online wildlife trade and [[invasive species]]
# Infectious disease
# [[Harmful algal bloom|Toxic algae blooms]]
# [[Hydropower]] damming and fragmenting of half the world's rivers
# Emerging contaminants, such as hormones
# Engineered [[nanomaterials]]
# [[Microplastics|Microplastic]] pollution
# Light and noise interference
# Saltier coastal freshwaters due to [[sea level rise]]
# Calcium concentrations falling below the needs of some freshwater organisms
# The additive—and possibly synergistic—effects of these threats

=== Invasive species ===
[[Invasive species|Invasive]] plants and animals are a major issue to freshwater ecosystems,<ref>{{Cite journal |last1=Capps |first1=Krista |last2=Flecker |first2=Alexander |date=22 October 2013 |title=Invasive aquarium fish transform ecosystem nutrient dynamics |journal=Proceedings of the Royal Society B: Biological Sciences |volume=280 |issue=1769 |doi=10.1098/rspb.2013.1520 |pmid=23966642 |pmc=3768308 }}</ref> in many cases outcompeting native species and altering water conditions. Introduced species are especially devastating to ecosystems that are home to [[Endangered species|endangered]] species. An example of this being the [[Asian carp in North America|Asian carp]] competing with the [[paddlefish]] in the [[Mississippi River|Mississippi river]].<ref>{{Cite web |last=Station |first=Julia Hampton, Great Rivers Field |title=Asian carp species |url=https://blogs.illinois.edu/view/7362/966400817 |access-date=2024-04-15 |website=blogs.illinois.edu |language=en-US}}</ref> Common causes of invasive species in freshwater ecosystems include [[Aquaculture|aquarium]] releases, introduction for [[Recreational fishing|sport fishing]], and introduction for use as a food fish.<ref>{{Cite web |title=Northern snakehead fish |url=https://nationalzoo.si.edu/animals/northern-snakehead-fish |access-date=2024-04-15 |website=Smithsonian's National Zoo and Conservation Biology Institute |language=en}}</ref>

=== Extinction of freshwater fauna ===
Over 123 freshwater fauna species have gone extinct in North America since 1900. Of North American freshwater species, an estimated 48.5% of mussels, 22.8% of [[gastropod]]s, 32.7% of crayfishes, 25.9% of amphibians, and 21.2% of fish are either endangered or threatened.<ref name="Ricciardi">{{Cite journal|last1=Ricciardi|first1=Anthony|last2=Rasmussen|first2=Joseph B.|date=1999-10-23|title=Extinction Rates of North American Freshwater Fauna|journal=Conservation Biology|volume=13|issue=5|pages=1220–1222|doi=10.1046/j.1523-1739.1999.98380.x|bibcode=1999ConBi..13.1220R |s2cid=85338348 |issn=0888-8892}}</ref> Extinction rates of many species may increase severely into the next century because of invasive species, loss of keystone species, and species which are already functionally extinct (e.g., species which are not reproducing).<ref name="Ricciardi" /> Even using conservative estimates, freshwater fish extinction rates in North America are 877 times higher than background extinction rates (1 in 3,000,000 years).<ref>{{Cite journal|last=Burkhead|first=Noel M.|date=September 2012|title=Extinction Rates in North American Freshwater Fishes, 1900–2010|journal=BioScience|volume=62|issue=9|pages=798–808|doi=10.1525/bio.2012.62.9.5|issn=1525-3244|doi-access=free|bibcode=2012BiSci..62..798B }}</ref> Projected extinction rates for freshwater animals are around five times greater than for land animals, and are comparable to the rates for rainforest communities.<ref name="Ricciardi" />  Given the dire state of freshwater biodiversity, a team of scientists and practitioners from around the globe recently drafted an Emergency Action plan to try and restore freshwater biodiversity.<ref> {{Cite journal|last1=Tickner|first1=David|last2=Opperman|first2=Jeffrey J|last3=Abell|first3=Robin|last4=Acreman|first4=Mike|last5=Arthington|first5=Angela H|last6=Bunn|first6=Stuart E|last7=Cooke|first7=Steven J|last8=Dalton|first8=James|last9=Darwall|first9=Will|last10=Edwards|first10=Gavin|last11=Harrison|first11=Ian|date=2020-04-01|title=Bending the Curve of Global Freshwater Biodiversity Loss: An Emergency Recovery Plan|url=https://doi.org/10.1093/biosci/biaa002|journal=BioScience|volume=70|issue=4|pages=330–342|doi=10.1093/biosci/biaa002|pmid=32284631 |issn=0006-3568|pmc=7138689}} </ref>

Current freshwater biomonitoring techniques focus primarily on community structure, but some programs measure functional indicators like biochemical (or biological) oxygen demand, sediment oxygen demand, and dissolved oxygen.<ref name="Friberg" /> Macroinvertebrate community structure is commonly monitored because of the diverse taxonomy, ease of collection, sensitivity to a range of stressors, and overall value to the ecosystem.<ref>{{Cite book|title=Freshwater biomonitoring and benthic macroinvertebrates, 40-158.|last1=Johnson|first1=R. K.|last2=Wiederholm|first2=T.|last3=Rosenberg|first3=D. M.|year=1993|pages=40–158}}</ref> Additionally, algal community structure (often using diatoms) is measured in biomonitoring programs. Algae are also taxonomically diverse, easily collected, sensitive to a range of stressors, and overall valuable to the ecosystem.<ref name="Stevenson">{{Citation|last1=Stevenson|first1=R. Jan|date=2003|pages=775–804|publisher=Elsevier|doi=10.1016/b978-012741550-5/50024-6|isbn=9780127415505|last2=Smol|first2=John P.|chapter=Use of Algae in Environmental Assessments|title=Freshwater Algae of North America}}</ref> Algae grow very quickly and communities may represent fast changes in environmental conditions.<ref name="Stevenson" />

In addition to community structure, responses to freshwater stressors are investigated by experimental studies that measure organism behavioural changes, altered rates of growth, reproduction or mortality.<ref name="Friberg" /> Experimental results on single species under controlled conditions may not always reflect natural conditions and multi-species communities.<ref name="Friberg" />

The use of reference sites is common when defining the idealized '''"'''health" of a freshwater ecosystem. Reference sites can be selected spatially by choosing sites with minimal impacts from human disturbance and influence.<ref name="Friberg" /> However, reference conditions may also be established temporally by using preserved indicators such as [[diatom]] valves, macrophyte pollen, insect chitin and fish scales can be used to determine conditions prior to large scale human disturbance.<ref name="Friberg" /> These temporal reference conditions are often easier to reconstruct in standing water than moving water because stable sediments can better preserve biological indicator materials.

=== Climate change ===
{{See also|Effects of climate change on the water cycle#Impacts on freshwater ecosystems}}
The [[effects of climate change]] greatly complicate and frequently exacerbate the impacts of other stressors that threaten many fish,<ref>{{Cite journal |last1=Arthington |first1=Angela H. |last2=Dulvy |first2=Nicholas K. |last3=Gladstone |first3=William |last4=Winfield |first4=Ian J. |date=2016 |title=Fish conservation in freshwater and marine realms: status, threats and management |journal=Aquatic Conservation: Marine and Freshwater Ecosystems |language=en |volume=26 |issue=5 |pages=838–857 |doi=10.1002/aqc.2712 |issn=1099-0755 |doi-access=free|bibcode=2016ACMFE..26..838A |hdl=10072/143075 |hdl-access=free }}</ref> invertebrates,<ref>{{Cite journal |last1=Prather |first1=Chelse M. |last2=Pelini |first2=Shannon L. |last3=Laws |first3=Angela |last4=Rivest |first4=Emily |last5=Woltz |first5=Megan |last6=Bloch |first6=Christopher P. |last7=Del Toro |first7=Israel |last8=Ho |first8=Chuan-Kai |last9=Kominoski |first9=John |last10=Newbold |first10=T. A. Scott |last11=Parsons |first11=Sheena |last12=Joern |first12=A. |date=2012 |title=Invertebrates, ecosystem services and climate change: Invertebrates, ecosystems and climate change |url=https://onlinelibrary.wiley.com/doi/10.1111/brv.12002 |journal=Biological Reviews |language=en |volume=88 |issue=2 |pages=327–348 |doi=10.1111/brv.12002|pmid=23217156 |s2cid=23578609 |url-access=subscription }}</ref> phytoplankton,<ref>{{Cite journal |last1=Winder |first1=Monika |last2=Sommer |first2=Ulrich |date=2012 |title=Phytoplankton response to a changing climate |url=http://link.springer.com/10.1007/s10750-012-1149-2 |journal=Hydrobiologia |language=en |volume=698 |issue=1 |pages=5–16 |doi=10.1007/s10750-012-1149-2 |bibcode=2012HyBio.698....5W |s2cid=16907349 |issn=0018-8158|url-access=subscription }}</ref> and other organisms. Climate change is increasing the average temperature of water bodies, and worsening other issues such as changes in [[Substrate (biology)|substrate]] composition, oxygen concentration, and other system changes that have ripple effects on the biology of the system.<ref name="Parmesan" /> Water temperatures have already increased by around 1&nbsp;°C, and significant declines in ice coverage have caused subsequent ecosystem stresses.<ref name="Parmesan" />