Editing IPCC:AR6/WGII/Chapter-2 (section)

===== 2.4.4.5.2 Observed changes in freshwater ecosystem productivity =====

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Temperature affects primary productivity by moderating phytoplankton growth rates, ice cover, thermal stratification and the length of growing seasons ( [[#Rühland--2015|Rühland et al., 2015]] ; [[#Richardson--2018|Richardson et al., 2018]] ). Global warming has reinforced eutrophication, especially cyanobacteria blooms ( [[#Wagner--2009|Wagner and Adrian, 2009]] ; [[#Kosten--2012|Kosten et al., 2012]] ; [[#O’Neil--2012|O’Neil et al., 2012]] ; [[#De%20Senerpont%20Domis--2013|De Senerpont Domis et al., 2013]] ; [[#Adrian--2016|Adrian et al., 2016]] ; [[#Visser--2016|Visser et al., 2016]] ; [[#Huisman--2018|Huisman et al., 2018]] ) ( ''very high confidence'' ) ''.'' Conversely '','' warming can reduce cyanobacteria in hypertrophic lakes ( [[#Richardson--2019|Richardson et al., 2019]] ). Freshwater cyanobacteria may benefit directly from elevated CO 2 concentrations ( [[#Visser--2016|Visser et al., 2016]] ; [[#Ji--2017|Ji et al., 2017]] ; [[#Huisman--2018|Huisman et al., 2018]] ; [[#Richardson--2019|Richardson et al., 2019]] ).

Macrophyte growth in freshwaters is likely to increase with rising water temperatures, atmospheric CO 2 and precipitation ( ''robust evidence'' , ''high agreement'' ) ( [[#Dhir--2015|Dhir, 2015]] ; [[#Hossain--2016|Hossain et al., 2016]] ; [[#Short--2016|Short et al., 2016]] ; [[#Reitsema--2018|Reitsema et al., 2018]] ). Nonetheless, primary productivity in rivers is variable and unpredictable ( [[#Bernhardt--2018|Bernhardt et al., 2018]] ) because seasonal variations in temperature and light are uncorrelated, frequent high-flow events reduce biomass of autotrophs and droughts can strand and desiccate autotrophs.

In large, nutrient-poor lakes, warming-induced prolonged thermal stratification can reduce primary production ( ''medium confidence'' ) ( [[#Kraemer--2017|Kraemer et al., 2017]] ). Warming may reduce phytoplankton concentrations when temperature-induced increases in consumption of phytoplankton outpace increases in phytoplankton production ( [[#De%20Senerpont%20Domis--2013|De Senerpont Domis et al., 2013]] ). These decreases in productivity may be under-recognised responses to climate change.

Summary: There is ''robust'' evidence of an increase in primary production along with warming trends. However, increases or declines of algae cannot entirely be attributed to climate change; they are lake-specific and modulated through weather conditions, lake morphology, salinity, land use and restoration and biotic interactions ( ''medium confidence'' ) ( [[#O’Beirne--2017|O’Beirne et al., 2017]] ; [[#Velthuis--2017|Velthuis et al., 2017]] ; [[#Rusak--2018|Rusak et al., 2018]] ; [[#Ho--2019|Ho et al., 2019]] ).

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