Editing River Lark (section)

==Ecology==
The Environment Agency assesses the water quality within the river systems in England. Each is given an overall ecological status, which may be one of five levels: high, good, moderate, poor and bad. There are several components that are used to determine this, including biological status, which looks at the quantity and varieties of [[invertebrate]]s, [[angiosperm]]s and fish. Chemical status, which compares the concentrations of various chemicals against known safe concentrations, is rated good or fail.<ref>{{cite web |url=http://environment.data.gov.uk/catchment-planning/help/glossary |title=Glossary (see Biological quality element; Chemical status; and Ecological status) |website=Catchment Data Explorer |publisher=Environment Agency |access-date=15 May 2017}}</ref>

The water quality of the River Lark system was as follows in 2019.
{| class="wikitable"
|-
! Section !! Ecological status !! Chemical status !! Length !! Catchment !! Channel
|-
| {{waterqual_title |desc=Lark (US Hawstead) |asset=GB105033042920 |accessdate=6 October 2017}} 
| {{waterqual_mod}} 
| {{waterqual_fail}} 
| {{convert|2.8|mi|km}}
| {{convert|5.79|sqmi|km2}}
|
|-
| {{waterqual_title |desc=Lark (Hawstead to Abbey Gardens) |asset=GB105033042940 |accessdate=6 October 2017}}
| {{waterqual_bad}} 
| {{waterqual_fail}}  
| {{convert|4.1|mi|km}}
| {{convert|11.24|sqmi|km2}}
| heavily modified
|-
| {{waterqual_title |desc=Lark (Abbey Gardens to Mildenhall) |asset=GB105033043051 |accessdate=6 October 2017}}
| {{waterqual_mod}} 
| {{waterqual_fail}} 
| {{convert|14.1|mi|km}}
| {{convert|33.60|sqmi|km2}}
| heavily modified
|-
| {{waterqual_title |desc=Lark downstream of Mill Street Bridge |asset=GB105033043052 |accessdate=6 October 2017}}
| {{waterqual_mod}} 
| {{waterqual_fail}} 
| {{convert|3.4|mi|km}}
| {{convert|8.21|sqmi|km2}}
| heavily modified
|-
| {{waterqual_title |desc=Ely Ouse (South Level) |asset=GB205033000070 |accessdate=6 October 2017}}
| {{waterqual_mod}} 
| {{waterqual_fail}} 
| {{convert|54.5|mi|km}}
| {{convert|180.02|sqmi|km2}}
| artificial
|}

Separate figures for the Lark downstream of Isleham are not available, and the final row covers the lower section of the Lark, parts of the River Great Ouse and a number of its other tributaries. The Hawstead to Abbey Gardens section deteriorated from poor quality in 2015 to bad in 2016. The Environment Agency have set a target for ensuring it is moderate by 2027. Like most rivers in the UK, the chemical status changed from good to fail in 2019, due to the presence of polybrominated diphenyl ethers (PBDE) and perfluorooctane sulphonate (PFOS), neither of which had previously been included in the assessment.<ref>{{cite web |url=https://environment.data.gov.uk/catchment-planning/help/usage#chemical-status |archive-url=https://web.archive.org/web/20240314040920/https://environment.data.gov.uk/catchment-planning/help/usage#chemical-status |archive-date=14 March 2024 |url-status=live |title=Chemical Status |publisher=Environment Agency |year=2023}}</ref>

The river hosts a large population of the invasive species, the [[signal crayfish]]. A study published in 2011 identified the decline in eels, which predate the crayfish, as the major factor in their dramatic increase.{{sfn |Longshaw |Stebbing |2016 |p=346}}