Seagrass extent is reported at several scales, and the numbers are not interchangeable. A 2026 global map identified 148,506 km² in clear, shallow coastal waters, while broader assessments report much larger ranges that include different methods and areas. Regional records show both substantial losses and measurable recovery.
Contents
- Global mapped extent and distribution
- Why global estimates vary
- Australian intertidal extent
- Florida and Gulf of Mexico extent
- Density and mapped change
- Recent losses, gains and degradation
Global mapped extent and distribution
The study titled Global high-resolution mapping of seagrass to support conservation identified 148,506 km² of seagrass in clear, shallow coastal waters. Its mapped periods were 2019–2020 and 2023–2024. The total included 5,961 km² of intertidal seagrass and 142,545 km² of subtidal seagrass.
| Measure | Extent or share | Measurement period |
|---|---|---|
| Total mapped seagrass | 148,506 km² | 2019–2020 and 2023–2024 |
| Intertidal seagrass | 5,961 km² | 2019–2020 and 2023–2024 |
| Subtidal seagrass | 142,545 km² | 2019–2020 and 2023–2024 |
| Five-country share | 69% | 2019–2020 and 2023–2024 |
| Area inside marine protected areas | 21% | 2019–2020 and 2023–2024 |
The Bahamas, Cuba, the United States, Australia and Indonesia together contained 69% of the mapped global extent. Only 21% of mapped global seagrass areas were inside marine protected areas during the mapped periods. These figures describe mapped habitat within the study’s clear, shallow-water scope; they are not a complete estimate of every seagrass meadow worldwide.
The UNEP Seagrass meadows page gives a broader global perspective. Its 2023 page reports more than 300,000 km² of seagrass meadows worldwide, distributed across 159 countries and 6 continents. The same source’s 2020 report summary states that almost 30% of global seagrass area had been lost since the late nineteenth century.
Why global estimates vary
Global extent depends on whether a source counts only mapped habitat, extrapolates unmapped areas, models potential habitat, or includes deeper areas omitted from a particular account. The IOC-UNESCO World Seagrass Day summary gives a 2020 global extent range of 177,000–600,000 km². The Second World Ocean Assessment, Volume II cites a 2021 global seagrass-area range of 160,387–266,562 km².
An earlier assessment, Accelerating loss of seagrasses across the globe threatens coastal ecosystems, estimated the historical measured global mapped area at 124,000 km² in its 2009 assessment. When unmapped seagrass was included, that assessment extrapolated at least 177,000 km². It also produced a model-based estimate of 4.3 million km² of potential global seagrass area using light, bathymetry and seagrass requirements. Potential area is a modeled suitability figure, not the same measure as observed meadow extent.
The same 2009 study estimated disappearance at 110 km² per year from 1980 onward, covering 1980–2006. Over the 127 years after the earliest records, it extrapolated more than 51,000 km² of global meadow loss for 1879–2006. Those historical estimates should be read as reported legacy findings, with their original measurement and extrapolation methods, rather than as independently verified current totals.
Australian intertidal extent
Australia’s National Ocean Account, Experimental Estimates reported 388.7 thousand hectares of intertidal seagrass in 2020. The account’s national estimate is specifically intertidal, and its scope does not make it directly interchangeable with global totals that include subtidal habitat.
| Australian area | Intertidal seagrass in 2020 |
|---|---|
| New South Wales | 10.8 thousand ha |
| Victoria | 16.6 thousand ha |
| Queensland | 121.5 thousand ha |
| South Australia | 40.5 thousand ha |
| Western Australia | 89.5 thousand ha |
| Tasmania | 32.3 thousand ha |
| Northern Territory | 72.3 thousand ha |
| Other Commonwealth waters | 5.2 thousand ha |
Queensland held 31% of Australia’s estimated intertidal seagrass extent in 2020. Western Australia held 23%, and the Northern Territory held 19%. The largest Australian sediment compartment identified in the account was Borroloola in the Gulf of Carpentaria, with more than 20 thousand hectares.
The state figures show why national extent needs geographic detail. Queensland’s 121.5 thousand hectares exceeded Western Australia’s 89.5 thousand hectares, while the Northern Territory’s 72.3 thousand hectares was greater than the combined reported amounts for New South Wales and Victoria. These are reported 2020 estimates, not a time series of annual change.
Florida and Gulf of Mexico extent
Florida’s five-region assessment, SIMM Report No. 2.0 Executive Summary, mapped 2,480,462 acres of seagrass in 2010. South Florida accounted for 1,620,441 acres. The Big Bend-to-Springs Coast region contained 617,921 acres, Southwest Florida contained 143,348 acres, Florida’s east coast region contained 58,270 acres, and the Florida Panhandle contained 40,482 acres.
These regional measurements are not evenly sized categories of one current survey: they are the five regions reported for the 2010 Florida assessment. Their values provide a geographic picture of mapped extent, with South Florida the largest reported region and the Panhandle the smallest.
For the wider Gulf, Status and Trends of Emergent and Submerged Vegetated Habitats, Gulf of Mexico, U.S.A. estimated Gulf of Mexico seagrass meadows at approximately 7,440 square miles, or 19,275 km², in a 1992 report. The Gulf states were estimated to contain 2.47 million acres, or about 1 million hectares, of seagrass. State-level figures in that report included 30,381 acres for Alabama, 4,940 acres for Mississippi and 10,127 acres for Louisiana.
The 1992 Gulf estimate and the 2010 Florida assessment use different dates, scopes and reporting units. They should therefore be used as historical regional extent measures, not combined into a single updated Gulf total.
Density and mapped change
Extent alone does not show whether mapped habitat is dense or sparse. The report Ecological monitoring in the Shark Bay marine reserves identified approximately 2,700 km² of seagrass habitat in 2002 and approximately 2,110 km² in 2014 within the marine reserves’ mapped program area. Dense seagrass covered about 1,925 km², or 72% of mapped seagrass habitat, in 2002. Dense seagrass represented 58% of the total mapped seagrass area in 2014 and 55% in 2016.
| Shark Bay measure | Reported value |
|---|---|
| Identifiable habitat, 2002 | approximately 2,700 km² |
| Identifiable habitat, 2014 | approximately 2,110 km² |
| Dense habitat, 2002 | about 1,925 km²; 72% |
| Dense share, 2014 | 58% |
| Dense share, 2016 | 55% |
West-central Florida provides a different pattern. The NOAA report on seagrass distribution, areal cover and changes from 1990–2021 found that Tampa Bay’s satellite-derived seagrass areal cover increased by 24.3 km², or 34.4%, from 1990 to 2021. St. Joseph Sound increased by 18.0 km², or 74.1%, and Clearwater increased by 6.3 km², or 182.6%, over the same period.
The Clearwater percentage is large alongside a smaller absolute area change, illustrating why both measures matter. The three results are satellite-derived areal-cover changes over 1990–2021, not direct comparisons with the 2010 Florida region totals.
Recent losses, gains and degradation
The 2026 global map recorded 5,969 km² of seagrass loss during its four-year comparison from 2019–2020 to 2023–2024. It also recorded 6,221 km² of tropical seagrass degradation from dense to sparse cover. Loss of mapped area and degradation of remaining cover are separate measurements: a meadow can become sparser without disappearing from the map.
The Australia State of the Environment 2021 coastal ecosystems and habitats report reported that more than 1,300 km² of seagrass was lost in Shark Bay after the 2011 heatwave and sediment-loading event. Recent Shark Bay mapping showed recovery of approximately 125 km² of meadow in the context of the 2021 state-of-environment report.
New South Wales records show that change can differ sharply among estuaries. Coastal and marine 2024 reported that transient seagrass extent across remapped NSW estuaries fell from 2,604 hectares in the early 2000s to 1,968 hectares in the latest four-year remapping period, identified as 2021–2024.
| NSW estuary or group | Reported change | Period |
|---|---|---|
| Remapped NSW estuaries | 2,604 ha to 1,968 ha | early 2000s to 2021–2024 |
| Smiths Lake | loss of 278 ha; 94% | early 2000s to recent remapping |
| Lake Macquarie | loss of 220 ha; 16% | early 2000s to recent remapping |
| Hastings River | loss of 113 ha; 78% | early 2000s to recent remapping |
| Hawkesbury River | gain of 30 ha; 77% | recent remapping |
| St Georges Basin | gain of 28 ha; 16% | recent remapping |
Smiths Lake lost 278 hectares, or 94%, relative to its early-2000s baseline. Lake Macquarie lost 220 hectares, or 16%, and the Hastings River lost 113 hectares, or 78%. In contrast, Hawkesbury River gained 30 hectares, or 77%, while St Georges Basin gained 28 hectares, or 16%, in recent remapping. The NSW figures describe transient seagrass and retain the periods reported by the state assessment.
Taken together, these records show why seagrass ecosystem extent statistics need three labels: the geography being measured, the observation period and the mapping definition. Global mapped totals, intertidal national accounts, historical regional surveys, density measures and transient-estuary remapping answer related but different conservation questions.