Definitions:

Potential kelp forest habitat: The amount of seafloor that could be suitable for kelp to grow.

Observed kelp forest habitat: The amount of seafloor where kelp has been observed and mapped.

7.2 Measuring the Kelp Biome

The area of kelp forest area around the world has been estimated several times. These estimates were obtained either by modelling kelp forest area based on environmental conditions where kelp is known to live (e.g., appropriate temperature, rocky reefs, < 60m depth) or by compiling recorded observations (observations, videos, sonar, photos) of kelp forest habitat from around the globe.

Models with large grid cells (coarse resolution) may overestimate habitat areas because they predict presence within large areas (e.g., 10 km x 10 km) and do not capture variability of coverage of kelp within a cell¹. Specifically, these models indicate that conditions are suitable for kelp somewhere within each cell, but do not specify that the entire cell is covered by kelp. Consequently, the reported area represents the number of suitable cells, not the actual continuous coverage of the habitat.

Conversely, only using observed data generates an under-estimate, as underwater data collection is technically challenging and prohibitively expensive. As a result, we only have incomplete observations of kelp forest ecosystems (50,000 - 100,000 km2)²,³.

This report is focused on which countries have the most kelp and the percentage of potential kelp habitat that is protected (See section 4). Because both these numbers are relative, we use the modelled (maximum available habitat) approach to estimate kelp area. We do not recommend using these numbers for setting area-based restoration targets because those numbers will not be feasible to achieve.

7.3 Kelp Breakdown

By area, five countries—Canada, the United States of America, Russia, and the United Kingdom—contain over 50% of globally estimated kelp habitat (946,000 km2). Much of this habitat lies in the high polar regions. In the Southern hemisphere, Australia, New Zealand, Chile, and Argentina have the most kelp habitat (278,000 km2 collectively). While other countries may have smaller kelp forests (South Africa, Mediterranean countries), many of those forests are unique and host biodiversity endemic to that region. Together, 25 countries are home to kelp forests (Figure 7.1).

Figure 7.1 Kelp Biome by Country

7.4 Regional Accuracy

The data used to generate the model influence the results, and data-poor areas may be under-represented. For instance, the modelled kelp biome for Australia missed regions along the East and West coasts⁴ where kelp is observed to exist.

7.5 Data Notes

These numbers represent the area in which kelp forest could grow and not necessarily the real distribution of kelp in any given year. They do not reflect local stressors such as pollution, habitat destruction, and other disturbances.

Kelp forests are dynamic ecosystems, and their boundaries may shift naturally from year to year. As a result, any estimate of kelp forest area will, at best, be an approximation. Further, intertidal kelps are not represented in these values because we cropped to the coastline, but they would only slightly increase the total area, as the vast majority of kelp is subtidal.

7.6 Methods

Kelp forests are found in 25 countries across all continents, but the actual area occupied by kelp forest ecosystems is currently unresolved, with observed estimates ranging from 5 M ha to modelled estimates of 150 M ha. Which number is used will necessarily influence any conservation targets that are produced (e.g., 30% of 5 M vs 30% of 150 M ha). Using an underestimate would mean that kelp habitat is left unprotected, and using an overestimate would create an unreachable goal. Additionally, detailed maps of observed or modelled kelp forests will also influence any assessment of the habitat overlap with MPAs. Ideally, this assessment would be based on the observed maps of kelp forest habitat, however, global maps of kelp, or even national-level maps, only exist for those species that form large surface canopies that can be detected with remote sensing (e.g., giant kelp - Macrocystis pyrifera). Even if such maps did exist, kelp forests are dynamic habitats, as mentioned above, and may shift in place from year to year. While the area-based target of the Kelp Forest Challenge was created using an observed estimate of the area of kelp forest (~10 M ha), corresponding maps do not currently exist. Therefore, this report uses the modelled kelp biome to provide its assessment⁵. Given this decision, this report provides an assessment of the amount of potential kelp forest habitat protected as opposed to the realised amount of kelp forest habitat protected. Future assessments may update this approach as more information becomes available.

The modelled biome comprises 147 species and 59 genera of kelp, the majority of which do not form a floating canopy at the ocean’s surface, like the more widely known giant kelp (M. pyrifera). We instituted a 60 m depth cutoff, using a global sub-ice bathymetry grid⁶ to restrict areas which would be likely too deep for significant kelp forests. We then vectorised the layer and snapped the landward edge to a detailed coastline shapefile⁷ to ensure congruency between boundaries. Next, we overlaid the EEZs (exclusive economic zones) of the world and calculated the area of kelp biome that lay within each country’s EEZ. Finally, we clipped the kelp forest data layer with the MPA layer to obtain the amount of kelp forest biome that was protected and under different levels of protection. Since the global MPA dataset is continuously updated to include newly designated MPAs, MPAs occasionally overlap with each other. Overlapping MPAs were hierarchically selected so only one MPA would be accounted for, and we prioritised the MPAs that had the highest level of protection.

7.7 Data Gaps

In a global analysis, there are inherent limitations to the country-level assessment. Notably, kelp distribution may be underestimated at the edge its distribution (e.g., the warmest areas of a population⁸) or in some remote areas where little information exists. For example, we observed that giant kelp and palm kelp in Baja California, Mexico and golden kelp in Northern New South Wales, Australia were modelled as absent despite known presence. In addition, kelp forest species from the Indian Ocean islands such as Kerguelen of France and the Prince Edward Islands of South Africa were also absent. This underestimate of kelp forest area may consequently overestimate how much kelp forest is protected in these countries. Future updates of this and other work will therefore need to refine the kelp distribution data. As a result, global and country-level kelp forest estimates may vary if the biome is refined to incorporate these regions, and results should be taken with care and validated with other datasets and regional experts.