Data from: Comparing passive and active kelp restoration techniques along an urbanised coast

Kelp forests have received increasing attention due to widespread declines
in many regions, alongside stability or expansions in others, and the
potential to reverse loss where it has occurred. Such reversal requires
understanding which techniques of intervention can restore their coverage
at large scales in cost-effective ways. We investigated two techniques of
kelp forest restoration along a metropolitan coast in southern Australia:
‘passive’ restoration, involving seascape-scale improvements to water
quality for multi-species restoration of kelp forests and seagrass
meadows, and ‘active’ restoration, which involves local-scale kelp
transplants to restore kelp forests. Passive restoration through
water quality improvements (i.e., wastewater treatment plant upgrades)
over 10 years yielded kelp recovery of 1.15 hectares. Active restoration
through transplanting local 1 m2 patches of kelp over 3 years yielded
0.0008 hectares. While each technique produced next-generation recruits to
sustain canopies, the annual rate of kelp expansion was ~386 times greater
via passive restoration than active restoration. The passive
approach required considerable initial investments and political will;
however, the cost per hectare (~USD$ 42,250 ha -1) is substantially lower
compared to the active approach of transplanting kelp (~USD$ 292,915 ha
-1), which appears to be a riskier process with lower returns on financial
investment. Passive restoration through water quality improvements
facilitates substantial kelp gains, offers increased scalability, and
provides broader ecosystem benefits across seascape scales, though the
initial investment required is substantial. Active restoration may provide
immediate canopies of kelp through transplantation, but provides a
negligible increase in kelp cover over time. Funding for upscaling remains
a significant limitation. The effectiveness of each restoration technique
depends on the local coastal ecology, with passive methods offering
benefits at broader spatial scales and active methods providing more
immediate results. However, sustained and large-scale recovery of kelp
ecosystems appears to require thoughtful approaches that directly address
the underlying drivers of coastal degradation.