Golden kelp Ecklonia radiata is growing on parts of Tasmania's Derwent Estuary rocky reefs, where scientists once thought recovery could be difficult.
Comparing surveys from 2010 and 2024, scientists from the University of Tasmania's Institute for Marine and Antarctic Studies (IMAS) found that dense mats of marine worms had largely disappeared, while kelp had increased in abundance, spread further upstream and extended into deeper water.
IMAS marine ecologist and co-author of the 2025 Derwent Estuary Report, Associate Professor Neville Barrett, had expected the degraded conditions recorded in the original surveys to persist.
"Our repeat surveys showed kelp growing on those reefs again. So I happily turned out to be completely wrong,” Associate Professor Neville Barrett said.
From worm mats to seaweeds
In 2010, extensive mats of tube-building marine worms, known as polychaetes, covered reefs through parts of the mid and upper survey area. At many surveyed sites between Bedlam Walls and Battery Point, the matting occupied almost all the reef surface at depths of around two to five metres.
These mats trapped fine sediment and appeared to prevent seaweeds from establishing. Golden kelp faced two barriers: a reef surface that offered little opportunity for young kelp to settle and grow, and limited light in the murky water.
"I suspected this could be an 'alternative stable state', with the worm mats continuing to dominate for long periods while the estuary remained enriched with organic matter," Associate Professor Barrett said.
"As long as the estuary remained enriched with organic matter, the worms could continue to dominate. But our repeat surveys showed a different outcome. The widespread matting had virtually disappeared, and seaweeds were occupying reefs that had previously supported little algal cover."
The changes were particularly clear around Rosny Point and Bellerive Bluff, where golden kelp increased markedly. At Rosny Point North, scientists recorded abundant kelp in the one to five metre depth band where they had recorded none in 2010.
Further downstream, at Tranmere Point and The Grange in Taroona, kelp also appeared in the five to nine metre depth band where the earlier surveys had not recorded it. These observations point to an expansion of suitable kelp habitat at several sites.
Red seaweeds changed too. Leaf-like forms, including Rhodymenia leptophylla, became prominent on reefs between Bedlam Walls and Rosny Point, previously covered by weed mats. Across these areas, the change involved the structure and cover of the seaweed community, rather than a uniform increase in the number of species.
More light and space to grow
IMAS scientists say the disappearance of the mats and the expansion of kelp are signs that silt and organic enrichment may have declined at these reefs. Reduced sediment loading could make conditions less favourable for the worms, while clearer water would allow sunlight to reach deeper reef surfaces.
"Golden kelp depends on light to grow. Its deeper distribution is consistent with improved light conditions, although the surveys do not directly establish what caused the change," Associate Professor Barrett said.
"The disappearance of the mats also appears to have freed reef surfaces for seaweeds to establish."
The surveys document these ecological changes, but do not establish a single cause. Estuaries respond to river flows, salinity, storms and local disturbance. The report suggests that the prolonged period without major flooding before the 2024 surveys may also have allowed some marine species to increase and spread upstream.
"The findings are encouraging. Conditions in parts of the Derwent have become more favourable for kelp, allowing recovery in places where it once seemed unlikely," Associate Professor Barret said.
The Derwent Estuary Program’s 2025 State of Timtumili Minanya/Derwent report also documents improvements in water clarity, particularly in the lower and middle estuary, based on its long-term water quality monitoring.
Recovery alongside continuing pressures
The kelp findings sit within a mixed picture for the estuary. Introduced seastars, including the Northern Pacific seastar Asterias amurensis, increased in abundance.
Introduced red seaweed Grateloupia turuturu, sometimes called Devil’s Tongue Weed, also became much more abundant and spread further upstream, with conspicuous growth just below the waterline at many sites.
"While we reported no evidence of this seaweed displacing native species at the surveyed sites, that doesn't rule out future impacts, and its continued spread remains a reason to monitor these reefs," Associate Professor Barrett said.
Some fish also declined.
"We recorded a marked reduction in Bastard trumpeter, a species previously found in resident schools on several reefs, and increased recreational spearfishing is likely to be contributing to this decline," said Associate Professor Barrett.
"It's important to note that variable recruitment, and survey visibility, may have influenced the results. But we suggest that restricting spearfishing in some areas could help protect Bastard trumpeter and improve opportunities for snorkelling and other recreation."
Tracking reef change
The IMAS study compared biodiversity across 27 sites using fixed transects and timed searches through different depths and habitats. Repeating the earlier methods allowed scientists to document changes that a single survey could not reveal.
Charlotte McAneney, Ashlee Bastiaansen and Neville Barrett prepared the report, with the Derwent Estuary Program initiating and funding the repeat survey and reporting.
The work builds on the 2010 baseline, whose contributors included IMAS researchers Graham Edgar, Elizabeth Oh and colleagues, supported through an ARC Linkage grant and a Caring for our Country grant to the program.
The Derwent Estuary in an example of kelp returning as local conditions change. Continued monitoring can show if these gains persist, how the animals associated with the reefs respond, and where management needs to focus next.
This adapted article was originally published by the Great Southern Reef Foundation.
Cover image by Stefan Andrews GSRF.