Belowground ecosystem engineers enhance biodiversity and function in a polluted ecosystem

Many important ecosystem functions are underpinned by belowground
biodiversity and processes. Marine sediments, one of the most abundant
habitats on earth, are essential to the mineralisation of organic matter.
However, they are increasingly polluted by urban activities leading to the
loss of biodiversity and the functions they provide. While traditional
sediment remediation strategies are focussed on microbial and engineering
solutions, we propose that the reintroduction of belowground ecosystem
engineers (bioturbators) is important to rehabilitate polluted sediments
and drive recovery of their functions in urban coastal ecosystems. We
tested this notion by introducing bioturbators to nutrient polluted
sediments to assess their survival, as well as their capacity to drive
biodiversity and oxygenation and their potential to remediate nutrient
pollution. Polychaete worms Diopatra aciculata and clams Katelysia sp.
were added to mesocosms (ex-situ), and the worms also added to
experimental plots in-situ. Potential for remediation was assessed with
measures of nutrient content. All animals survived when introduced to
polluted sediments and showed no evidence of sub-lethal effects. Worms
oxygenated sediments and reduced organic matter content by up to 50%
in-situ. The worms also drove shifts in the receiving communities at all
locations and increased the number of taxa at one location. On the other
hand, the effects of clams were variable, showing opposite effects in
organic matter content at different sites and levels of pollution.
Synthesis and applications. Global seafloor habitats are becoming
increasingly degraded and novel strategies that combine biodiversity
restoration with remediation are urgently needed to return function.
Tube-building bioturbators can stimulate nutrient processing in sediments
proving multiple functional outcomes, but these effects are dependent on
the receiving environment. In areas with medium levels of pollution, they
can kick-start recovery in a feedback loop whereby bioturbation increases
oxygenation and nutrient remediation, shifting sediment biodiversity and
contributing to further recovery. This can drive long-term changes in
sediment communities, particularly in urban areas where unvegetated
sediments are conspicuous.