Macroinvertebrate and environmental responses to dredging and submerged macrophytes transplantation

Eutrophication of freshwater ecosystems is a major global problem, but
restoration can be difficult due to ongoing problems relating to water
pollution, sedimentary nutrient stores, and altered aquatic biodiversity.
Mitigation of water quality stressors is often conducted alongside
transplantation of submerged macrophytes and dredging, but knowledge of
ecosystem response to post-dredging transplantation of submerged
macrophytes is limited.  Here, we report a long-term (2008-2018)
in-situ monitoring study to evaluate the effects of two different
restoration measures: dredging only (Dredged) and dredging with
post-dredging transplantation of submerged macrophytes (Dredged with
macrophytes) conducted in five subtropical eutrophic lakes in Lake Taihu
basin, China. Water and sediment nutrients, bloom-forming algae
Microcystis, and macroinvertebrate were monitored every two years for each
treatment and compared with reference areas (Control) established in
unrestored parts of the same lakes.  Dredging only decreased
sediment nutrients (nitrogen, phosphorus, total carbon, and water total
phosphorus significantly, however, this effect diminished about five years
later. Dredged with macrophytes had a stronger, longer-lasting positive
effect on water quality than Dredged alone. Disturbance caused by dredging
(without macrophytes transplantation) decreased the biomass of
Microcystis, while transplantation of submerged macrophytes shortly after
dredging did not contribute to the decrease of Microcystis biomass. The
biomass of Microcystis in Dredged with macrophytes areas was always
similar to Control over the period of our monitoring.  A positive
effect of submerged macrophytes transplantation post-dredging was found
for macroinvertebrate abundance and diversity: Dredged with macrophytes
areas had significantly higher macroinvertebrate biomass and richness than
Dredged areas after 9 years’ recovery. Macroinvertebrate richness in
Dredged with macrophytes areas nearly doubled compared to Control; while
Dredged areas were just restored to Control levels.  Synthesis
and applications. Our study provides an in-situ long-term field monitoring
with new findings about the benefits and caution of submerged macrophytes
transplantation post-dredging, and the effect of partial restoration,
which could inform eutrophic waterbody restoration schemes.