Abstract
Eutrophication of lakes often results in dominance of cyanobacteria, which may potentially lead to serious blooms and toxic water. However, cyanobacterial detritus may act as an important carbon source for aquatic organisms. Using stable isotope carbon (13C) as a tracer, we assessed the carbon transfer from cyanobacteria to pelagic and benthic consumers in a 28-day outdoor mesocosm (~130 L) labelling experiment established in Lake Taihu, China, during a Microcystis aeruginosa bloom. The different organisms were labelled differently after addition of the labelled Microcystis detritus to the water. δ13C of particulate organic matter and of cladoceran zooplankton peaked earlier than for larger invertebrate consumers. Among the pelagic species, Daphnia similis had the highest Δδ13C, while the two snail species Radix swinhoei and Bellamya aeruginosa had lower but similar Δδ13C. The bivalves showed relatively modest changes in δ13C. The δ13C of Anodonta woodiana and Unio douglasiae showed a marginal though not significant increase, while a marked increase occurred for Arconaia lanceolate peaking on day 20, and Corbicula fluminea a slight increase peaking on day 9. Our results suggest that carbon from cyanobacteria can be incorporated by pelagic and some benthic consumers and eventually be transferred to higher trophic levels. Cyanobacterial carbon may, therefore, be considered an important carbon source supporting the entire food web during blooms, even if the cyanobacteria are not consumed directly.
Highlights
In many lakes, phytoplankton dominates primary production [1,2] and previous food web studies have, traditionally focused on phytoplankton-based food sources [3,4,5]
Some studies suggest that certain zooplankton species can feed on cyanobacteria [32,33,34], and numerous biomanipulation experiments have shown a drastic reduction in cyanobacteria abundance following a return of large Daphnia after removal of plankti-benthivorous fish, indicating enhanced grazer control of the cyanobacteria [35,36]
Both the abundance and biomass of macroinvertebrates were much higher in the northern part of Lake Taihu, with recurrent cyanobacteria blooms, than in the southern part [46], which indicates that Microcystis is used as a food source, either directly or indirectly as detritus
Summary
Phytoplankton dominates primary production [1,2] and previous food web studies have, traditionally focused on phytoplankton-based food sources [3,4,5]. Microcystis turned out to be an exploitable carbon source for this species and for Daphnia [39,40] In this way, cyanobacteria may contribute as an energy source to the higher trophic level in the food web even if they are not grazed upon directly. Microcystis blooms in Lake Taihu are often associated with relatively high abundances of small zooplankton such as Bosmina [45]. Both the abundance and biomass of macroinvertebrates were much higher in the northern part of Lake Taihu, with recurrent cyanobacteria blooms, than in the southern part [46], which indicates that Microcystis is used as a food source, either directly or indirectly as detritus. We hypothesised that cyanobacterial detritus constitutes a significant proportion of the carbon source for both pelagic and benthic consumers
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