Are cyanobacterial blooms trophic dead ends?

Area of Science:

• Aquatic ecology
• Limnology
• Food web dynamics

Background:

• Cyanobacterial blooms pose increasing ecological and economic challenges.
• Zooplankton grazing resistance is a key factor in bloom persistence, but research findings are contradictory.
• Direct in situ observations are needed to understand cyanobacterial fate in aquatic food webs.

Purpose of the Study:

• To investigate the transfer of cyanobacterial organic matter from Planktothrix rubescens blooms to higher trophic levels in a lake ecosystem.
• To quantify the contribution of cyanobacteria to the diet of zooplankton and young fish using advanced analytical techniques.

Main Methods:

• Fatty acid (FA) and fatty acid stable isotope analyses (FA-SIA) were employed to trace cyanobacterial carbon.
• Zooplankton and young-of-the-year perch (Perca fluviatilis) were sampled from Lake Bourget, France, during a Planktothrix rubescens bloom (May-November 2009).
• Analysis focused on α-linolenic acid (18:3(n-3)) as a key indicator of cyanobacterial contribution.

Main Results:

• Planktothrix rubescens blooms were rich in 18:3(n-3), but direct grazing by zooplankton did not show increased levels of this fatty acid.
• FA-SIA indicated that cyanobacteria supplied over 25% of 18:3(n-3) to small zooplankton (<500 μm), but not to larger cladocerans (>500 μm).
• Approximately 15% of the 18:3(n-3) in young perch was traced back to the P. rubescens bloom, indicating dietary transfer.

Conclusions:

• Despite lower transfer efficiency compared to other micro-algae, Planktothrix rubescens blooms significantly contribute to pelagic secondary production.
• Cyanobacterial blooms are not a 'trophic dead end' and provide crucial nutritional benefits, especially to juvenile fish during critical recruitment periods.
• The studied cyanobacterial bloom served as a significant dietary resource for young perch, enhancing their survival prospects.