Stratification, nitrogen fixation, and cyanobacterial bloom stage regulate the planktonic food web structure

Area of Science:

• Aquatic ecology
• Marine biology
• Biogeochemistry

Background:

• Increasing sea surface temperature and water column stratification are predicted to alter aquatic food web complexity.
• Filamentous, nitrogen (N2)-fixing cyanobacterial blooms are expected to become more prevalent, significantly impacting planktonic food webs.
• The feeding behavior of mesozooplankton during these blooms remains unclear, with conflicting evidence suggesting herbivory or carnivory dominance.

Purpose of the Study:

• To investigate the planktonic food web structure and nitrogen sources during filamentous cyanobacterial blooms.
• To understand how microplankton community structure and bloom stage influence mesozooplankton feeding strategies.
• To unify contrasting findings on mesozooplankton trophic roles in cyanobacterial bloom conditions.

Main Methods:

• Utilized amino acid nitrogen stable isotopes from two mesozooplankton size fractions as proxies for nitrogen source and trophic position.
• Analyzed 124 biotic and abiotic variables using multivariate statistics to identify key environmental drivers.
• Examined microplankton diversity and community composition in relation to bloom age and environmental conditions.

Main Results:

• Mesozooplankton feeding shifted from herbivory to carnivory along a gradient from eutrophic to oligotrophic sites, correlating with microplankton diversity and bloom aging.
• Salinity was a major driver for non-N2-fixing microplankton biomass, while stratification, N2 fixation, and bloom stage regulated microplankton diversity and mesozooplankton trophic dynamics.
• Heterotrophic microbial food webs play a significant role in supporting mesozooplankton during aging cyanobacterial blooms.

Conclusions:

• Carnivory, not herbivory, is the dominant feeding mode for mesozooplankton during aging and decaying cyanobacterial blooms.
• These findings resolve conflicting evidence regarding mesozooplankton trophic roles and highlight the importance of microbial food webs.
• The shift towards carnivory has implications for the biogeochemical functions performed by mesozooplankton in future aquatic ecosystems.