Examination of the role of the microbial loop inregulating lake nutrient stoichiometry and phytoplankton dynamics

[摘要] The recycling of organic material through bacteria and microzooplankton tohigher trophic levels, known as the "microbial loop", is an importantprocess in aquatic ecosystems. Here the significance of the microbial loop ininfluencing nutrient supply to phytoplankton has been investigated in Lake Kinneret(Israel) using a coupled hydrodynamic–ecosystem model. The model was designedto simulate the dynamic cycling of carbon, nitrogen and phosphorus throughbacteria, phytoplankton and zooplankton functional groups, with each poolhaving unique C : N : P dynamics. Three microbial loop sub-modelconfigurations were used to isolate mechanisms by which the microbial loopcould influence phytoplankton biomass, considering (i) the role of bacterialmineralisation, (ii) the effect of micrograzer excretion, and (iii) bacterialability to compete for dissolved inorganic nutrients. The nutrient fluxpathways between the abiotic pools and biotic groups and the patterns ofbiomass and nutrient limitation of the different phytoplankton groups werequantified for the different model configurations. Considerable variation inphytoplankton biomass and dissolved organic matter demonstrated thesensitivity of predictions to assumptions about microbial loop operation andthe specific mechanisms by which phytoplankton growth was affected.Comparison of the simulations identified that the microbial loop mostsignificantly altered phytoplankton growth by periodically amplifyinginternal phosphorus limitation due to bacterial competition for phosphate tosatisfy their own stoichiometric requirements. Importantly, eachconfiguration led to a unique prediction of the overall communitycomposition, and we conclude that the microbial loop plays an important rolein nutrient recycling by regulating not only the quantity, but also thestoichiometry of available N and P that is available to primary producers.The results demonstrate how commonly employed simplifying assumptions aboutmodel structure can lead to large uncertainty in phytoplankton communitypredictions and highlight the need for aquatic ecosystem models to carefullyresolve the variable stoichiometry dynamics of microbial interactions.

[学科分类] 地球化学与岩石