The importance of model structural complexity when simulating aquatic food webs

Abstract

The problem of simulating microbial interactions in aquatic systems is receiving considerable attention and has resulted in recent advances to ecological models for understanding and forecasting algal blooms. However, these ecological models often simplify microbial diversity and do not always provide an accurate picture of the nutrient flux pathways that occur in food webs due to the complicated nature of microbial interactions. This study used the FABM (Framework for Aquatic Ecosystem Models) framework to develop three ecological models of different structural complexity, that sequentially build on the classic 'Nutrient-Phytoplankton-Zooplankton-Detritus' (NPZD) model, to better understand the significance of specific microbial interactions ecosystem dynamics, namely the 'microbial loop' and 'viral shunt' (Figure 1). The results of a 'Nutrient-Phytoplankton-Zooplankton-Detritus+Viruses' (NPZD+V) model were used to compare the influence of zooplankton mediated mortality relative to virus mediated mortality of phytoplankton. The results showed that virus mediated mortality via infection and lysis of phytoplankton can be as important as zooplankton mediated mortality via grazing under typical conditions. Next the results of the 'Nutrient-Phytoplankton-Zooplankton-Detritus+Viruses+Bacteria' (NPZD+VB) model indicated that the viral shunt can short circuit the microbial loop via viral infection and lysis of phytoplankton and bacteria, and thereby increase the transfer of material to the detrital pool. Furthermore, the more complex model structure that include the viral shunt and microbial loop pathways illustrated the importance of 'bottom-up' (resource) control of algal production via microbial interactions in aquatic ecosystems. These results help provide an improved mechanistic understanding for viral-bacterial-phytoplankton-zooplankton interactions in aquatic ecosystems and can help to guide decisions about appropriate model conceptualisations that can be used. Further work on systems analysis of the model structures is required to better understand their resilience and stable states that are likely to form under a range of nutrient enrichment conditions.