Beyond competition: food availability shapes trophic interactions between benthic harpacticoid copepods and free-living nematodes

Species interactions are fundamental drivers of ecosystem functioning, yet their role in structuring trophic relationships among benthic meiofauna remains poorly understood. We examine biotic interactions between the two most abundant taxa in estuarine environments, copepods and nematodes, as a case study of interspecific competition for trophic niches. Using controlled microcosm experiments with varying food regimes (diatoms alone vs. diatoms with bacteria), we assessed whether food composition or interspecific interactions influence fatty acid composition and food assimilation in our test species, the harpacticoid copepod Platychelipus littoralis. Our results demonstrate that trophic dynamics of intertidal benthic copepods are primarily structured by food resource diversity (single vs. mixed diet) and quality, rather than direct interspecific competition. The absence of interaction effects on copepod survival, fatty acid composition, and food assimilation suggests that niche overlap does not necessarily result in competitive exclusion, at least under moderate food availability. Copepods showed reduced diatom assimilation when bacterial food was available, but bacterial uptake was unaffected by nematode presence, indicating trophic flexibility. These findings underscore the importance of food quality and microbial composition in shaping meiofaunal interactions and energy flow, and the need to integrate microbial–meiofaunal linkages into studies of benthic ecosystem functioning.