Climate warming rewires aquatic reciprocal feedbacks: environmental controls on phytoplankton-nutrient symmetry in lakes

Abstract Aim: Understanding feedback mechanisms between phytoplankton and nutrients is fundamental to predicting aquatic ecosystem responses to global climate change. While unidirectional nutrient-phytoplankton interactions have been well studied, mutual influences (reciprocal causality) and the balance (symmetry) of these pairwise feedbacks (PairFB) remain poorly explored at the global scale. This study aimed to empirically assess the prevalence and symmetry of PairFB between phytoplankton and macronutrients in lakes, and to elucidate the environmental controls, particularly climate warming, mediating their structure and balance across diverse biogeographical contexts. Location: 136 sampling sites from 93 lakes distributed across Asia, Europe, North America and Oceania, covering diverse climatic, morphometric and trophic regimes over a broad latitudinal gradient. Time Period: Long-term in-situ observational time series ranging from 34 to 660 months. Major Taxa Studied: Phytoplankton (quantified as chlorophyll a concentration). Methods: We used convergent cross mapping to detect significant reciprocal feedbacks between chlorophyll-a and each macronutrient and quantified the relative strength and symmetry of bidirectional interactions at the level of each individual feedback, in addition to the pooled relationship. We then applied Bayesian generalized linear mixed-effects models to evaluate how global drivers (warming rate, absolute latitude) and local lake attributes (mean water depth, trophic status, nutrient limitation type) control the probability of occupying each feedback state, and how they relate to the symmetry of the pooled relationship. Results: Our analysis revealed that PairFB were highly prevalent (54-75% across nutrient types) and often fundamentally symmetrical across global lakes. However, symmetry varied substantially among individual feedbacks and was best described as a continuous property rather than a binary state. The probability that a feedback pair sustains bidirectional coupling was in turn sensitive to environmental context. Accelerated warming raised that probability and increased the magnitude of directional imbalance, most clearly for NOx-1 links. Trophic status and nitrogen limitation, but not latitude or mean water depth, were associated with the coupling state: hypereutrophic lakes were more likely than eutrophic lakes to sustain bidirectional coupling, whereas the nine to eleven N-limited records were markedly less likely to do so. Main Conclusion: This study provides global empirical evidence that ongoing climate warming is reshaping the core dynamic structure of phytoplankton-nutrient interactions in lakes. Interaction asymmetry can be a stable configuration as well as a transitional state. The environmental contingency of reciprocal feedback symmetry revealed here therefore reframes how we interpret lake ecosystem responses to climate change. Keywords: Causal inference, lake dynamics, asymmetrical feedback, trophic status, nutrient limitation, freshwater biogeography