<p>This study experimentally assessed the joint effects of light attenuation, allelopathy, and nutrient masking on phytoplankton productivity and carrying capacity in Laguna del Cisne (Uruguay), a dystrophic subtropical shallow lagoon. Using a fractional factorial design, we analyzed phytoplankton growth under eight treatment combinations of water type, nutrient availability, and light attenuation. A logistic growth model was applied to estimate intrinsic growth rate (r) and carrying capacity (K), while statistical significance and ecological relevance of pairwise comparisons were evaluated. Our results indicate that light attenuation alone cannot fully explain the extremely low phytoplankton productivity observed in the lagoon. Instead, the cumulative effects of nutrient masking and allelopathy play critical roles. Enriched nutrient conditions mitigated the masking effect of dissolved organic compounds while removing allelopathic compounds significantly increased both r and K. High attenuation levels resulted in negligible growth, emphasizing the conjunction with other factors. The study highlights the ecological importance of these controlling mechanisms and their implications for dystrophic lakes management in the context of increasing anthropogenic pressures. This research provides insights into the ecosystem functioning of subtropical dystrophic shallow systems and offers valuable information for the sustainable management of Laguna del Cisne as a source of potable water.</p>