A combination of global changes such as species invasions, climate change, and nutrient pollution have altered ecosystems, food webs, and the bioenergetic processes that control growth. These changes are especially pronounced in freshwater ecosystems and often lead to rapid variation in fish growth and dependent ecosystems services such as fishery yield. Understanding the mechanisms driving growth responses to environmental change is important for interpreting past dynamics and sustainably managing ecosystems. This study uses integrated bioenergetics and growth modeling to understand how nutrient dynamics, species invasions, and changing temperatures have altered growth of the keystone pelagic whitefish (Coregonus wartmanni) in Lake Constance, Germany from 1925 to 2020. Growth variation was modeled by allowing covariates to alter temperature-dependent consumption, while size-specific metabolism varied only with temperature. Consumption and growth increased strongly to a maximum with phosphorous, and this effect was stronger when intraspecific competition (measured as whitefish biomass) was low. Increasing whitefish biomass reduced growth under mesotrophic conditions, but had no effect under oligotrophic conditions. In contrast, increasing competition with invasive three-spined stickleback (Gasteosteus aculeauts) was predicted to reduce growth even under oligotrophic conditions. The invasion has effectively turned summer into winter for whitefish, with older fish ceasing to grow and younger fish losing up to 10% of their body weight during the normal growing season in subsequent years. Warming is predicted to further reduce whitefish growth due to competition with invasive stickleback, which would further alter zooplankton food availability and reduce already low fishery yields. These results demonstrate the importance of considering biotic interactions and synergistic effects in global change studies, as well as the value of mechanistic-based models for understanding effects. Similar growth responses to ecosystem change are likely within and across ecosystems, and bioenergetic models can help understand effects to support informed ecosystem management.