Climate change causes temperature and light to change drastically in Arctic fjords, being the main drivers for ecosystem-engineering seaweeds (kelps; Laminariales, Phaeophyceae). Climate projections on kelps are often based on static performance curves, treating species as one homogenous unit with similar tolerances within their entire biogeographical range. This might lead to mis-extrapolations. We assessed how Arctic kelp specimens are conditioned by their specific in-situ environment. Therefore, we sampled Saccharina latissima sporophytes from eight fjords along the west coast of Svalbard, Norway. Analysing their biochemical response variables (pigment content and composition; antioxidative activity; total carbon and nitrogen content), we found a distinct clustering of the biochemical composition of S. latissima, which correlated significantly with their environment. S. latissima responded strongly to changes in run-off induced turbidity, i.e., light availability. High light availability correlated with a significant reduction of photosynthetic pigments indicating high light protection. Nevertheless, the kelps’ total carbon content increased. The kelps’ total nitrogen content increased with increasing turbidity, which might be a response to nutrients being washed into the fjord by run-off. We found no stress response to suboptimal temperatures (3 °C vs. 7 °C). This is a further indication of the importance of light as a driver for high-latitude kelp populations, and the necessity to include it in climate projections. In conclusion, we found a high site-specific plasticity of Arctic S. latissima sporophytes. This has to be considered when projecting the responses of kelps towards climatic changes and local management activities.