Carbon flow through ecosystems is determined by the nature, availability and incorporation of basal resources (BRs) to higher trophic levels of food webs. The occurrence of abundant supplementary BR often diversifies trophic niches within food webs. Blooms of opportunistic macroalgae, which may act as additional BRs, have increased in intensity, and are expected to amplify further because of global change. Understanding the effects of high biomasses of opportunistic BRs on the functioning of naturally unvegetated coastal ecosystems is thus crucial. We set out to assess whether and how green algae (opportunistic BR) modifies macrotidal sandy beach trophic structure and functioning. We also investigated whether these effects propagate up through the web, looking at different scales and trophic levels (i.e. primary producers, specific primary consumers, and the general higher-consumer assemblage). To achieve these objectives, we used a high-resolution field sampling (over 1200 individual stable isotope-, and 180 biodiversity samples) at pristine and macroalgal-bloom-affected macrotidal sandy beaches, together with recent and novel analytical approaches (biomass-weighted isotope signatures, Isotopic Functional Indices). We found that the opportunistic BR, both directly and indirectly affects the entire beach food web across several trophic levels, by inducing shifts in trophic interactions. Overall, we found that the dominance of a BR in the form of opportunistic algae simplifies the entire food web over time. Our findings challenge the paradigm that addition of BR inherently diversifies trophic interactions within an ecosystem, and have implications for the interpretation of shifts in dynamic system food webs and eutrophicated environments.