Top-down cascading effects counteract de-eutrophication efforts in coastal ecosystems

Deteriorating water quality, driven by eutrophication, is a major concern for global coastal ecosystems. Although mitigating nutrient loads (\enquote{de-eutrophication}) is the de-facto restoration strategy, it has not consistently reduced excess phytoplankton biomass. Here, we combine satellite observations, in situ data, and mechanistic modelling to show that (1)~coastal phytoplankton biomass has increased globally over recent decades, while nutrient concentrations declined at higher latitudes ($>23^\circ$) but increased elsewhere; and (2)~the effects of de-eutrophication can be overridden by top-down trophic cascades. In turbid coastal waters, planktivorous fish, including small and/or juvenile individuals, seek refuge from predators and intensively graze on zooplankton, reducing herbivory on phytoplankton and increasing their biomass. Coastal warming is likely to intensify these cascades, potentially driving further phytoplankton increases. Our findings demonstrate that phytoplankton stocks are strongly influenced by top-down processes, underscoring the need to incorporate trophic interactions into ecosystem models and management strategies. De-eutrophication alone may therefore be insufficient.