In China, crucian carp (Carassius auratus) are dominant fish species in various freshwater ecosystems. As an Omni-benthivorous fish, on one hand, crucian carp may affect water nutrient and turbidity levels via sediment disturbance and thereby enhance phytoplankton growth, a "bottom-up" route. On the other hand, crucian carp may also affect algal growth and nutrient cycling via predation on zooplankton and thereby reducing zooplankton grazing on phytoplankton, a "top-down" process. For shallow lakes, the relative importance of both mechanisms remains to need to be elucidated. Here, we designed an outdoor mesocosm experiment to compare the effects of crucian carp on water turbidity, nutrient and phytoplankton growth between mesocosms with (via adding a sieve above the sediments) and without sediment disturbance. The experiment was conducted using 16 armoured glass tanks (400 L) and lasted for 36 days (from August 6 to September 11 in 2019). The results showed that:1) in mesocosms with sediment disturbance (without sieve), crucian carp caused severe sediment resuspension, reflected by the large increases of total suspended solids (TSS) and inorganic suspended solids (ISS). Correspondingly, light attenuation efficient (K_d), total nitrogen (TN) and total phosphorus (TP) concentrations were significantly higher in mesocosms with crucian carp than ones without fish. 2) In mesocosms without sediment disturbance (sieve present), the effects of crucian carp on suspended solids (TSS, ISS) and K_d were both insignificant. However, TN and TP concentrations were significantly reduced by crucian carp. 3) no significant effects of crucian carp on chlorophyll-a concentration and zooplankton biomass were observed. Our study suggested that crucian carp promote water turbidity and nutrient levels only when they could access the sediments. Thus, in shallow lakes without submerged macrophytes, "bottom-up control" may be the main mechanism of crucian carp affecting water quality adversely.