Prolonged (80-day) exposure to waste drilling fluids alters the structure and trophic networks of marine plankton communities in a microcosm study

The ecological risks associated with the direct discharge of waste drilling fluids (WDFs) into the ocean must be systematically assessed. Through an 80-day microcosm experiment, we assessed main pollution characteristics and seawater quality after introducing WDF water extracts at three concentrations and examined the prolonged dynamic effects of continuous exposure on natural plankton communities. WDF input altered key water quality parameters, increased Cr and As, and caused Cr and Hg in the HIGH group to exceed GB 3097-1997 Class I seawater standards. Chlorophyll a (Chl a) and dissolved oxygen (DO) levels and the pH decreased, whereas turbidity increased. WDF input also changed phytoplankton and zooplankton species compositions, increased spatial differentiation within communities, and generated dose-dependent effects. Phytoplankton responded more quickly than zooplankton, and toxicity verification experiments confirmed that phytoplankton were more sensitive. RDA indicated that Cr and Hg were the primary variables that explained changes in the community structure of phytoplankton and zooplankton, respectively. Co-occurrence network analysis revealed decreases in the total network and subnetwork topologies and the connectivity and abundance of key taxonomic groups, suggesting general network degradation. Based on the random forest model and Spearman's correlation analyses, cross-trophic interactions dominated the total network responses, followed by the phytoplankton subnetwork. These results indicate that the central mechanism underlying WDF ecological risk is weakening planktonic interactions via direct interspecies interference and impaired phytoplankton supply. These findings provide a systematic foundation and theoretical support for establishing an ecological risk assessment system for WDFs and promoting their eco-friendly use and emission management.