Effects from maritime scrubber effluent on phytoplankton and bacterioplankton communities of a coastal area, Eastern Mediterranean Sea

The few studies on the impact of vessel scrubber effluents on the marine environment indicate adverse effects. Phytoplankton and bacterioplankton, the most abundant organisms of pelagic ecosystems, are the first responders to scrubber water discharge in seawater. Understanding the scrubber effluent effects on these communities could improve our knowledge of shipping impacts on marine ecosystems. In the present work, mesocosm experiments were conducted to examine scrubber effluent effects on phytoplankton and bacterioplankton communities sampled in the Thermaikos Gulf, Eastern Mediterranean, taken from an unpolluted site and a site polluted by other sources of pollution. Diverse shifts of the phytoplankton and bacterioplankton communities were observed at high (HS: 10% v/v) and low (LS: 1% v/v) scrubber effluent treatments. Diatoms in the polluted area in HS treatments, and particularly, the dominant species Pseudo-nitzschia cf. pungens forming a harmful bloom, exhibited acute population decline in the first 24 h of exposure followed by a stationary phase with steady cell numbers till the end of the experiment. These acute adverse effects were not masked by the nitrate enrichment caused by nitrogen contents of the scrubber effluent, although a weak eutrophication signal was evident in control treatments with nitrate additions. In the unpolluted site, the dominant pico-chlorophytes were not significantly impacted, while in LS treatments no adverse effects were observed in phytoplankton and bacterioplankton abundance and composition. Phytoplankton composition changed slightly with exposure to the scrubber effluent, while a pronounced shift was observed in the bacterioplankton community towards genera known to degrade Polycyclic Aromatic Hydrocarbons (PAHs) in both treatments (LS, HS). Network analysis indicated high functional-response diversity of PAHs degrading bacteria in the unpolluted site community, which is critical for community resistance and resilience that may sustain the ecosystem's ecological status to the previously unimpacted state.

Automated summary

The few studies available on the impact of vessel scrubber effluents on the marine environment have shown adverse effects. In this study, mesocosm experiments were conducted to examine the effects of scrubber effluents on phytoplankton and bacterioplankton communities in the Thermaikos Gulf, Eastern Mediterranean. High and low concentrations of scrubber effluents caused diverse shifts in the composition of these communities, with significant declines in diatoms and changes in bacterioplankton towards PAHs degrading genera. The results highlight the importance of understanding the effects of shipping activities on marine ecosystems.