Individual species’ reactions to eutrophication can vary widely. For example, some organisms, such as primary producers, can adapt very quickly and even thrive under anoxic conditions. However, most organisms are extremely susceptible to slight changes in aquatic oxygen levels. Simply put - if a respirating organism is presented with little to no oxygen, its chances of survival will decrease. Therefore, eutrophication and anoxic conditions in water leads to a decrease in biodiversity.
For example, the soft coral Xenia umbellata can resist some anoxic conditions for short periods of time, but after about 3 weeks, mean survival decreases to about 81% and about 40% of surviving species experience size reductions, lessening in coloration, and compromised pinnate structures (Simancas-Giraldo et al., 2021). Another example of a susceptible organism is observed with The Sydney Cockle, Anadara trapezia. Enriched sediments have lethal and sublethal effects on this Cockle and, as stated in [Vadillo Gonzalez et al., 2021], “movement of cockles was reduced in enriched sediments compared to natural treatments.” These are just a few examples of the hundreds of thousands of aquatic species that exist, but these and other examples show important results.
A study collecting over 850 published experiments "reporting oxygen thresholds and/or lethal times for a total of 206 species spanning the full taxonomic range of benthic metazoans."
Individual species will have different adaptive responses to anoxic conditions based on their biological makeup and the condition of their habitat. While some are able to pump oxygen from higher water levels down into the sediment, other adaptations include specific hemoglobins for low oxygen environments, slow movement to reduce rate of metabolism, and symbiotic relationships with anaerobic bacteria. In all cases, the prevalence of excess nutrients results in low levels of biologic activity and a lower level of species diversity if the area is not normally anoxic.