The most notable effects of eutrophication are vegetal blooms, sometimes toxic, loss of biodiversity and anoxia, which can lead to the massive death of aquatic organisms.
Due to the hypoxic conditions present in dead zones, marine life within these areas tends to be scarce. Most fish and motile organisms tend to emigrate out of the zone as oxygen concentrations fall, and benthic populations may experience severe losses when oxygen concentrations are below 0.5 mg l O. In severe anoxic conditions, microbial life may experience dramatic shifts in community identity as well, resulting in an increased abundance of anaerobic organisms as aerobic microbes decrease in number and switch energy sources for oxidation such as nitrate, sulfate, or iron reduction. Sulfur reduction is a particular concern as Hydrogen sulfide is toxic and stresses most organisms within the zone further, exacerbating mortality risks.
Low oxygen levels can have severe effects on survivability of organisms inside the area while above lethal anoxic conditions. Studies conducted along the Gulf Coast of North America have shown hypoxic conditions lead to reduction of reproductive rates and growth rates in a variety of organisms including fish and benthic invertebrates. Organisms able to leave the area typically do so when oxygen concentrations decrease to less than 2 mg l. At these oxygen concentrations and below, organisms that survive inside the oxygen deficient environment and are unable to escape the area will often exhibit progressively worsening stress behavior and die. Surviving organisms tolerant of hypoxic conditions often exhibit physiological adaptations appropriate for persisting within hypoxic environments. Examples of such adaptations include increased efficiency of oxygen intake and use, lowering required amount of oxygen intake through reduced growth rates or dormancy, and increasing the usage of anaerobic metabolic pathways.
Community composition in benthic communities is dramatically disrupted by periodic oxygen depletion events, such as those of seasonal dead zones and occurring as a result of Diel cycles. The longterm effects of such hypoxic conditions result in a shift in communities, most commonly manifest as a decrease in species diversity through mass mortality events. Reestablishment of benthic communities depend upon composition of adjacent communities for larval recruitment. This results in a shift towards faster establishing colonizers with shorter and more opportunistic life strategies, potentially disrupting historic benthic compositions.