International Journal of Marine Science, 2024, Vol.14, No.4, 285-294 http://www.aquapublisher.com/index.php/ijms 288 anoxic conditions in the bottom waters. This process illustrates that the initial increase in productivity masks the severe hypoxia that follows, highlighting the profound impact of eutrophication on oxygen distribution in water bodies. The ecological impacts of hypoxic and anoxic zones are significant, leading to the mortality and emigration of fish, crustaceans, and other marine organisms. These zones disrupt normal biological functions and can lead to shifts in community structure and ecosystem function (Altieri and Díaz, 2019). Moreover, the interaction between hypoxia and other stressors, such as temperature, can exacerbate the impacts on marine life. For instance, low DO conditions combined with higher temperatures can increase metabolic demand and reduce the availability of thermal refuges, further stressing aquatic organisms (Roman et al., 2019). 4 Ecological Impacts on Coastal Marine Ecosystems 4.1 Effects on primary producers Eutrophication significantly impacts primary producers in coastal marine ecosystems. Increased nutrient concentrations in seawater lead to higher phytoplankton biomass and promote the growth of opportunistic macroalgal species at the expense of canopy-forming species (Gerakaris et al., 2022). This shift is evident in the increased coverage of fast-growing, ephemeral algae over perennial macroalgae and seagrasses, causing habitat degradation (Östman et al., 2016). Additionally, climate change exacerbates these effects by promoting the growth of filamentous green algae, which are often associated with intensive algal blooms (Takolander et al., 2017). 4.2 Changes in species composition and biodiversity 4.2.1 Decline of sensitive species Eutrophication often results in the decline of sensitive species. For instance, the structural traits of seagrass meadows, such as Posidonia oceanica, show negative trends with increasing levels of nutrient-related pressure indicators like ammonium, nitrate, and phosphate (Gerakaris et al., 2022). Similarly, the loss of biologically engineered habitats and the decline of marine-originating foundation species such as fucus are severe biodiversity impacts linked to eutrophication (Takolander et al., 2017). 4.2.2 Proliferation of opportunistic species The proliferation of opportunistic species is a common consequence of eutrophication. Increased nutrient levels favor fast-growing, opportunistic macroalgae and phytoplankton, leading to shifts from perennial seagrass to these opportunistic species (Schmidt et al., 2017). This shift is further intensified by climate change, which promotes the growth of filamentous green algae (Takolander et al., 2017). 4.2.3 Shifts in community structure Eutrophication-induced changes in primary producers lead to shifts in community structure. For example, in Atlantic Canada, eutrophication has caused distinct shifts in species composition, with increases in filter feeders, epibenthic detritivores, and some herbivores, while more hypoxia-sensitive species have declined (Schmidt et al., 2017). These changes are associated with differences in food availability and predation refuge offered by phytoplankton and opportunistic macroalgae. 4.3 Disruption of food webs and trophic interactions Eutrophication disrupts food webs and trophic interactions by altering the distribution and flow of energy and biomass throughout the ecosystem. The introduction of invasive species, which often differ functionally from native species, generates ecological impacts that propagate along the food web, leading to changes in species abundance and interactions (Gallardo et al., 2016). Additionally, eutrophication can erode habitat gradients and behavioral mechanisms that maintain ecological separation and reproductive isolation among species, resulting in reduced ecological specialization and genetic homogenization (Alexander et al., 2017). This disruption of food webs and trophic interactions can have far-reaching consequences on the structure and functionality of aquatic ecosystems.
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