Impacts on Marine Life
Impacts to Coral Reefs
Coral bleaching and disease are common problems for reefs in waters contaminated by sewage pollutants. Sewage pollution can also indirectly affect marine life by altering ocean temperature, pH, and salinity as well as increasing disease in many organisms, such as corals, fish, and shellfish. Some common stressors found in sewage and their impacts on corals are listed in the table below (adapted from Wear and Vega-Thurber, 2015).
|STRESSORS||IMPACTS TO CORALS|
|Nutrients||Increased coral bleaching, increased coral disease (prevalence and severity), decreased coral reproductivity, algal overgrowth, decreased coral skeletal integrity, decreased coral cover and biodiversity, and increased phytoplankton shading.|
|Endocrine disruptors||Reduction in coral egg-sperm bundles, slowed coral growth rates, coral tissue thickening.|
|Pathogens||Source of white pox disease pathogen for corals and associated mortality, and increased pathogenicity in corals.|
|Solids||Reduced photosynthesis of coral symbionts, coral species richness, coral growth rates, coral calcification, coral cover, and coral reef accretion rates, and increased coral mortality.|
|Heavy metals||Coral mortality, coral bleaching, reduction of basic functions such as respiration and fertilization success; Fe2+ may increase growth of coral disease.|
|Toxins||Lethal and sublethal effects on corals – highly variable and dependent on specific toxin. Reduced photosynthesis of coral symbionts, coral bleaching, coral mortality, reduced coral lipid storage, reduced coral fecundity, death of coral symbionts, and decreased coral growth.|
Algal blooms on the surface block access to sunlight required by photosynthetic zooxanthellae in corals. Oxygen is required for and generated by photosynthesis as well as respiration and calcification and is therefore imperative to coral survival.
Hypoxia has been shown to cause bleaching events. Bleaching potential and severity are increased by sewage pollution, leading to increased damage and recovery capacity of corals. ref Local sewage pollution mitigation strategies to enhance resilience to bleaching for corals are increasingly critical. ref
Coral diseases are another threat intensified by sewage pollution. Outbreaks of two of the most common coral diseases have been linked with pollution. For instance, white pox is directly caused by the human gut pathogen Serratia marcescens, while black band disease is strongly associated with macroalgal cover that increases in polluted waters. ref
Impacts to Fish and Shellfish
Nutrients, typically from land-based sources such as agriculture or sewage, are essential building blocks for marine life. However, excess nutrients in the marine environment cause algae blooms that can coat the water surface, blocking sunlight and hampering photosynthesis, and contributing to ocean warming and acidification. Algal overgrowth presents competition for corals and can inhibit recovery after die-off and disease events. After algae die, their decay uses up oxygen, depleting it from the water and making it unavailable for other marine life. This eutrophication creates dead zones, characterized by low levels of dissolved oxygen, which are projected to increase in frequency and severity with climate change. ref The image below shows this process in more detail, beginning with nutrient input and leading to eutrophication, hypoxia, and die-off events.
Sewage pollution and excess nutrients in the ocean also lead to the generation of toxins that compromise ecosystem integrity, marine life, and human health. ref Different species of algae produce different toxins, resulting in a wide range of severity and impacts. These toxins bioaccumulate, building up in the tissues of organisms across the food web. In combination with disrupting photosynthesis, hazardous toxins create conditions that are uninhabitable for many fish and shellfish essential to both marine food webs and human livelihoods. ref
Marine life response to mild and severe hypoxia, including changes in physiological processes, habitat choices, and survivorship. Note: BBD stands for black band disease. Source: Nelson and Altieri 2019
In addition to toxins generated by algae, many others are present in sewage. These include pharmaceuticals, such as endocrine disruptors and synthetic compounds, that are not removed during treatment. By ingesting these toxins, marine organisms can then become toxic for human consumption as well, presenting a significant health hazard to humans in addition to the threat of biodiversity loss. See the case study from Puako, Hawaii where sewage pollution was identified as the biggest contributor to declining fish biomass and the community worked to identify and address the sources sewage pollution.