Impacts on Marine Life

Sewage pipe. Photo © Joe Miller
Traditionally thought of as only a human health concern, the impacts of sewage on marine life have been understated and unaddressed. Sewage pollution has many detrimental consequences for corals and fish species. Ongoing nutrient loading and algal blooms are particularly devastating for marine life and are increasing.

red tide

Red Tide in California, USA. Photo © Flickr

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).

 

STRESSORSIMPACTS TO CORALS
NutrientsIncreased 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 disruptorsReduction in coral egg-sperm bundles, slowed coral growth rates, coral tissue thickening.
PathogensSource of white pox disease pathogen for corals and associated mortality, and increased pathogenicity in corals.
SolidsReduced photosynthesis of coral symbionts, coral species richness, coral growth rates, coral calcification, coral cover, and coral reef accretion rates, and increased coral mortality.
Heavy metalsCoral mortality, coral bleaching, reduction of basic functions such as respiration and fertilization success; Fe2+ may increase growth of coral disease.
ToxinsLethal 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.

 

Processes that contribute to decreases in dissolved oxygen and the impact of subsequent hypoxia on marine life (left image). Hypoxia and bacteria concentrations impact marine life across trophic levels. Larger fish species require higher levels of dissolved oxygen while microfauna, such as worms, can tolerate lower oxygen levels. Dead zones occur when survivorship in a habitat is reduced by hypoxia (right image). Source: Boesch 2008

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.

opens in a new windowChemicals of Emerging Concern (CECs) are being found in treated sewage effluent more frequently and in higher concentrations. These chemicals include compounds found in medicines and personal care products and are not removed through current sewage treatment mechanisms. Antidepressants have been shown to impact fish behavior and mortality in low concentrations, while synthetic hormones and endocrine disruptors can impair reproductivity and contribute to aggressive tendencies. In addition, overall survivability is impaired for fish exposed to sewage contaminated waters. Recent studies show bioaccumulation of many CEC compounds in fish tissue and raise concerns over the lack in understanding and research of potential CEC impacts on marine life.

 

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