Wastewater pollution and climate change stressors can have synergistic, as well as additive, impacts on ocean and human health. Wastewater pollution can make ecosystems more vulnerable to warmer temperatures, acidification, species invasions, and sea-level rise, among other stressors.
Take salt marshes, for example. As the sea-level rises, marshes affected by high levels of wastewater pollution can sink and drown. When polluted with high levels of nitrogen from wastewater, marsh plants shift their resources from below-ground (building roots) to above-ground (building leaves and stems). The result is weakened marsh soils that are no longer held together by thick root mats. This increases rates of erosion on the marsh surface and on creek bank edges, which ultimately causes creek banks to calve and shorelines to erode at a growing pace. Marshes with high levels of nutrient pollution also experience increased invasion by exotic plants. Since studies show that both nutrient pollution and sea-level rise increase decomposition and CO2 emissions, it is also likely that these factors interact to make salt marshes less effective as carbon sinks.
For oyster reefs, anoxic or hypoxic zones generated by wastewater pollution make them more vulnerable to multiple climate stressors. The connection is time delayed and is set in motion when anoxic events kill off large areas of oysters (Lenihan et al., 2001). These events greatly lower the number of live oysters on reefs, decreasing the overall ability of that reef to buffer against increasing temperatures, sea level rise and storm action. Sublethal hypoxic events also interact with oyster diseases and water cycles that have been altered by climate change. For example, an increase in freshwater runoff into estuaries increases parasite load in oysters, while exposure to sublethal hypoxia induces immunosuppression in oysters and increases their vulnerability to pathogens (Barnett et al., 2020).
Interactions between nutrient pollution and climate stressors also imperil coral reefs. When nutrient pollution is elevated on reefs, corals are more susceptible to both disease and bleaching and are less likely to fully recover (Osborne et al., 2017; Vega Thurber et al., 2014). For example, on the Great Barrier Reef and in the Florida Keys, large-scale surveys and experiments found that nutrient pollution increased disease incidence and bleaching in corals (Vega Thurber et al., 2014). Importantly, after termination of nutrient additions, there was a return to pre-enrichment water quality, followed by reduction in disease and bleaching – a result that bodes well for a future where wastewater pollution is reduced. The mechanisms underlying the intensifying effect of wastewater pollution on climate stress in corals are three-fold. First, wastewater pollution fuels growth of disease-causing bacteria by providing a ready supply of nitrogen. Second, wastewater carries pathogens into the ocean that attack corals directly. And third, the heavy metals and toxic chemicals found in wastewater effluent suppress immune response in corals (Tracy et al., 2020). These results suggest that reefs experiencing high loads of wastewater effluent are far less likely to resist and recover from climate change stressors such as warming. This predicted outcome has recently been shown in a large-scale study in French Polynesia that examined climate-nitrogen pollution interactions. Here, researchers found that nitrogen pollution increases susceptibility to temperature stress and increases bleaching across seascapes (Donovan et al., 2020).
Wastewater pollution is an even greater problem in the context of increased atmospheric carbon dioxide deposition. The normal pH of the ocean is around 8.1. As the pH lowers, the waters become more acidic, straining metabolic activity in marine organisms. The increasing concentrations of carbon dioxide in the atmosphere lowers pH in the oceans, because as carbon dioxide dissolves, a small portion interacts with water to form carbonic acid. While most research on ocean acidification has focused on climate change as the culprit, recent research shows that wastewater pollution also lowers pH in coastal waters, often has a bigger impact locally than global climate change (He & Silliman, 2019; Wallace et al., 2014). The primary mechanism at work is that wastewater pollution generates algal blooms that eventually die, thereby releasing massive amounts of carbon dioxide into the water which lowers the pH, sometimes below 7.
It is very likely that wastewater pollution and climate change will act synergistically to drive extreme drops in coastal water pH, as both forces fuel growth of HABs (EPA, 2013; He & Silliman, 2019). Increased wastewater pollution in coastal waters will fuel more HABs, while climate change will likely promote the growth and dominance of HABs through a variety of mechanisms including: 1) warmer water temperatures, 2) changes in salinity, 3) increases in atmospheric carbon dioxide concentrations, 4) changes in rainfall patterns, 5) intensifying coastal upwelling, and 6) sea-level rise. The synergistic interaction between wastewater pollution and climate change in driving HABs, is especially likely in semi-enclosed basins with high-density human populations, such as the Baltic Sea, Mediterranean Sea, San Francisco Bay, Chesapeake Bay, Puget Sound, and Hudson Bay.