Three Decades Long Study Shows Why Florida's Coral Reefs Are Dying

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Three Decades Long Study Shows Why Florida's Coral Reefs Are Dying

For a long time, it was expected that climate change is the major reason why coral reefs are dying. Yet, the results of a three-decade-long study show that humans actually play a much bigger part in marine contamination.

The research paper, published online in the journal Marine Biology , is based on data collected over three decades from the Looe Key Sanctuary Preservation Area in the Florida Keys. The research proves that coral coverage declined from 33% in 1984 to just 6% in 2008. Even as temperatures have trended upward globally, average local temperatures didn't change much during the study period, but reef continued bleaching.

Abstract:

Increased loadings of nitrogen (N) from fertilizers, topsoil, sewage, and atmospheric deposition are important drivers of eutrophication in coastal waters globally. Monitoring seawater and macroalgae can reveal long-term changes in N and phosphorus (P) availability and N:P stoichiometry that is critical to understanding the global crisis of coral reef decline. Analysis of a unique 3-decade data set for Looe Key reef, located offshore the lower Florida Keys, showed increased dissolved inorganic nitrogen (DIN), chlorophyll a, DIN:soluble reactive phosphorus (SRP) ratios, as well as higher tissue C:P and N:P ratios in macroalgae during the early 1990s.

These data, combined with remote sensing and nutrient monitoring between the Everglades and Looe Key, indicated that the significant DIN enrichment between 1991 and 1995 at Looe Key coincided with increased Everglades runoff, which drains agricultural and urban areas extending north to Orlando, Florida. This resulted in increased P limitation of reef primary producers that can cause metabolic stress in stony corals. Outbreaks of stony coral disease, bleaching, and mortality between 1995 and 2000 followed DIN enrichment, algal blooms, and increased DIN: SRP ratios, suggesting that eutrophication interacted with other factors causing coral reef decline at Looe Key.

Although water temperatures at Looe Key exceeded the 30.5 °C bleaching threshold repeatedly over the 3-decade study, the three mass bleaching events occurred only when DIN: SRP ratios increased following heavy rainfall and increased Everglades runoff. These results suggest that Everglades discharges, in conjunction with local nutrient sources, contributed to DIN enrichment, eutrophication, and increased N:P ratios at Looe Key, exacerbating P limitation, coral stress, and decline. Improved management of water quality at the local and regional levels could moderate N inputs and maintain more balanced N:P stoichiometry, thereby reducing the risk of coral bleaching, disease, and mortality under the current level of temperature stress.

Citation :

Lapointe, B.E., Brewton, R.A., Herren, L.W. et al. Mar Biol (2019) 166: 108. DOI: 10.1007/s00227-019-3538-9

Source: Springer

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