8. Sep, 2017

Ghost reefs of the past: decline of Florida’s coastal ecosystems

Coral reefs form a beautiful and important part of our ocean, made up of thousands of tiny animals—coral “polyps”—that are related to anemones and jellyfish. The polyp uses calcium and carbonate ions from seawater to build itself a hard, cup-shaped skeleton made of calcium. Some  stony corals of the family Acroporidae like Staghorn coral and Elkhorn coral can reach enormous proportions. Corals reproduce once a year, on cues from the lunar cycle and the water temperature. Entire colonies of coral reefs then simultaneously release their tiny eggs and sperm, called gametes, into the ocean. When a coral egg and sperm join together as an embryo, they develop into a coral larva, called a planula. Planulae float in the ocean, some for days and some for weeks, before dropping to the ocean floor. 

Comparison of coral areas near Key West (FL) identified on an old sea chart from 1775 (left) and a modern satellite chart (right) where the same coral is missing (adapted from McClenachan et al. 2017 *) 

Then, depending on seafloor conditions, the planulae may attach to the substrate and grow into a new coral colony at the slow rate of about 10 centimeters a year. Unfortunately, this wonderful subtle process of reproduction is also vulnerable and easily affected by external influences like tropical storms, changes in water temperature or environmental pollution. 

Decline of the coral reefs The ecology of coral reefs have been a matter of concern of  marine biologists, since their conditions deteriorated  considerably in the last 50 years. Anthropogenic causes of dying coral reefs are land erosion, fertilizers used in agriculture, draining of  swamps and mangroves, motor ways connecting chains of small island,  deforestation, building projects, cyanide fishing and release of waste and chemicals by the industries. Together they form a serious threat for survival of great reef formations, such as the Great  Barrier reef in NE Australia and the Coral Triangle in the Indo pacific Ocean. But also less spectacular reef formations in the Caribbean and Southern Florida regions have shown a decline,  which has made these reefs  a 'worst case scenario' in a recent IUCN report. The Elkhorn coral, one of the most important Carribean corals in terms of its contribution to reef growth, will be hard to find today.

Ghost reefs of Florida Humans have fundamentally  altered coastal ecosystems over the years. Deterioration of seawater quality goes hand in hand with damage of coral formation, and mostly so in coastal areas were effects of pollution tend to be more severe. Recently, a team of  American and Australian investigators discovered that in the last two centuries many coral formations along the southern Florida coastline had vanished, especially close to the shore (*). They had the ingenious idea to use 18th century British imperial charts of the Florida coast made by cartographer George Gauld. His precise sea charts have the reputation to provide useful  ecological information  especially with regard  to corals that are of interest as a navigational hazard. Using these charts they identified  143 coral formations on two charts that spanned from Key Largo to the Marquesas Keys.  Most of the observations fell into the three interior zones: the nearshore patch reef, the offshore patch reef, and the reef crest. Their overall findings were then  compared with modern satellite charts of the same region (the Millennium Coral Reef Mapping Project, the Benthic Habitats South Florida Map, and the Unified Florida Coral Reef Tract Map). 

This revealed that overall 52% of the corals on the seafloor around the Florida Keys was lost.  Strongest losses were in the Florida Bay and  nearshore areas were coral had declined  by 87.5% and 68.8% respectively (see picture above for an example), whereas offshore areas of coral had remained largely intact.  The investigators conclude that surveys  looking only at species within the know extant range may overlook loss of coral formation over a longer time range resulting in an overly optimistic view of the current conservation status. The danger of a shifting baseline syndrome is that not the orginal (forgotten) situation but the current or more recent situation is taken as a baseline to evaluate environmental changes. Another example is climate change and global warming, which could have started much earlier than normally assumed. Namely with the onset of industrial revolution some 250 years ago  when its  global manifestations were less conspicuous.  But the  Florida study also has some good news: it highlights the restoration potential of  coral species in nearshore areas  that hitherto were considered unsuited for coral growth.

Sources

*Loren McClenachan et al. Science Advances, 2017 6 September. Ghost reefs: Nautical charts document large spatial scale of coral reef loss over 240 years research article

J. B. C. Jackson, Reefs since Columbus. Coral Reefs 16, S23–S32 (1997).