Red Sea coral reefs: A Pleistocene-Recent comparison

Coral reefs are complex ecosystems that provide important goods and services to society, including protection of the shoreline, seafood, biodiversity and recreational opportunities. Since the 1980s, acute heat stress and associated coral bleaching have become more frequent, leading to a decrease in the live cover and diversity of coral reefs. It has therefore become crucial to predict the response of reef corals to future temperature rise, which includes adaptation and acclimatization on site, biogeographic shifts to higher latitudes, decline at lower latitudes, or global decline because of decreasing capability to produce aragonitic skeletons. All these potential responses to climate change can be studied from the Pleistocene fossil record of coral reefs. Martin Zuschin and Andreas Kroh and their team of international cooperation partners propose to study community changes along a latitudinal gradient in the last interglacial episode (i.e., the Eemian, approximately 125,000 years ago), when temperatures and sea level were higher than today. This will yield important insights into ecosystem stability and biogeographic shifts of coral reefs and therefore promote our understanding of responses of coral reefs to climate changes. We plan to study fossil reefs along a latitudinal gradient in the Red Sea and then compare our quantitative results with datasets from the modern reefs of the region. The data from the modern Red Sea consist of older published and new data provided by our international partners. Comparing them has the added value that we can test if homogenization of the faunal composition due to anthropogenically induced environmental degradation has already occurred over the last few decades. Specifically, we address the following research questions: Do Red Sea coral reefs show a stable community composition over the last 125,000 years? Did Red Sea coral reef communities shift northward during the Eemian climate warming? Did the diversity of central Red Sea coral reef communities decrease during the Eemian climate warming? Is the diversity of Eemian coral reefs in the Red Sea primarily determined by habitat complexity? Do Eemian reefs support the idea that there is a recent homogenization of modern coral communities in the Red Sea? The chosen Pleistocene reef terraces will be radiometrically dated and fossil reef assemblages will be mapped with the same method used to study modern reefs. We will incorporate published and unpublished, modern and Pleistocene coral datasets from the Red Sea and evaluate the data matrix with modern statistical methods in terms of changes in diversity and community structure in space and time.

Community changes were studied at the Red Sea coast of Egypt along a latitudinal gradient in the last interglacial episode (i.e., MIS5e, Eemian), when temperatures and sea level were higher than today. The goals were to get insights into ecosystem stability and biogeographic shifts of coral reefs and to learn about the responses of coral reefs to climate changes. Standard ecological techniques were applied to fossil coral reefs, to facilitate comparability between modern and fossil datasets. Three field trips were conducted to Egypt to sample fossil reefs between Marsa Alam and Hurghada and two field trips to Sudan to study modern offshore reefs. Modern and fossil reefs were studied with the same well-established method, line-intercept transects, where everything along a 20 m long transect is recorded in the field and at some localities additionally with photo transects, which were studied in more detail in the lab. Fossil reefs were age-dated with the the Uranium-Thorium method. Sixty coral taxa were identified (mostly to species-, some to genus level) in fossil reef terraces along the Red Sea coast of Egypt. The most abundant taxon is Porites sp. at all fossil sites, while the consecutive coral taxa differed in abundance depending on the locality and and on the palaeoenvironment. Diversity, reef cover and community composition in fossil reefs evaluated from line transects differ strongly from results gained with photo quadrats. We were consistently able to distinguish between reef edge and slope and can demonstrate a latitudinal gradient of slightly decreasing diversity from south to north in these two fossil reef habitats. The fire coral Millepora is a key genus to distinguish exposed from protected reef environments in the fossil reefs. Fire corals are typically rarely found in fossil reefs and we can show that a lack of preservation of habitats preferred by Millepora instead of the preservation potential of the skeleton itself is the most plausible reason for the mismatch between modern and fossil abundances. Quantitative, semiquantitative and qualitative collections of molluscs from the studied reef terraces yielded a total of 244 species. Some molluscan species are indicator taxa for MIS5e terraces in the Red Sea and do not occur in the Red Sea today. The MIS5e locality Sharm El Luli, near Marsa Alam is characterized by a variety of reefs and reef-associated habitats; molluscan diversity is highest in coral patches and at the reef top. The studied modern Sudanese offshore reefs showed overall negligible coral bleaching, and relatively high coral cover (~ 30 - 40 %) and diversity (> 100 taxa). High abundances of soft corals and high cover of sponges, however, indicate that all investigated reefs show signs of disturbance and stress.