Deep-time Climate Change Impact on Marine Food Webs

Link to project homepage: https://sites.google.com/view/kagiadi/projects/paleoweb Marine ecosystems around the world are changing today at a degree unprecedented in human history. The eastern Mediterranean Sea in particular is a unique ecosystem; this small-scale ocean is currently one of the areas most affected by climate change, the invasion of alien species from the Red Sea, and habitat deterioration due to human activities. Although we observe the ecosystem changing, the ultimate outcome of this change is unknown to us, because the historical and archaeological data are not sufficient to cover the extent of the environmental changes happening today, nor are they suitable to distinguish the effect of climate change alone. A way to circumvent this problem is by drawing data from the geological past. In this project, we use the fossils inside the rocks that form the beaches of Rhodes Island (Greece) to reconstruct the intricate food web of the eastern Mediterranean Sea one million years ago. The study time interval, which is part of the Pleistocene geologic epoch, was characterized by recurrent shifts between glacial and interglacial intervals that can be used as analogues to the temperature change of 24C that are predicted for our near future. However, unlike today, the eastern Mediterranean at that time was not affected by human activities or biological invasions. Therefore, it is suitable for isolating the climate change impact. The fossils of the various marine organisms in the rocks of Rhodes Island are proof of their existence in the eastern Mediterranean at that time. As glacial and interglacial intervals alternated, many of these organisms migrated in and out of the Mediterranean, in response to the rising and falling temperatures. In addition, their growth rates were affected, and some of them modified their food and lifestyle preferences in order to survive. Delving into these adaptation mechanisms, we derive knowledge on how a marine ecosystem responds to climate change, and potentially identify ways through which we can prevent and mitigate the impact of changing climate.

Ecosystems are structured into intricate food webs of interacting organisms that are simultaneously predators and prey. Today, marine ecosystems are heavily impacted by unprecedented climate change, which affects not only the individual organisms but also ecosystem structure and functioning. However, long-term data that is necessary in order to predict the future state of the oceans does not exist because systematic efforts to monitor the oceans did not start until the 1950s. Looking instead into the geological past, at intervals of extreme climate change, offers insights into the limitations of natural systems. The aims of the PALEOWEB project were to reconstruct the marine food web of the eastern Mediterranean Sea during glacial and interglacial periods in the Pleistocene, 700-800 thousand years ago, and to test hypotheses about the impact of the intense climatic changes that took place at that time on marine animal communities and the food web structure. The challenge in reconstructing ancient food webs is that only the hard parts of organisms (bones, teeth, scales, otoliths) are usually fossilized. As a result, many marine organisms that do not have such hard parts, such as jelly fish, do not leave fossil remains. In order to overcome this challenge, we used an ecological modeling approach that is based on mass balance. According to this, if a predator is present (based on the fossils we find), enough of its prey must also have been present at the same time. In this way, it is possible to infer the presence and abundance of organisms that have are not represented in the fossil record at all. The results of the PALEOWEB project confirmed our hypothesis that the marine food web of the eastern Mediterranean Sea was impacted by the climatic changes between glacial and interglacial periods, but these impacts were not irreversible. Although the stocks of fishes and shellfishes decreased, the marine food web did not collapse despite a 4C warming. However, it is important to stress that climate warming today is taking place much more quickly than ever before and it is leading to much higher temperatures. Therefore, the marine ecosystem may not have enough time to adapt to this change, as it did in the Pleistocene.