A carbon perspective on higher trophic levels - How do animals contribute to the organic carbon cycle and stock in the Baltic Sea?

Abstract

The world's oceans are an integral part of the Earth's climate system, contributing significantly to the mitigation of human-induced climate change through carbon cycling and storage. Marine organisms of all kinds, from the smallest virus to the largest whale, play various roles in both cycling and storage. Traditionally however, organisms higher up in the food web such as fish and marine mammals, have been given little to no consideration in this context, as their contribution was considered quantitatively negligible. Recent studies, conversely, show that these organisms can indeed make a significant contribution. However, despite increasing research, our understanding of how fish and marine mammals affect the ocean carbon cycle is still incomplete due to a lack of data, concepts and descriptions that encompass these organisms.

This incomplete understanding is particularly critical as populations of higher trophic levels are in steep decline worldwide. The contribution of these organisms may be changing as a result, while at the same time it has not even been fully understood, let alone quantified, especially in regional ecosystems. The Baltic Sea is one of the regional ecosystems where fish and marine mammals have not yet been accounted for in the carbon cycle and are simultaneously subject to significant population changes.

The Baltic Sea is a unique brackish ecosystem in Northeastern Europe. Exposed to high anthropogenic exploitation pressures and changing environmental conditions, the Baltic Sea has undergone several major transitions in the last decade. One consequence of these transitions includes the drastic reduction of the cod stock, one of the key higher trophic level species within the Baltic Sea. The sharp decline in the piscivorous cod population has implications for the food web, ecology of the Baltic Sea, and the livelihoods of many people who depend on this system. However, how the decline of cod affects carbon cycling and storage is unknown.

In this thesis, we advance the understanding of the contribution of higher trophic levels to organic carbon (OC) cycling and storage both globally and specifically within the Baltic Sea ecosystem. Although our investigations are mainly theoretical and conceptual, they fit into a picture that is increasingly emerging: Fish and other higher trophic levels can most likely make a non-negligible contribution to the marine carbon cycle and thus potentially to climate change mitigation. To account for the roles and contribution of these organisms in the future, scientific gaps need to be filled and a perspective shift in policy and management needs to be advanced.