How trophic cascades and photic zone nutrient content interact to generate basin-scale differences in the microbial food web

T Frede Thingstad

doi:10.1093/icesjms/fsaa028

Abstract

Abstract In linear food chains, resource and predator control produce positive and negative correlations, respectively, between biomass at adjacent trophic levels. These simple relationships become more complex in food webs that contain alternative food chains of unequal lengths. We have used a “minimum” model for the microbial part of the pelagic food web that has three such food chains connecting free mineral nutrients to copepods: via diatoms, autotrophic flagellates, and heterotrophic bacteria. Trophic cascades from copepods strongly modulates the balance between the three pathways and, therefore, the functionality of the microbial food web in services such as food production for higher trophic levels, DOM degradation, and ocean carbon sequestration. The result is a theoretical framework able to explain, not only apparent conflicts in Arctic mesocosm experiments, but also biogeochemical features of the Mediterranean. Here, the fundamental difference between Arctic and Mediterranean microbial food webs is the way they are predator driven by seasonal migration of large copepods in the Arctic, but resource driven due to the anti-estuarine circulation in the Mediterranean. In this framework, global change effects on microbial ecosystem functions are more like to come indirectly through changes in these drivers than through direct temperature effects on the microbes.

Highlights

The abstract to this article can be seen as a state-of-the-art summary of research in marine microbial ecology rooted in a single event >40 years ago: the 1977 blowout from the Bravo platform, releasing something like 15 000 tonnes of crude oil (Gunkel et al, 1985) into the North Sea
I here argue for a somewhat similar difference in the external drivers for the photic zone microbial food web, i.e. with copepod predators being the central driver in cold Arctic waters, as opposed to a dominating resource-driven microbial food web in the warm Mediterranean Sea
The arguments are based on published observations for the west-to-east oligotrophication gradient in the Mediterranean Sea (Santinelli et al, 2012) and our use of trophic cascades to explain seemingly conflicting results in Arctic mesocosms (Larsen et al, 2015), The marine microbial food web

Summary

Introduction

The abstract to this article can be seen as a state-of-the-art summary of research in marine microbial ecology rooted in a single event >40 years ago: the 1977 blowout from the Bravo platform, releasing something like 15 000 tonnes of crude oil (Gunkel et al, 1985) into the North Sea. I here argue for a somewhat similar difference in the external drivers for the photic zone microbial food web, i.e. with copepod predators being the central driver in cold Arctic waters, as opposed to a dominating resource-driven microbial food web in the warm Mediterranean Sea. The arguments are based on published observations for the west-to-east oligotrophication gradient in the Mediterranean Sea (Santinelli et al, 2012) and our use of trophic cascades to explain seemingly conflicting results in Arctic mesocosms (Larsen et al, 2015), The marine microbial food web

Results

Conclusion