Deoxygenation impacts on Baltic Sea cod: Dramatic declines in ecosystem services of an iconic keystone predator

Karin Limburg,Alessandro Orio,Yvette Heimbrand

doi:10.1007/s13280-021-01572-4

Abstract

The intensified expansion of the Baltic Sea’s hypoxic zone has been proposed as one reason for the current poor status of cod (Gadus morhua) in the Baltic Sea, with repercussions throughout the food web and on ecosystem services. We examined the links between increased hypoxic areas and the decline in maximum length of Baltic cod, a demographic proxy for services generation. We analysed the effect of different predictors on maximum length of Baltic cod during 1978–2014 using a generalized additive model. The extent of minimally suitable areas for cod (oxygen concentration ≥ 1 ml l−1) is the most important predictor of decreased cod maximum length. We also show, with simulations, the potential for Baltic cod to increase its maximum length if hypoxic areal extent is reduced to levels comparable to the beginning of the 1990s. We discuss our findings in relation to ecosystem services affected by the decrease of cod maximum length.

Highlights

Ecosystem services are defined as ‘‘the benefits people obtain from ecosystems’’ (Millennium Ecosystem Assessment 2005) and are mainly divided into provisioning, regulating and maintenance, and cultural services (HainesYoung and Potschin 2013)
We attempt to link the increase in hypoxic areas with the decrease in maximum length of Baltic cod
Our model results show that the extent of suitable areas for cod have substantial explanatory power in predicting the changes in the maximum length of cod

Summary

Introduction

Ecosystem services are defined as ‘‘the benefits people obtain from ecosystems’’ (Millennium Ecosystem Assessment 2005) and are mainly divided into provisioning, regulating and maintenance, and cultural services (HainesYoung and Potschin 2013). Decreased levels of dissolved oxygen (hypoxia and anoxia) in global oceans and coastal zones is a growing problem around the world generated primarily by eutrophication and anthropogenic emissions of greenhouse gases (Diaz and Rosenberg 2008; Breitburg et al 2018). Hypoxic waters can affect organisms through direct mortality, alteration of metabolism and growth, forced migration, habitat contraction, increased susceptibility to predation, or changes in prey availability (Rabalais et al 2001, 2002; Breitburg 2002; Diaz and Rosenberg 2011; Hinrichsen et al 2011; Levin 2018). There are different mechanisms that link fish growth to oxygen concentrations as, for example, physiological stress due to exposure to hypoxia increasing metabolic costs, or overcrowding in normoxic areas resulting in density-dependent reduction of growth rates from resource depletion or interference competition (Breitburg 2002; Eby et al 2005; Pollock et al 2007)

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