Abstract
Nutrient loading and climate change affect coastal ecosystems worldwide. Unravelling the combined effects of these pressures on benthic macrofauna is essential for understanding the future functioning of coastal ecosystems, as it is an important component linking the benthic and pelagic realms. In this study, we extended an existing model of benthic macrofauna coupled with a physical-biogeochemical model of the Baltic Sea to study the combined effects of changing nutrient loads and climate on biomass and metabolism of benthic macrofauna historically and in scenarios for the future. Based on a statistical comparison with a large validation dataset of measured biomasses, the model showed good or reasonable performance across the different basins and depth strata in the model area. In scenarios with decreasing nutrient loads according to the Baltic Sea Action Plan but also with continued recent loads (mean loads 2012-2014), overall macrofaunal biomass and carbon processing were projected to decrease significantly by the end of the century despite improved oxygen conditions at the seafloor. Climate change led to intensified pelagic recycling of primary production and reduced export of particulate organic carbon to the seafloor with negative effects on macrofaunal biomass. In the high nutrient load scenario, representing the highest recorded historical loads, climate change counteracted the effects of increased productivity leading to a hyperbolic response: biomass and carbon processing increased up to mid-21st century but then decreased, giving almost no net change by the end of the 21st century compared to present. The study shows that benthic responses to environmental change are nonlinear and partly decoupled from pelagic responses and indicates that benthic-pelagic coupling might be weaker in a warmer and less eutrophic sea.
Highlights
Coastal seas around the world are facing multiple anthropogenic pressures (Cloern et al, 2016; Gray, Wu, & Or, 2002; Griffiths et al, 2017)
Unravelling the combined effects of nutrient loads and climate change on benthic macrofauna is important for understanding the future functioning of coastal ecosystems, as it is an important component linking the benthic and pelagic realms (Graf, 1992; Grall & Chauvaud, 2002; Griffiths et al, 2017)
For the first time, a dynamic model of benthic fauna coupled to a hydrodynamic–biogeochemical model to estimate the biomass and carbon processing of benthic fauna in a changing coastal sea over more than a century
Summary
Coastal seas around the world are facing multiple anthropogenic pressures (Cloern et al, 2016; Gray, Wu, & Or, 2002; Griffiths et al, 2017). Timmerman et al (2012) estimated the balance between biomass lost and gained with changes in nutrient loading at six stations in the deeper parts of the Baltic Sea using a physiological macrofauna model driven by outputs from a physical–biogeochemical model, but did not investigate the effects on carbon cycling This model was further developed to simulate the biomass and metabolic carbon processing of benthic fauna in two coastal areas in the near past (Ehrnsten, Norkko, Timmermann, & Gustafsson, 2019) and to systematically investigate the effects of temperature, food supply and hypoxia on local benthic communities in the Baltic Proper (Ehrnsten, Bauer, & Gustafsson, 2019). We extend these developments to explore the response in biomass and metabolic carbon processing of benthic macrofauna to the combined effects of different scenarios of changing nutrient loads and climate on the scale of the Baltic Sea over a century
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