The meagre future of benthic fauna in a coastal sea—Benthic responses to recovery from eutrophication in a changing climate

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.

Seasonal Variability in Benthic–Pelagic Coupling: Quantifying Organic Matter Inputs to the Seafloor and Benthic Macrofauna Using a Multi-Marker Approach

“…The consequences of such changes for the seafloor communities are unknown, although we can expect that many benthic biological processes will be affected. It has recently been suggested that climate change can potentially weaken benthic-pelagic coupling in the future due to shifts in the composition of pelagic production and the reduction in POC export to the seafloor (Ehrnsten et al, 2020). These changes in the pelagic system will have direct consequences for benthic macrofaunal biomass and coastal ecosystem functioning.…”

“…However, coastal areas are under increasing pressure from multiple stressors, such as eutrophication and climate change that are affecting the timing, quality, and intensity of phytoplankton blooms (Griffiths et al, 2017;Tamelander et al, 2017). Changes in the occurrence and nature of the pelagic blooms will alter the duration, quality and quantity of organic material transfer to the benthos, and consequently the function of benthic macrofauna communities (Griffiths et al, 2017;Ehrnsten et al, 2020). Hence, there is a need to gain insights into the seasonal variations in organic matter inputs to understand the role of seafloor sediments in the coastal ecosystem functioning.…”

Baltic Earth Assessment Report on the biogeochemistry of the Baltic Sea

“…In the Baltic Proper, climate change will increase primary production because warming and reduced bottom-water oxygen levels will intensify nutrient turnover. Higher pelagic regeneration will benefit phytoplankton, whereas benthic production depending on export from the euphotic zone, will decline (Ehrnsten et al, 2020). In the more oligotrophic Bothnian Sea and Bothnian Bay primary production might decrease because higher DOC inputs reduce transparency and favour heterotrophic bacteria (Andersson et al, 2015).…”

Human impacts and their interactions in the Baltic Sea region

“…dinoflagellates, certain copepods) (Wasmund et al, 2019). Changes in benthic communities have been related to warming and eutrophication (Ehrnsten et al, 2020). A prominent potential victim of the anticipated reductions in sea ice in the northern Baltic Sea is the ringed seal which breeds on sea ice (Meier et al, 2004), while other marine mammals e.g.…”