Organic matter processing in a [simulated] offshore wind farm ecosystem in current and future climate and aquaculture scenarios.

The rapid development of blue economy and human use of offshore space triggered the concept of co-location of marine activities and is causing diverse local pressures on the environment. These pressures add to, and interact with, global challenges such as ocean acidification and warming. This study investigates the combined pressures of climate change and the planned co-location of offshore wind farm (OWF) and aquaculture zones on the carbon flow through epifaunal communities inhabiting wind turbines in the North Sea. A 13C-labelled phytoplankton pulse-chase experiment was performed in mesocosms (4 m3) holding undisturbed hard-substrate (HS) communities, natural sediment with infauna, and mobile invertebrate predators. Carbon assimilation was quantified under current and predicted future-climate conditions (+3 °C and -0.3 pH units), as well as a future-climate co-use scenario with blue mussel (Mytilus edulis) aquaculture. Climate change induced an increase in macrofaunal carbon assimilation as well as an organic enrichment of underlying sediments. Dynamic (non-)trophic links between M. edulis and other HS epifauna resulted in shifts among the species contributing most to the phytoplankton-derived carbon flow across climate scenarios. Increased inter- and intraspecific resource competition in the presence of M. edulis aquaculture prevented a large increase in the total assimilation of phytoplankton by HS fauna. Lower individual carbon assimilation rates by both mussels and other epifauna suggest that if filter capacity by HS epifauna would approach renewal by advection/mixing, M. edulis individuals would likely grow to a smaller-than-desired commercial size. In the same scenario, benthic organic carbon mineralisation was significantly boosted due to increased organic matter deposition by the aquaculture set-up. Combining these results with in situ OWF abundance data confirmed M. edulis as the most impactful OWF AHS species in terms of (total) carbon assimilation as well as the described stress responses due to climate change and the addition of bivalve aquaculture.

Degradation of macroalgal detritus in shallow coastal Antarctic sediments.

Glaciers along the western Antarctic Peninsula are retreating at unprecedented rates, opening up sublittoral rocky substrate for colonization by marine organisms such as macroalgae. When macroalgae are physically detached due to storms or erosion, their fragments can accumulate in seabed hollows, where they can be grazed upon by herbivores or be degraded microbially or be sequestered. To understand the fate of the increasing amount of macroalgal detritus in Antarctic shallow subtidal sediments, a mesocosm experiment was conducted to track 13C- and 15N-labeled macroalgal detritus into the benthic bacterial, meiofaunal, and macrofaunal biomass and respiration of sediments from Potter Cove (King George Island). We compared the degradation pathways of two macroalgae species: one considered palatable for herbivores (the red algae Palmaria decipiens) and other considered nonpalatable for herbivores (the brown algae Desmarestia anceps). The carbon from Palmaria was recycled at a higher rate than that of Desmarestia, with herbivores such as amphipods playing a stronger role in the early degradation process of the Palmaria fragments and the microbial community taking over at a later stage. In contrast, Desmarestia was more buried in the subsurface sediments, stimulating subsurface bacterial degradation. Macrofauna probably relied indirectly on Desmarestia carbon, recycled by bacteria and microphytobenthos. The efficient cycling of the nutrients and carbon from the macroalgae supports a positive feedback loop among bacteria, microphytobenthos, and meiofaunal and macrofaunal grazers, resulting in longer term retention of macroalgal nutrients in the sediment, hence creating a food bank for the benthos.