Degradation of macroalgal detritus in shallow coastal Antarctic sediments

Abstract

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.

Benthic primary production and respiration of shallow rocky habitats: a case study from South Bay (Doumer Island, Western Antarctic Peninsula)

“…While data in the Arctic remain scarce, Antarctic research on the mentioned topics is even harder to come by, with some areas even lacking basic information on, e.g., the bathymetry (e.g., Gattuso et al 2006). The existing studies have mainly targeted soft-sediment communities by applying sediment incubations (Nedwell et al 1993; Nedwell and Walker 1995; Shim et al 2011; Hoffmann et al 2018; Braeckman et al 2019) and O 2 microprofile measurements (McMinn et al 2010; Hoffmann et al 2018). However, hard substrate, i.e., rocky habitats are a dominant feature in Antarctic coastal waters, that cannot be targeted by traditional measuring approaches and they have thus been overlooked or ignored for assessing coastal element cycling (e.g., Glud et al 2010).…”

The Depleted Carbon Isotopic Signature of Nematodes and Harpacticoids and Their Place in Carbon Processing in Fish Farm Sediments

“…Overall, the meiofauna organisms incorporated approximately 0.2% of the total label added at reference experimental units and 0.1% at cage experimental units. This is in the same order of magnitude (or an order more) as the relative consumption of macroalgae by meiofauna of shallow Antarctic sediments (Braeckman et al, 2019), and about an order of magnitude less than the relative consumption of microphytobenthos by intertidal macrofauna (Herman et al, 2000) in similar time frames (few days). A generally higher specific uptake of diatoms was observed in reference samples (2way ANOVA) possibly because they are not oversaturated with organic load, as was the case in the farm samples.…”

Die Wälder der Antarktis

“…Makroalgen bedecken große Teile des Hartsubstrates und können Biomassen von 10 kg Frischgewicht m –2 erreichen . Viele der großen Braunalgenarten (Desmarestiales) sind durch Sekundärmetabolite vor direktem Fraß geschützt, werden aber regelmäßig durch Sturm, Erosion oder Eisgang abgerissen und als tote organische Substanz dem System zugeführt . Als Detritus sind sie für die Organismen besser verwertbar, da Zellwände teilweise mechanisch aufgebrochen werden und entweder durch Fauna (z.…”

“…Der aufgenommene Kohlenstoff kann dann wieder respiriert oder ein Teil davon als refraktorisches Material im Sediment als „blue carbon“ gespeichert werden. Eine Quantifizierung dieser Prozesse gestaltet sich als äußerst schwierig und ist erst in Ansätzen erfasst . Man geht aber davon aus, dass der Kohlenstoff der Makroalgen sehr wichtig für das antarktische Ökosystem ist, da er anders als der Kohlenstoff vom Phytoplankton als Langzeitspeicher auch im Winter für die Organismen zur Verfügung steht.…”