Carbon export is facilitated by sea urchins transforming kelp detritus

Abstract

With the increasing imperative for societies to act to curb climate change by increasing carbon stores and sinks, it has become critical to understand how organic carbon is produced, released, transformed, transported, and sequestered within and across ecosystems. In freshwater and open-ocean systems, shredders play a significant and well-known role in transforming and mobilising carbon, but their role in the carbon cycle of coastal ecosystems is largely unknown. Marine plants such as kelps produce vast amounts of detritus, which can be captured and consumed by shedders as it traverses the seafloor. We measured capture and consumption rates of kelp detritus by sea urchins across 4 sampling periods and over a range of kelp detritus production rates and sea urchin densities, in northern Norway. When sea urchin densities exceeded 4 m-2 , the sea urchins captured and consumed a high percentage (ca. 80%) of kelp detritus on shallow reefs. We calculated that between 1.3 and 10.8 kg of kelp m-2 are shredded annually from these reefs. We used a hydrodynamic dispersal model to show that transformation of kelp blades to sea urchin feces increased its export distance fourfold. Our findings show that sea urchins can accelerate and extend the export of carbon to neighbouring areas. This collector-shredder pathway could represent a significant flow of small particulate carbon from kelp forests to deep-sea areas, where it can subsidize benthic communities or contribute to the global carbon sink.

Ocean temperature controls kelp decomposition and carbon sink potential

“…Also, it could be because the material, even as detritus, can remain viable and photosynthetically active for extended periods in shallow subtidal areas with su cient light to maintain net photosynthesis 50 . Although critical information about export of this detrital material is still lacking in many regions 11 , our ndings show that kelp detritus has long residence times in the coastal zone, and therefore high potential to be transported to deeper regions 36,47 . This is consistent with evidence that a substantial amount of kelp reaches deep marine sinks where it can be sequestered in the long-term 11,32,51 .…”

“…Each cage was tethered with cable ties to a weight on the sea oor at ~8-m depth. In order to accurately quantify the impact of ocean climate on decomposition, we selected this cage size to exclude grazing by large herbivores in our experiments, which can drive localized increases in the turnover, size, and availability of kelp detritus in some areas 36 and could overwhelm measures of turnover in areas where they were locally abundant. All kelp pieces were kept damp after collection, stored in a dark cooler and deployed within 24 hours of collection.…”

“…For example, macroalgal detritus that reaches open ocean depths >1000 m is considered trapped in water masses where the CO 2 is retained for signi cant time periods (i.e., >1000 years) before returning to the ocean surface and eventually the atmosphere 9,32 . Detritus that is retained in some nearshore areas, such as deep fjords or basins with high rates of sedimentation may also be buried for 100s to 1000s of years, effectively removing it from the short-term carbon cycle [33][34][35][36] .…”

The differences in the trophic structure of semi‐enclosed and open coastal communities under the influence of an alien top predator (red king crab in the Barents Sea)

“…Sea urchins Strongylocentrotus spp. are known as active opportunistic grazers with the preference for macroalgae (Briscoe & Sebens, 1988; Evseeva, 2016; Filbee‐Dexter et al, 2020; Kuznetsov, 1946; Paar et al, 2019). Studies of the metabolism and demography of S. droebachiensis on the Murman coast led Propp (1977) to the conclusion that the food demands of sea urchins cannot be fulfilled by kelp production and has to be compensated by consuming fucoids and other algal material transported from the intertidal to the subtidal zone.…”

Effect of a cold event on population and community of pit-inhabiting sea urchins in Western Pacific coasts

“…In some places, such regime shifts have been accompanied by a hysteresis effect of approximately one order of magnitude in urchin biomass, which has maintained barrens between the critical threshold of overgrazing and recovery (Ling et al, 2015). Sea urchin barrens can persist in the absence of seaweeds because surviving urchins can shift their food source to less nutritious microalga or invertebrates growing on the open rocky substrates (Ling & Johnson, 2009), or acquire floating algae, algae fragments and detritus via their spines and tube feet (Campbell et al, 1973;Russo, 1980;Filbee-Dexter et al, 2020). Previous research has shown that predation pressure by sea urchins with high population densities continues to exceed seagrass growth and prevent recovery of kelp forest (Sivertsen 1997;Filbee-Dexter & Scheibling, 2014), and that the absence of sea urchin predators by fisheries prevents seaweed recovery (Johnson et al, 2005).…”