Low Contribution of the Fast‐Sinking Particle Fraction to Total Plankton Metabolism in a Temperate Shelf Sea

Abstract

Temperate shelf seas are productive areas with the potential to export high quantities of particulate organic carbon (POC), as sinking particles, to the sediments or off‐shelf to the open ocean. The amount of carbon which can be exported depends partly on the amount of POC produced and on the remineralization processes occurring on the sinking material. Here, we assessed the relative seasonal importance of microbial respiration and bacterial production associated with suspended, slow‐ and fast‐sinking particle fractions. The three fractions were collected in the Celtic Sea above and below the seasonal thermocline in November 2014, April and July 2015 using Marine Snow Catchers. The slow‐sinking fraction had higher microbial respiration and bacterial production rates than the fast‐sinking fractions, and these two fractions sustained rates of microbial respiration and bacterial production between 1 and 3 orders of magnitude lower than the suspended fraction. This low contribution of the slow‐ and fast‐sinking fractions was consistent with their low contribution to the POC concentration at the two depths sampled. The POC‐specific respiration rates associated with the slow‐ and fast‐sinking fractions were low (median 0.17 and 0.08 d−1, respectively), indicating low‐sinking particle degradation. Our results indicate that ∼5% of the POC in surface waters can be exported below the thermocline.

Respiration rate scales inversely with sinking speed of settling marine aggregates

“…Our results supported García‐Martín et al [ 8 ] in that respiration rate was highest in slower sinking aggregates, but note that the division between the sinking speed fractions were not identical as García-Martín et al divided into three groups: suspended, slow sinking (<24 m d-1) and fast sinking (>24 m d-1) aggregates [ 6 ]. Our division into different sinking speed fractions was different between the years, which could possibly have shifted the results slightly.…”

“…There are different characteristics of sinking aggregates that make them sink and decompose at different rates. The contrasting results of García‐Martín et al [ 8 ] and Alcolombri et al [ 7 ] for slow-sinking aggregates (0–40 m d -1 ) may depend on the way the measurements were carried out. Alcolombri et al [ 7 ] used uniform aggregates made from agar placed in a flow cuvette, whereas García‐Martín et al [ 8 ] collected natural sinking material with a marine snow catcher, and the collected material was subsequently differentiated into different sinking fractions.…”

“…The contrasting results of García‐Martín et al [ 8 ] and Alcolombri et al [ 7 ] for slow-sinking aggregates (0–40 m d -1 ) may depend on the way the measurements were carried out. Alcolombri et al [ 7 ] used uniform aggregates made from agar placed in a flow cuvette, whereas García‐Martín et al [ 8 ] collected natural sinking material with a marine snow catcher, and the collected material was subsequently differentiated into different sinking fractions. Here we took a similar approach as the latter but with the difference that we collected naturally settling aggregates from mesocosms.…”

“…In 2018, the overall experimental setup consisted of nine KOSMOS mesocosms [ 15 ], which were 15 m deep (Ø = 2 m), each with a volume of 38 m 3 [ 16 ]. Out of the nine mesocosms, one was an untreated control, and the rest had different addition of deep water, either as a single addition or the same amount of deep-water distributed as multiple additions throughout the experiment [ 8 ]. For this study, we used material sampled from three of the mesocosms: the control mesocosm and the two mesocosms that received the highest amounts of nutrient-rich deep-water.…”

“…On the one hand, Alcolombri et al [ 7 ] demonstrated that slowly moving aggregates (up to ~40 m d -1 ) are respired faster than barely moving (1–2 m d -1 ) or motionless aggregates, and attributed this to oligomeric breakdown products being more rapidly flushed away at higher flow rates. On the other hand, García‐Martín et al [ 8 ] found that respiration rate and bacterial production was higher in the suspended aggregate fraction (assumed to be non-sinking) than in the slow- and fast-sinking fractions (sinking speeds of <24 and >24 m d -1 ).…”