The Ultimate Breakdown Beneath the Surface – Zooplankton Influence on C, N and P Degradation in Diatom Aggregates

Abstract

In the ocean, carbon export is driven by the biological carbon pump where organic matter is transported from the surface layer to the deep sea. The sinking particulate organic matter is mainly a product of phytoplankton aggregates and faecal pellets from zooplankton & fish, together also known as “marine snow”. Marine snow is continuously being degraded by microbial and zooplankton activities as it sinks to the depth, resulting in attenuation of vertical flux. The attenuation of vertical flux has mostly been investigated in terms of carbon, whereas less is known about the nitrogen and phosphorus (C:N:P ratio) in descending marine snow. Here I investigated the degradation of two diatom aggregates made from Skeletonema marinoi and Chaetoceros affinis to quantify the changing size and C:N:P ratio over a two-week period, examining how microbial and zooplankton activities influence this process. This was done by incubating the aggregates with and without the presence of the copepod Amonardia normani. In addition, aggregate sinking velocities were measured using video recordings from the diatom aggregates. The results from the degradation experiments and sinking velocity were used in a transport matrix model to simulate the vertical flux of carbon and nitrogen over depth on a global scale.

The results showed a significantly different sinking velocity between the two aggregate types, similar to what has been reported in the literature. I also found significantly different fragmentation over time with and without the presence of copepods, suggesting that the feeding behaviour of copepods is important for modifying the vertical flux. However, in contrast to expectations the results did not show a significant increase in C:N or C:P ratio over time, instead we mostly found a decrease in the C:N and C:P ratio over time.

It is generally assumed, that while organic matter is being degraded, nitrogen and phosphorus are remineralized faster than carbon, resulting in an increase in the C:N:P ratio. The lack of a decrease in C:N:P ratio could have been due to an insufficient incubation time and testing with a longer degradation time might be advisable in the future. The use of our data in a transport matrix model resulted in values that differ from what has been reported in literature. The carbon reservoir I calculated using my data (6800 GtC) was approximately 3 times of what has been reported in literature (2800 GtC). I did experience some issues with the data which may have resulted in a miscalculation of the carbon reservoir. This highlights even more the importance of the variables ‘degradation rate’ and ‘sinking velocity’ when it comes to modelling the carbon sequestration in the ocean on a global scale. Learning more about these variables and their variability is important in order to improve flux models, especially in the context of the changing climate.

Popular Abstract

The Ultimate Breakdown Beneath the Surface

In the ocean, carbon export is driven by the biological carbon pump where organic matter is transported from the surface layer to the deep sea. The sinking particulate organic matter is mainly a product of algae clumps (aggregates) and faecal pellets from zooplankton & fish, together also known as “marine snow”. Marine snow is continuously being degraded by microbial and zooplankton activities as it sinks to the depth. Zooplankton are important organisms in the bottom of the marine food web. They actively effect how much carbon is being transported to the deep sea.

I was investigating the degradation of aggregates made by two algae species (Skeletonema marinoi and Chaetoceros affinis) with and without the presence of copepods by measuring their carbon, nitrogen and phosphorus content over a two-week period. I further measured their sinking speed by making video recordings. In the end I used the data in a model to illustrate the global export of carbon in the ocean.

In this study I found that aggregates are being fragmented by copepods making the aggregates smaller, which alters their sinking speed. I was also able to detect that the aggregates formed by Skeletonema marinoi sink faster than the aggregates formed by Chaetoceros affinis. This is most likely due to the fluffier structure of the Chaetoceros affinis aggregates what slowed the sinking process down. I detected a decrease in carbon, nitrogen and phosphorus content in the aggregates over the two-week period, an indicator for a degradation of the aggregates. However, we expected to see a faster remineralization of nitrogen and phosphorus due to their importance in metabolism and growth in copepods. We were unable to show that, and, in the future, we would try to use a longer experimental duration in order to enhance the probability to proof our hypothesis.

Nevertheless, we were able to use our data in a model and illustrate the carbon export in three different parts of the ocean. This model predicted an export of carbon of 4 000 to 14 000 g carbon per meter, depending where in the ocean carbon is being exported. In our predictions the carbon could stay in the deep sea for up to 31 years, before it recycles back up to the surface.

The more carbon is being exported in the ocean and stored in the deep ocean the better, as it helps remove CO2 from the atmosphere. CO2 is increasing due to climate change, and the ocean is one important storage place for it. Understanding the factors that influence the export of carbon in the ocean, helps making models better in calculating the efficiency at which carbon can be exported and stored in the ocean. This will help making better assumptions for the future and it will help us take actions in fighting climate change.


Master’s Degree Project in Biology 60 credits 2025
Department of Biology, Lund University

Advisor: Andre Visser and Marja Koski
Advisors Unit/Department (or Company or Authority): DTU Aqua

• author: Sommer, Alma Madita
• supervisor: Andre Visser; Marja Koski; Erik Selander
• organization: Degree Projects in Biology
• course: BION03 20242
• year: 2025
• type: H2 - Master's Degree (Two Years)
• subject: Biology and Life Sciences
• language: English
• id: 9212886
• date added to LUP: 2025-09-19 15:14:30
• date last changed: 2025-09-19 15:14:30