What makes kelp more palatable for mesograzers? Effects of urchin grazing, light and aging on kelp detritus palatability, and its role in carbon sequestration

Abstract

Kelp forests are among the most productive ecosystems on Earth, releasing detritus year-round through direct blade erosion, storm-induced detachment, wave breakage, seasonal shedding, as well as indirect urchin grazing. Fragmental kelp detritus is transported from the euphotic zone to deeper parts of the ocean such as fjords, where decomposition rates are very low and detritus may contribute to long-term carbon storage. However, as detritus moves along the seafloor, detrital fragments may become more palatable to small benthic consumers. Fragmentation, such as shredding through sea urchin grazing, increases availability. Different irradiance levels could for example influence photosynthetically active fragment cells. Aging could increase palatability through microbial colonization. Increased palatability and linked consumption reduce kelp fragment carbon sequestration potential through recirculation of carbon.

This study examined how fragmentation origin (mechanical versus urchin gut passage), aging, and irradiance influences chemical composition (C:N ratios) and through a series of feeding trials how this is linked to fragment palatability for mesograzers. Using C:N ratio as a proxy for nutritious value and palatability, a combination of no-choice and multiple-choice feeding trials were performed to assess separate and combined effects of experimental factors on mesograzer consumption rates and preferences.

Fragmentation method and aging duration were significant predictors of C:N ratio, with mechanically fragmented material developing strongly elevated C:N ratios at long aging duration, while urchin-grazed material maintained lower C:N ratios. The higher irradiance treatment accelerated carbon fixation in mechanically fragmented material. Chemical differences between treatments translated into detectable feeding preferences only partially. In multiple-choice feeding trials, Gammarus sp. showed a significant preference for urchin-grazed, long-aged material while in Littorina littorea aging significantly predicted consumption. Comparison between fresh and reconstituted material indicated that both morphological and chemical properties of detritus contribute to palatability.

The findings contribute to a better understanding of Blue Carbon dynamics and the role of kelp-derived detritus in marine carbon sequestration and help improve estimates of the ocean’s Blue Carbon capacity.

Popular Abstract

What makes kelp more palatable for mesograzers?

Effects of urchin grazing, light and aging on kelp detritus palatability, and its role in carbon sequestration.

Kelp forests are among the most productive ecosystems on Earth. Beyond feeding many species directly, the three-dimensional structures kelp individuals create serve as habitat for invertebrates, fish, and marine mammals. But the importance of kelp doesn't end when a blade breaks off. Kelp fragments are produced year-round, through wave breakage, storms, blade erosion, natural shedding, and the work of so-called "marine shredders" like sea urchins, which consume large pieces and transform them into smaller, nutrient-rich fecal pellets. Small grazers, like amphipods, snails, and limpets consume those, recirculing carbon back into the food web.

If not consumed kelp fragments drift from shallow, sun-lit waters down into deeper fjords, where cold temperatures and low oxygen slow decomposition. There, the carbon locked inside kelp tissue can be buried and stored for centuries, a process known as Blue Carbon sequestration. Overfishing of key predators of sea urchins like cod and sea otters has tipped the balance between kelp and urchins, allowing urchin populations to explode, intensifying shredding activity and increasing carbon recirculation. This can potentially shift the ocean's role as a carbon sink.

This study set out to understand what makes kelp detritus more or less appealing to small consumers, focusing on three factors: How the fragment was created (through urchins grazing versus mechanical fragmentation), how long it had been aging, and how much irradiance it was exposed to. The chemical ratio of carbon to nitrogen (C:N) was used as a proxy for nutritional quality. A lower C:N ratio signals a higher nutritious value, meaning a more palatable material.

Looking at the C:N rations, urchin-processed detritus and mechanically fragmented detritus started out chemically similar, but diverged over time. After long aging, mechanically fragmented kelp developed very high C:N ratios, meaning fragments became carbon-rich but nitrogen-poor. Urchin-processed material, by contrast, maintained low C:N ratios throughout, likely because gut passage physically disrupts kelp cells, increases surface area, and introduces nitrogen-fixing bacteria, making the material increasingly nutritious as it aged. Higher irradiance exposure further widened this gap, accelerating carbon fixation in mechanically fragmented material and increasing its C:N ratio even higher.

The translation of chemical differences in feeding preference was more complex and less connected than expected: When grazers were given only one type of food with no alternative, a so-called no-choice trial, most of these chemical differences did not translate into obvious changes in how much they consumed. It seemed more like individuals tended to eat what they were given when there's nothing else offered. However, when given a choice between multiple food types simultaneously, preferences emerged for some species. The amphipod Gammarus sp. showed a consistent preference for urchin-processed, long-aged detritus, which is precisely the food with the lowest C:N ratio. Other tested species (Littorina littorea and Patella pellucida) did not show the same discrimination, suggesting more predictors of palatability than initially expected.

Interestingly, by comparing trials using intact fragments versus homogenized material with the conserved chemical composition, isolated from morphological properties, the study also showed that palatability is shaped by both the physical structure of fragments as well as their chemical composition, not chemistry alone.

These findings add a piece to the puzzle of how much carbon kelp ecosystems can truly lock away, and highlight that the ocean's Blue Carbon capacity depends not just on how much kelp grows, but on what happens to every last fragment of it.

• author: Tichawa, Sarah
• supervisor: Markus Molis; Erik Selander
• organization: Degree Projects in Biology
• course: BION02 20252
• year: 2026
• type: H2 - Master's Degree (Two Years)
• subject: Biology and Life Sciences
• keywords: Palatability, sea urchin grazing, carbon sequestration, blue carbon, herbivory, detritus
• language: English
• id: 9227941
• date added to LUP: 2026-05-26 09:33:36
• date last changed: 2026-05-26 09:33:36