| 16 Jan 2026
Colored and Fluorescent DOM in the Sea-Surface Microlayer: Response to a Phytoplankton Bloom and Photodegradation in a Mesocosm Study
Abstract. A month long mesocosm study at the Institute for Chemistry and Biology of the Marine Environment (Wilhelmshaven, Germany) examined how a phytoplankton bloom and photodegradation influence colored and fluorescent dissolved organic matter (CDOM and FDOM) in the sea-surface microlayer (SML) and underlying water (ULW). The SML, a thin (< 1000 µm) interface between ocean and atmosphere, plays a key role in air-sea exchange processes, but temporal mechanisms behind organic matter enrichment remain unclear. To isolate biogeochemical processes from environmental variability, daily SML and ULW samples were analyzed using spectral fluorometric and photometric methods, with supporting data e.g. on irradiance, temperature, and chlorophyll-a. The study covered bloom onset, peak, and decay of two partially overlying phytoplankton blooms. Samples were taken alternatively in the morning and in the afternoon, varying the exposure time to UV-light. Changes in composition and quality of organic matter were tracked using CDOM and FDOM derived parameters. Changes on the FDOM component composition were investigated using PERMANOVA. Protein-like FDOM components increased in both layers during bloom progression, while humic-like FDOM components decreased throughout the study. The significant influence of the bloom phases and the layer (SML or ULW) on the component composition was confirmed, however, their interaction was not significant. It’s likely that the change in FDOM component composition is a joint result of the influences of the phytoplankton bloom and photodegradation effects. Based on the slope ratio (SR) of CDOM absorption slopes S275-295 and S350-400, photodegradation was confirmed as the dominant sink of organic matter over microbial alteration. Generally, photodegradation represented a major sink for aromatic DOM during the mesocosm study, yet its effects were similar in the SML and ULW. Strong vertical mixing, shallow depth, and high light penetration likely prevented surface-specific photochemical gradients from forming.
This manuscript presents the time-course of the concentrations of the colored (CDOM) and fluorescent (FDOM) fractions of dissolved organic matter (DOM) in the sea-surface microlayer (SML) and the underlying water (UWL) during a controlled mesocosms experiment. The impact of a phytoplankton blooms stimulated by nutrient additions and photodegradation processes on the variability of the optical properties of dissolved organic matter in the two layers is assesed.
Overall, this is a well-designed study providing useful insights into the dynamics of colored and fluorescent dissolved organic matter in the SML, specifically regarding its biological production and photochemical decomposition under conditions that allow for isolating both driving forces from the multiple physical and biogeochemical factors involved.
I find the manuscript to be very thorough, which makes it quite long and occasionally difficult to follow; however, I recognize that this level of detail may be beneficial for readers who are not experts in the optical properties of dissolved organic matter.
Something that strikingly caught my attention is the lack of data on dissolved organic carbon (DOC) concentrations. My recommendation is that, if these were measured, they should be included. If they were not, a brief explanation should be provided.
Please find a list of detailed comments and recommendations below:
Line 44 – replace 0.4 microm by 0.7 microm
Lines 70 – 74. This is not the right place for these sentences. They should be moved to the last paragraph of the Introduction.
Lines 83 – 86. Same as above.
Line 93 – What do you mean with “high resolution” in this context?
Lines 96 – 98 (Figure 1). Proccesses excluded in mesocosms studies should be colored differently, e.g. inflow, upwelling, outflow, winds, mixing waves, etc...
Line 101-102. Please specify what is the depth of the outdoor basin (0.8 m). What is the color of the basin? (relevant issue in a study about photodegradation).
Lines 104-105. How was constant mixing of the ULW achieved?
Line 123. It surprises me that such large water volumes are needed to count bacterial abundance.
Line 135-136. While storing filtered samples at 4°C is standard practice for FDOM, the duration here extends to several months. Could you specify the exact timeframe? Additionally, were any tests performed to ensure sample integrity after such prolonged storage?
Line 137. Please, specify the excitation and emission slit widths.
Line 149. No need to include the equation for inner filter correction.
Lines 154 and 157. No need to include the equation for Raman normalization.
Line 180 – Table 1. IC1, IC2 and IC3 not described yet. Explain meaning in the table caption.
Line 207. Refer to the paper by Bibbi et al. (2025) for nutrient concentrations. A sentence indicating the levels observed and the would help the readers to follow the evolution of the mesocosm.
Line 2014. Which paper of Bibi et al (2025)? 2025a?
Line 223. These temperatures seems unrealistic for the Jade Bay. Please comment.
Line 276. It sems more logical to introduce first the variation of the CDOM and FDOM concentrations and later the %FDOM.
Line 347. Explain how the sum was calculated. Is just the sum of the Fmax of each componente or the sum of the average fluorescecence of each component?
Line 405 – Figure 8. You may calculate the derivative of Chla in the panels on the right.
Lines 536-537. a254 should not change susbtantially because natural UV of this wavelengh does not arrive to the Earth Surface so it does not directly decompose chromophores absorbing at this wavelengh.
Line 542-543. This sentence is repeated in the previous paragraph. It could be erased.
Lines 555-561. This paragraph about a paper in preparation could ne omttied.
Line 574. The conclusions may be drastically reduced. Lines 576–581 and 591–594 can be omitted.